|The Impacts of Opencast Mining on the Rivers and Coasts of New Caledonia (UNU, 1984, 53 pages)|
During October 1982, surveys were carried out to assess the impacts of opencast mining on rivers and coastal systems in New Caledonia. Features shown on vertical air photographs taken by the Institut Géographique National in 1954-1955 (scale 1 :40,000) were compared with those on air photographs taken in 1976 (scale 1 :20,000), and with the present configuration examined in the field. In northern New Caledonia in 1982 many of the rivers still showed the effects of torrential runoff during Cyclone Gyan (24-25 December 1981), when large quantities of sediment of varying calibre were carried downstream. Severe flooding was recorded for the Diahot, Koumac, and louanga rivers in the north-west, and there was torrential flow down ungauged streams to the west coast between Voh and Koumac.
The problem of deciding how far opencast mining has influenced the form and sediment content of river channels and to what extent it has modified features at river mouths and in adjacent coastal and nearshore environments was considered on the basis of each of the major river systems draining to New Caledonia's western and eastern coastlines (fig. 2). These assessments are given below, taking the river systems in counterclockwise order from the north.
Opening north-westward into the Baie d'Harcourt, the Diahot (catchment: 620 sq km) carries a substantial load of fine-grained sediment, mainly sand and silt, derived from the Mesozoic sedimentary formations of the Central Ranges and from the mica-schists of the Panié massif. Sediment delivered to the river mouth, mainly during flood episodes, has been built into channelside levees prolonged as sedimentary jetties forming the framework of a delta that includes extensive mangrove swamps as well as dry saline plains. The rate of sediment yield may have been increased in historical times by the cutting and burning of forest and woodland vegetation and the introduction of grazing, resulting in accelerated soil erosion on friable materials.
This river system drains a basin of 229 sq km in north-western New Caledonia, its northern tributaries flowing from the schistose highlands of the Forêt d'Ougne Range, while the southern tributaries include streams flowing from the ultrabasic massif of Tiébaghi. This massif has been mined for chromium ore, at first opencast, then mainly in shafts and edits. The Néhoué and its tributaries have water stained red with ferruginous clay in suspension, and it is likely that their loads have been augmented by mining waste and material eroded from areas where the surface has been disturbed by prospecting. However, although some red mud has reached the deltaic region, features at the mouth of the Néhoué have so far been little modified by the effects of mining in the catchment.
The Koumac and its tributaries drain an extensive catchment (area: 258 sq km) in northern New Caledonia, where the geological formations include schists, sandstones, limestones, and phthanite chert, a form of hard cryptocrystalline silicified sediment. This latter material has broken up into coarse gravels, present in the Pleistocene river terraces and piedmont fans bordering the upper valleys, and abundant in the river channel downstream past Koumac to the delta region. During episodes of flooding, gravel has been carried to the mouth of the river, and longshore drifting has thence swept it northwards along the coastline to form beaches, culminating in a long recurved spit at Tangadiou. Although there has been mineral prospecting within this catchment, it has not been affected by opencast mining, and the presence of gravel in the river channel and along the coastline north of the delta cannot be attributed to mining impact. It is thus evident that if a source of coarse-grained sediment (in this case Pleistocene terrace and fan gravels) is naturally available within such a river catchment, such material can be carried down to the coast, notably during episodes of floodwater discharge that occur during tropical cyclones. Delivery of coarse sediment loads to river mouths is thus not in itself a reliable indication of the impact of opencast mining in New Caledonia.
Nevertheless, it appears that the addition of gravelly beaches to the coastline at and north of the Koumac delta is a very recent phenomenon, for the older parts of the delta are dominated by sediments no coarser than sand. A probable explanation is that slope wastage, and especially the erosion of Pleistocene river terraces and piedmont fans, results from increased and accelerated runoff due to the reduction and impoverishment of the vegetation cover, mainly forests, within
this catchment by bushfires. The Melanesian people used fire extensively in this region, and bushfires have occurred frequently in the era of European occupation. If this is the case, the delivery of a gravelly load to the coast by the Koumac during floods is indeed an anthropogenic impact, even though it is not due to opencast mining.
South of Koumac the ultrabasic Kaala massif is fronted by a broad piedmont fan composed of a variety of coarse to fine sediments washed down from the steep slopes during Pleistocene times. In the Holocene this has become a relatively stable landform, crossed by a series of parallel small watercourses, but one of these, the Oué Bouameu (catchment: 13 sq km), has become larger and deeper than the others, and is carrying a load ranging up to cobbles and small boulders, much of which has been derived from spilling and slumping waste fans around the opencast nickel mines on Mount Kaala. The Oué Bouameu has taken a variety of courses across a lobate fan at Karembé (fig. 13), where recurrent flooding of a farmstead has prompted the cutting of an artificial channel to deflect the river southwards. During Cyclone Cyan, when over 430 mm of rain fell within a period of 24 hours at Koumac, the Oué Bouameu flowed torrentially and carried large quantities of sediment ranging from fine red clay up to cobbles 12 cm in diameter downstream to the coast (plate 6), where the coarser sand and gravel have accumulated as a beach in front of a shoreline that was previously mangrove-fringed and fronted by tidal mudflats (plate 7).
This beach has grown northwards (plate 8), past the Karembé wharf, where a rock breakwater is under construction to the north to intercept the drifting beach material and prevent it spreading to the Koumac shore. Wave action is thus redistributing material that was brought down to the coast in the last flood. Near the southern end of the deposited sand and gravel, southerly waves have built a cuspate spit, which is being driven back into mangrove-covered clay flats in the form of a chenier.
Red clay washed down from the hilltop mine has been deposited in the nearshore zone, and the sea is stained red-brown by this sediment in suspension. Local people believe that this pollution has impeded coral growth in nearshore waters, and that the inshore fishery has also been impoverished, both in terms of quality and quantity of fish available. But apart from structural damage during the flood (plate 9), the mangrove areas have been little affected and mangrove growth is continuing on red mud accretion. Construction of barrages and sediment traps around the opencast mine is now reducing the sediment yield, but substantial masses of waste material have already spilled and slumped down slopes in the headwaters of the Oué Bouameu, and much of this will be carried down to the coast during future episodes of flooding.
Like the Koumac, the louanga (catchment: 522 sq km) drains an area of varied geology, mainly schists, sandstones, and phthanite cherts. it has so far been little affected by the opencast nickel mining on Mount Kaala and on the Massif d'OuazangouTaom; its channel load at Kaala-Gomen is a mixture of sand and gravel, the latter mainly of chart. Some of this material reaches the delta region, where ridges of fluvial sand and gravel, with some shelly and coralline material, have been built up as fringing beaches or cheniers emplaced on clay plains. However, the seaward fringes of this delta are mainly mangrove swamp, and gravel beaches and spits of the kind seen north of the Koumac delta have not yet developed. In the past few years there have been changes in the meandering channel downstream from the highway, associated with aggradation and bank erosion, but this does not seem to be a direct consequence of anthropic factors.
The Tinip stream (fig.14) drains a small catchment (area: 12 sq km) on the western slopes of Mount Ouazangou. Like the Oué Bouameu, it has been strongly affected by opencast nickel mining in its headwaters, the stream channel having been widened to accommodate an increased and coarsened sediment yield derived from the waste products of mining. An air photograph taken in September 1976 shows a large mass of mining waste in the valley on the western slopes of Mount Ouazango (see fig.14), and this has subsequently moved down the river. In October 1980 the Tinip had an aggraded channel at the highway crossing (plate 10), mining waste having reached this sector, but by October 1982 much of this material had been sluiced down-stream, re-exposing the trunks of trees that had been killed and buried by the earlier aggradation (plate 11). This change implies some success in attempts to control the flow of overburden discharged from the hilltop mine, the last major floods (Cyclone Freda in March 1981, then Cyclone Gyan in December) having driven channel fill down the river where the channel has widened and aggraded (plate 12). In the Baie de Gomen a delta of red mud, sand, and fine gravel is developing. The inshore waters of the Baie de Gomen are stained red with suspended clay, and the prop-roots of Rhizophora are also coated with red clay, although this has not prevented their continued growth. A cloud of suspended clay extending south from the river mouth is visible on an air photograph taken in 1976, but is absent from a 1954 air photograph of the same area, taken before the periods (1955-1964 and 1967-1977) of intensive mining on Mount Ouazangou.
The Tinip is thus an example of a stream that has been modified along the whole of its length by the inflow of mining waste materials from its headwater regions. The coastline at its mouth has been modified by the formation of an "anthropic delta," and the nearshore waters show the effects of red clay pollution, which has not proved adverse to mangrove growth but may well have depleted local fish populations.
Like the Tinip, the Ouaco (catchment area: 35 sq km) has been much modified by the effects of opencast mining on Mount Ouazangou. Its sediment yield has been increased and its deltaic shoreline is prograding as the result of the additional sediment supplied to the river mouth. The nearshore sea water here is generally discoloured by red clay washed down the river and retained in suspension.
Draining the southern slopes of Mount Ouazangou, the Poué Koué (basin area: 32 sq km) has also been greatly affected by the opencast hilltop mining. In fact, the mining waste is received from three small peridotite massifs in the headwaters (Ouala, Tsiba, and Oualéat), which have been intensively mined. Being only four to six km inland, their wastes have quickly reached the river mouth. The river channel is overloaded with a mixture of sediment ranging from red clay to small boulders, and the lobate piedmont fan that formed this part of the coastline is being prograded by the addition of this material to form a delta around the river mouth. Extensive red staining has polluted the coastal waters off the Poué Koué.
Draining the southern slopes of the Massif d'Ouazangou-Taom, this river (basin area: 105 sq km) has been less affected by the inflow of mining waste, which has not spread as far downstream as in the Poué Koué. Mining has occurred at only three sites 15 to 20 km upstream, so that the effects have been less dramatic than in the smaller Tinip and Poué Koué basins. At the highway bridge the watercourse is slightly incised and gravelly, but the sediment yield to the mouth of the river has been small and the effects of mining activities on coastal and nearshore environments have so far been only slight.
This river drains a deep gorge between Mount Taom and Mount Témala. The upper part of its catchment (area: 92 sq km) has not been affected by hilltop nickel mining, but the summit area and the eastern slopes of Mount Taom have been intensively prospected, leaving a dense network of trackways (visible on 1976 air photographs). Below Boyen the river flows across a Pleistocene alluvial plain, and although its channel load at the highway crossing contains peridotite boulders from the massif, the augmentation of sediment yield to the coast has so far been meagre and the effects of mining activities remain very slight.
Although nickel mining has taken place in the coastal Mount Katépahié and Massif de Voh regions, the sediment yield from the Témala catchment (352 sq km), like that from the adjacent Oué Pouanlotch, has been more affected by the soil erosion that has resulted from the grazing of pastures on basaltic hill country in the relatively dry environment around Témala (Iltis 1979). Occasional heavy rains have washed substantial quantities of grey clay and silt from these eroding hillsides into the Témala River, and downstream to the extensive marshes and mangrove swamps on its delta plains west of Voh. At the highway bridge the river is clear, with shoals of grey sand and gravel, and the impact of the opencast mining activities here and at the river mouth has also been slight.
The Voh River (catchment area: 227 sq km) drains an extensive upland region dominated by the large ultrabasic Massif de Koniambo, the north-western part of which is called the Massif de Voh. Hilltop mining was intensive here from 1900 to 1940, when the region was the leading producer of nickel in New Caledonia, exceeding even the Thio area. Hillsides remain scarred by this mining activity and there is no doubt that the Voh and its tributaries had augmented sediment loads during the mining phase, but the inflow of mining waste has diminished, and the river channel, which was probably widened as the augmented sediment yield passed through, now appears relatively stable. It is likely that the delta at the mouth of the Voh has been prograded as the result of augmented sediment discharge, but these effects have diminished and the present mangrovefringed coastline is no longer strongly affected by these historical mining activities in the hinterland.
Massif de Koniambo
This ultrabasic massif, between Voh and Koné, is one of the larger areas that has so far not been mined extensively for nickel; it is regarded as a reserve for future nickel production and will be exploited in due course. Meanwhile the summits and slopes are almost intact, their weathered lateritic mantle held in place beneath maquis scrub, and the Piedmont apron on their seaward flank shows a number of narrow, gravelly watercourses (e.g., Le Pandanus, La Confiance) that have not been modified by mining activities. Their present dimensions and sediment loads are thus contrasted with those of the Oué Bouameu and Tinip streams, which were once like this.
Koné and Pouembout
Ultrabasic terrains at the eastern end of the Massif de Koniambo form only a small proportion of the catchment (area: 272 sq km) of the Koné and its tributaries, and at the highway bridge the river carries a mainly sandy load downstream to a mangrove-fringed delta that is not prograding as rapidly as the deltas of rivers strongly affected by mining activities. The same applies to the adjacent Pouembout river system (catchment basin: 339 sq km). However, the left bank tributaries of the Pouembout are influenced by inflow of graveily peridotite from former mines on the crest of the Massif de Kopéto, notably during floods that accompany cyclones. This coarse material extends down to the confluences with the Pouembout, finer sediment having been washed further downstream.
The Tiaoué has a relatively small catchment (66 sq km) on the western side of the Massif de KopetoBoulinda. Although there has been some prospecting and hilltop mining on the upper fringe of this catchment, the river system has been little affected. It carries a sparse load of grey gravel, sand, and silt, largely derived from the erosion of basaltic hill country, and has not yet built a delta.
The Népoui, and its tributary, the Rivière Salée (combined catchment area: 177 sq km), drain deeply dissected ultrabasic massifs where opencast nickel mining has been extensive (fig. 15). In the 1970s "boom" period this was the most productive mining region in New Caledonia, but it has been overtaken by Kouaoua, and in fact production ceased from the Massif de Kopéto on I January 1983. As a result of mining, vast masses of overburden have flowed and slumped down the slopes bordering hilltop mines in the Massif de Kopéto-Boulinda (and especially in the Massif de Graunda-Boulinda), and the river channels of the Népoui (as of the neighbouring Ouha to the south-east) have been greatly modified, widening to accommodate large quantities of sediment ranging from red clay through sand and gravel to cobbles and boulders. Figure 16 shows details of the impact of this opencast mining on the geomorphology of Mount Graunda, between the Népoui and Ouha valleys. Mining has here removed much of the summit plateau, and hillside waste fans form depositional cones, one of which has partly blocked the Ouha valley, forming a ponded river. The channels of the Népoui and Ouha have become wide and gravelly, the streams flowing through braided channels. Plates 2, 3, and 13 show the features produced by the inflow of mining waste to the Népoui, in contrast with the unmodified river channel upstream (plate 14).
The Népoui River flows into an elongated embayment (fig. 17) alongside the limestone Presqu'île de Népoui, and at its mouth mangroves are extending rapidly on to broad red mudflats (plate 15) produced by the deposition of iron-stained clay washed down from the disturbed lateritic overburden in the hilltop mines. Comparison of air photographs taken in 1954 and 1976 show that the mangrove fringe advanced seaward by up to 400 metres on the shores of the Népoui delta in this period, and this advance is continuing. The waters of the Baie de Népoui have been shallowed by this sedimentation and red clay staining is extensive. Successive floods are washing coarser sediment down the Népoui. In addition to red clay, some sandy material has arrived at the river mouth, pebble gravels have extended to the lower reaches, and eventually coarser gravel will arrive at the coast. Already severely affected by mining activities, the Népoui. delta region is destined to receive much more sediment of gradually increasing calibre. Mangrove growth has not been impeded, and has probably been stimulated by the accretion of red mud, but as coarser sediment reaches the delta area, it is likely that the mangrove cover will be disrupted.
As mentioned already, the Ouha (catchment: 84 sq km) has also been affected by opencast mining on the crests of the Massif de Graunda-Boulinda. At the highway crossing it carries a sandy load stained by red clay, which is being delivered to its mouth. The sea is stained red in the Baie de Muéo off the growing delta of the Ouha. However, the advance of this delta is clearly less rapid than that of the Népoui. Instead of a general spreading of mangroves on to accreting mudflats, the advance is here by way of scattered mangrove establishment.
Baie de la Poya
The Poya and its tributaries (catchment: 418 sq km) drain a broad lowland flanked by the Massif du Boulinda to the north and the Massif du Me Maoya to the east. Both areas have undergone extensive hilltop nickel mining, and the upper tributaries have received sediment yields augmented by mining waste; but the Poya is so far little affected. By contrast, the Muonio to the north-west flows directly from the southern slopes of the Massif du Boulinda into the broad, shallow Baie de la Poya, and has an aggraded channel of coarse material stained by red clay. The Moindah River (catchment: 165 sq km), draining directly from the mined Me Maoya massif to the west, is also delivering red-stained silt and clay derived from mining waste into the Baie de la Poya. Coarser material (pebbles and boulders of peridotite) has been transported five to six km down from the massif along a deep gorge, but is deposited well upstream from the river mouth.
Rivière du Cap
Backed by the Me Maoya massif, the catchment of the Rivière du Cap (area: 174 sq km) is the largest of several river systems draining to the coast between Poya and Bourail. Its right bank tributaries have built broad piedmont fans and terraces, while the main course flows alongside the siliceous sandstones and cherts of the Montagnes Blanches to enter the sea by way of a small delta built into an embayment. Downstream the channel load includes peridotitic gravel and cobbles supplied by the right bank tributaries. The lower watercourse and the delta receive red lateritic clay derived from former opencast nickel mining on the southern slopes of Mount Djiaouma. The river has thus been moderately affected by mining activities.
The Néra and its tributaries form one of the larger river systems on the west coast (catchment: 546 sq km), uniting near Bourail to enter the Baie de Gouaro by way of a deltaic plain fringed by sandy beach ridges. Much of the catchment consists of hilly terrain on basalts and sedimentary rocks, but ultrabasic massifs occur on the higher inner margins, which form the eastern edge of the Massif du Me Maoya thrust outliers.
Abandoned cultivation terraces (tarodières) are extensive in this and other catchments on the central west coast (plate 16), indicating a precolonial phase of land management by indigenous people, who may have perceived that their utilization of steep slopes was resulting in soil erosion; they imply a response to an earlier phase of anthropogenic erosion on slopes that had been cleared and cultivated, but the details of this have not been researched.
Of the various Néra tributaries, the most affected by mining is the Téné, a tributary of the Boguen, which has its headwaters in the Massif de Téné, but the effects of this do not yet extend far downstream and the Néra estuary is a clear river with shoals of grey silty sand and some gravel. Sandy material washed into the Baie de Gouaro has been built into beaches and beach ridges by ocean swell that enters by way of a gap in the outlying coral reefs.
The Nessadiou (catchment: 87 sq km) follows a meandering course westward from the hill country north of Moindou to reach the sea by way of a small bay-head delta south of Bourail. Its catchment consists mainly of Mesozoic and Eocene volcanic and sedimentary formations, and its yield is a mixture of silt, sand, and fine gravel. It is an example of a river system unaffected by mining activities, but with a sediment yield probably somewhat augmented by the clearing and burning of vegetation on hilly country in its upper catchment. Here, as elsewhere on the west coast, recurrent burning to maintain pastures has converted extensive areas of sclerophyllous forest into grassy savannas with areas of niaouli scrub, formerly confined to low-lying areas. This weakened vegetation cover results in accelerated runoff and soil erosion.
The Moindou (catchment: 125 sq km) is another basin without ultrabasic formations and opencast mining. There has been underground mining of coal in the Moindou region, but this has had a negligible impact on river sediments. The Moindou carries mainly clay, silt, and sand down to its marshy mangrove-fringed bay-head delta.
This extensive river system (catchment: 438 sq km) has headwaters in Palaeozoic and Mesozoic volcanic and sedimentary formations. Some serpentinite outcrops also occur, but have not yielded exploitable minerals. La Foa has not been affected by mining activities. In its lower reaches it carries mainly clay, silt, and sand into a winding estuary opening into the Baie de Téremba.
This river system drains hilly and mountainous country (catchment: 175 sq km) north-west of Bouloupari, including the western slope of an ultrabasic massif, Mont Do, where hilltop mining has not taken place. The Ouaméni delivers a load of fine-grained sediment, no coarser than sand, to its delta in the northern part of the Baie de St. Vincent.
Opencast hilltop nickel mining has been carried out on the western end of the peridotitic Grand Massif du Sud on the margins of the Ouenghi catchment (area: 270 sq km), and the sediment load of this river has been augmented by red clay and by gravelly material derived from mining waste. These effects have been reduced, however, by systematic mechanical settling of colloidal iron hydroxides and by the building of large sediment-trapping boulder dams around the opencast mines of the Massif de Kongouhaou, and the impact of mining activities in the lower reaches of the river, on the Ouenghi delta, and in the adjacent Baie de St. Vincent have been slight. Nevertheless, recent attempts to introduce oyster farming in the eastern part of this bay and the building of shrimp ponds in the Baie de Kuara, adjacent to the Ouenghi delta (fig. 18), are activities that are periodically impeded by red-clay pollution from recurrent river flooding.
Like the Ouenghi, the Tontouta and its tributaries (catchment: 476 sq km) drain a peridotite hinter-I des Roussettes, North of Bourail, Date and where opencast mining has been extensive on mountain crests, and on hillslopes, some only few kilometres from the coast. The river carries extensive gravelly shoals as well as sand and finer material, and has shallowed nearshore areas round its mouth in the southern part of the Baie de St. Vincent. The impact of mining activities as been stronger than in the Ouenghi catchment because of the lack of conservation works and ecause of the intensity of mining and waste generation in the Tontouta catchment in the last 20 years. The sediment yield has increased at the river mouth, where mangroves have become ore extensive, spreading back on to the delta below Mont N'dui, as seen when 1954 and 1976 air photographs are compared. Dombéa dome small-scale localized nickel mining has ocurred in the headwater regions of the Dumbéa River (catchment: 233 sq km), but the effects on river loads have been slight; as in other drainage basins near Nouméa; mineral prospecting has been forbidden in order to maintain water quality. The sediment yield at the river mouth, where mangrove and marsh-covered delta (fig. 19) is rowing, consists mainly of clay, silt, and sand, and the effects of mining activities have been only slight. Some mangroves alongside the new highway have been killed by waterlogging due to road construction, rather than by changes resulting from hinterland mining.
With a catchment (area: 92 sq km) almost entirely in the ultrabasic Grand Massif du Sud, La Coulée carries sands and gravels stained by red clay, a sediment yield augmented by hilltop mining activities which continued into the 1970s. The river is extending its delta into the southern part of the Baie de Boulari, where tidal mudflats are being deposited in a red-stained sea. Similar features are seen on a smaller scale along the Plum stream to the east of Mont Dore, where the hillsides are scarred by gullies and landslides in the weathered red lateritic mantle.
Rivière des Pirogues
This river (catchment: 152 sq km) has its headstreams adjacent to the Upper Yaté basin, a sparsely vegetated region of peridotitic karst, in the vicinity of Ouénarou, where river piracy is in progress. The headwater region shows features formed by differential erosion of gabbros and granodiorites associated with the peridotite Massif du Sud, and has not been mined. The lower part of the catchment has been much affected by opencast mining, and by gullying and slumping of red ferruginous lateritic mantles on valley-side slopes. More than 60 years ago there were reports of rapid erosion as a sequel to mining in this area (Compton 1917). The Rivière des Pirogues has a load of sand, gravel, and cobbles stained by red clay, which is rapidly filling its estuary in the lee of a sand spit built at the river mouth. The adjacent sea, shoaly with down-washed sediment, is red-stained. Similar features occur in the small Rivière Ngo catchment to the south, opening into Baie Ngo (plate 17).
Baie de Prony
The branching ria of the Baie de Prony receives a number of small streams, including the Rivière Bleue and the Ruisseau de la Bergerie, which carry increased loads of red clay as a sequel to opencast mining between 1955 and 1968 of the ferruginous crust of the laterite mantle as an iron ore. Extensive areas north-west of the Baie de Prony have been denuded of vegetation and soil by this extraction, and persist as a red clay desert, within which gully erosion is developing. The streams are washing red clay down to stain beaches and nearshore waters in the small embayments (e.g., Baie de la Somme) which branch from the Baie de Prony. Divers have reported unusually large madrepore species growing in turbid waters here, indicating that some corals may be able to adapt to the changed environment. The deeper waters appear to have been little affected, and coral growth remains vigorous offshore.
Draining southward from the escarpment bordering the Plaine des Lacs, this river (catchment: 150 sq km) has some headstreams in the area affected by opencast iron ore mining, but does not yet show significant modification of its channel, which opens into a winding ria.
Typical of streams descending from headwaters in the peridotitic karst region of the Plaine des Lacs, the Kuébéni (catchment: 38 sq km) carries a load of red clay and pisolitic ironstone sand, much of which is being deposited in the small inlet at its mouth, in the lee of the emerged reef islands of Nou and Néaé. Opencast iron mining in the Goro region between 1937 and 1941 and the subsequent bulldozing of superficial deposits have increased this sediment load and accelerated deposition at the river mouth. Systematic prospecting in the 1970s in this southern part of New Caledonia revealed extensive reserves of low-grade nickel ore in the superficial formations, and it is possible that large-scale mining will proceed here in the future.
The original Yaté dam was completed in 1927 and has been replaced by the modern barrage, built between 1955 and 1958. As a result, the Yaté (catchment: 450 sq km) has delivered little sediment to the coast. The extensive, branching reservoir acts as a sediment trap, receiving red clay and pisolitic sands from the bordering slopes, a supply that has been augmented by nickel and some chromium mining, road-making, prospecting, and the depletion of vegetation by recurrent bushfires.
This basin (catchment area: 98 sq km) occupies the southern part of the eastern slopes of the Grand Massif du Sud; the watershed rises about 1,000 metres above sea level and slopes are steep. Erosion is a dominant process, especially in the middle and lower catchments, where the forest cover thins out. Mining activity has been limited to nickel prospecting and local exploitation of chromium. At the river mouth a small sandy barrier nestles at the head of an embayment and has been slightly enlarged by the accumulation of ferruginous and chromitic sands during the past 20 years, without much effect on the outlines of the embayment.
This catchment (146 sq km) is even more mountainous than that of the Pourina. The river mouth consists of a small delta built into a lagoon behind a large sand barrier on which the mining settlement of Ouinné stands. The configuration of the delta has been modified by the cutting of channels to allow barges to load mineral ore close to the mines. Nickel-bearing formations occur between 200 and 300 metres above sea level on hillsides overlooking the delta and were mined in the late 1960s. Air photographs taken in 1976 show that the mouth of the Ouinné had not yet been affected by augmented sediment supply, but the small Ué River to the south, draining coastal hills where mining occurred, had been almost blocked by the deposition of waste material. It is feared that the effects of nickel mining will increase here during the next few years in spite of the reduction in mining activity and the building of dykes to partly enclose the river mouth.
This very mountainous small basin (catchment: 77 sq km) is dominated by the ridge of the Massif de Kouakoué, which rises to 1,501 metres above sea level. It has been subject to exploration but not to mineral exploitation. The lower part of the valley widens into a delta. Although the basin has not been mined, sedimentation appears to have increased at the river mouth, where a sand spit built of fluvial sediment reworked by wave action grew substantially between 1954 and 1976.
Mineral prospecting has occurred within the basin of the Ni (catchment: 173 sq km), and construction of exploration roads has strongly increased sediment yield. Comparison of air photographs taken in 1954 and 1976 shows sand accretion in the delta, where the river splits into three distributaries. Two of the three outlets are now partially obstructed by a large fluvial sand and gravel barrier beach built during this period.
Nickel mining has occurred at several places within this 93-square-kilometre catchment, but the effects on the river and its delta have so far been slight. The delta, which (like the Kouaoua delta) includes successively formed beach ridges, is bordered by a littoral spit, which grew between 1954 and 1976 to reduce the outflow channel to the sea to a narrow pass.
Dominated by the high ranges of the Humboldt massif, this basin (catchment: 180 sq km) has also been subject to mineral prospecting, which has greatly increased fluvial sediment yields. Large quantities of fine-grained sediment have also been derived from serpentinite hills up to 300 metres high in the lower valley, where ravine dissection has been in progress since before the prospecting era, and perhaps from pre-colonial times. Nourished by these inputs, the Comboui has a widened channel leading to a substantial delta, with several distributaries, one of which has been sealed off and two others narrowed as a result of growth during the last 20 years of sand spits built of wave-worked fluvial sediment.
In this small basin (85 sq km) there has been only local nickel mining and some prospecting. These activities do not appear to have greatly affected the form of the river channel or its mouth, where a small littoral spit built of wave-worked fluvial sand has been driven landwards in recent years.
In the northern part of this basin (catchment: 72 sq km) there were two nickel mines at a low level about 3 km inland, worked intensively at the beginning of the century. Later a chromium mine was established, and nickel prospecting extended southwards. Sand and gravel discharged from the mines and areas of disturbance by prospectors aggraded the river channel and led to accretion on the delta, but these effects have diminished during the last few decades. Between 1954 and 1976 there were only minor changes on the deltaic shoreline of the To N'deu.
The Thio and its tributaries drain a region of 404 sq km, dominated partly by the ultrabasic uplands of the Grand Massif du Sud and partly by the volcanic and sedimentary formations of the Central Chain. Opencast hilltop mining has been extensive, especially on the slopes of Mount Ningua, and at the Mines Bornet above the Nakalé and Nembrou tributaries, where the river loads have undoubtedly been increased and coarsened as the result of these activities (plate 18). The most extensive mines, however, are in the coastal region, notably on the Thio Plateau, immediately west of the Thio township, where large-scale opencast nickel mining has been in progress for 80 years (fig. 20). The Thio Plateau contains the largest nickel deposit in the world. It has been mined continuously since 1901, and in eight decades has yielded almost 20 million tonnes of ore, producing about 450,000 tonnes of nickel and cobalt. In 1981 the Thio region was the second most productive mining centre (the first being Kouaoua) in New Caledonia.
The outcome has been massive generation of mining waste. Substantial masses of sediment have slumped down the hillside and have been washed into the Thio River. It is believed that this river was formerly navigable by canoe upstream to Nakalé and by larger boats to a point near Les Pétroglyphes, about four km from the mouth, but infilling by sediment derived from mining waste has been rapid and such navigation is no longer possible. Below St. Pierre-Nakalé the impact of mining waste flowing into the Thio channel has been obvious, especially since the 1960s, although some changes had been observed twenty years previously, when mechanization of mining greatly increased the rate of waste production and downslope movement of discharged debris. Now the banks and bedload of the Lower Thio consist entirely of sand and gravel washed down from the Thio Plateau. An alluvial island 400 metres long has been completely blanketed by this debris, its scrubby vegetation cover destroyed.
The Thio delta, developed on either side of the rocky ridge of Bota Mere, has shown many changes in configuration on photographs taken in recent decades, and its present outlines differ substantially from those shown on the 1976 air photographs (plate 19). Each flood brings additional sediment, ranging up to sand and gravel, down to the river mouth, where it is sorted and built into spits and bars by wave action. Successive ground and air photographs indicate progradation relative to Bota Mere, with the formation, growth and widening of spits, the development of deltaic islets, the silting of distributary channels, and the infilling of small lagoons permitting mangrove establishment. Mining activities have thus accelerated delta development and augmented the sand supply to the beaches that extend northward from the mouth of the Thio. In recent years, rock dams and interception basins have been built to reduce the amount of sediment yield from the mining areas.
The Dothio (catchment: 77 sq km) drains a small basin north-west of the Thio that has a head-water region of volcanic and sedimentary rocks and several ultrabasic massifs near the coast where hilltop mining is active or has been carried out in recent decades. The slopes of the Thio Plateau have several areas where mining waste has spilled down and some large landslides. An attempt has been made to control the discharge of coarse waste material down the valley of the Wellington, tributary to the Dothio, by bulldozing boulder levees and a diversion dam (plates 20, 21, 22), but floodwaters will continue to deliver some of the finer material to the Dothio. The mouth of the river threads through a beach ridge plain to a small sandy delta, off which there are shoals deposited from floodwaters in a sea that is stained red by suspended clay in the nearshore zone. Much of the impact of mining activities on the Dothio and the coastline adjacent to its mouth derives from the lower catchment. At present the river mouth is enclosed by a barrier spit that has grown northward from the Thio delta and become attached to the coast by deposition of sediment, mainly from the Dothio.
The lower part of the Nakéty catchment (area: 106 sq km) consists of ultrabasic massifs subject to extensive opencast nickel mining, and the delta has received an augmented yield of red clay, sand, and fine gravel. Comparison of 1954 and 1976 air photographs showed the growth of a littoral spit attached to the beach of St. Paul and the formation of an alluvial island, related to augmented sedimentation produced by mining activity from the late 1950s to the 1970s. Subsequently the beach ridge plain that borders the river mouth has been truncated by wave erosion, undermining some of the coconut trees that grew on the sandy terrain (plate 23). In general, beach erosion is unusual adjacent to the mouths of New Caledonian rivers that have sediment yields augmented by the effects of opencast mining, as river floods have generally supplied sufficient sand and gravel to maintain or prograde such beaches. It is possible that the beach erosion behind the Baie de Nakéty is a temporary phenomenon induced by the storm surge that accompanied Cyclone Gyan in December 1981, and that in due course material from sandy shoals off the river mouth will be carried shore-ward by wave action to rebuild the eroded sector.
The Canala river system (catchment: 290 sq km) drains a mountainous area north of the Plateau de Dogny, within which Mesozoic sedimentary and volcanic rocks predominate. In the lower catchment, above Boakaine, there are areas of opencast nickel mining which have produced waste material spilling down slopes and into lateral valleys. However, as only this part of the drainage basin is in ultrabasic rocks, the effects on the Canala River have so far been moderate in the lower reaches; moreover, antipollution works have recently been introduced in the Boakaine area. Some down-washed red clay has been deposited in the extensive mangrove swamps that border the Canala delta, built into the head of the Baie de Canala, but the mangroves remain healthy and are advancing seawards as this accretion proceeds. Between 1954 and 1976 an alluvial islet in one of the distributaries grew from about 180 x 100 metres to about 260 x 120 metres as the result of mud accretion and mangrove spread.
North-west of Canala, the Ouango drains a small but steep catchment (area: 16 sq km) on the eastern side of the Me Aiu Plateau. Opencast mining was extensive in the headwater region in the 1960s and 1970s, and vast masses of sand and gravel have been spread across the valley floor in a wide fan (plate 24) that extends down to the delta, which is growing out into a branch of the Baie de Canala. The delta consists of lobes of sand and gravel bordered to seaward by wide red mudflats formed by the accumulation of downwashed ferruginous clay (plate 25). The effects of nickel mining on this valley and delta have been severe.
The hilltop nickel mining that has discharged such large masses of waste material into the Ouango valley has also affected the Karoipa valley to the north. Although the catchment is small (area: 23 sq km), the river channel has been much widened, and a delta of sandy sediment, fringed seaward by red clay, is growing into the southern part of the Baie de Kouaoua (plate 26).
The Kouaoua river basin, with its large tributary, the Kakenjou (combined catchment: 253 sq km), drains an area dominated by the ultrabasic massifs of Me Aiu to the south and Ménazi to the west. In the latter, opencast hilltop nickel mining has been very extensive during the last 15 years. In 1981 the Ménazi massif was the chief mining centre in New Caledonia, the SLN mine producing 779,000 tonnes of ore and Pentecost Kouaoua a further 462,000 tonnes. In consequence, the Kouaoua and especially the Kakenjou have received substantial amounts of coarse sediment, their channels have widened, and deposition of sand and gravel is building up spits and bars in the deltaic region near Aoumou. The configuration of the coastline at the mouth of the Kouaoua has changed considerably in recent years, as shown on successive air photographs, and the present mouth is almost enclosed by a long curving sand spit, built up by wave action in the months since the floods generated by Cyclone Gyan washed out the previous bordering spits from the river mouth. Red-stained sea and offshore shoals in the Baie de Kouaoua off the river mouth are further indications of changes due to the impact of mining activity in the hinterland.
The Koua catchment (195 sq km) extends back to the crest of the Central Ranges, consisting of Palaeozoic and Mesozoic schists. The upper basin includes part of the peridotite Grand Massif du Sud, relatively wide in this region. The nickel deposits on the north slope of the Ménazi massif have been intensively mined for two decades, but the mines are more than 15 km inland and mining waste from this source has not yet extended far downstream. In the coastal regions there are areas of active and historical mining north of the river at a low level, and waste material from these has aggraded the channel, inducing bank erosion and meander displacement. Arrival of additional sediment at the river mouth has built up the delta, the bordering sandy beach having been widened and extended along the whole length of the delta shoreline between 1954 and 1976 as the result of augmented accretion.
Baie de Poro
The coastal peridotite massifs have here been subject to open-cast nickel mining by the Société Le Nickel. In 1981 this region was the fourth largest mineral producer in New Caledonia, but in 1982 mining ceased here. By then large quantities of red clay, sand, and gravel had moved directly down coastal slopes to the present shoreline. Much of the augmented sediment yield, however, is travelling to the sea by way of the overloaded Goua stream (catchment: 9 sq km), at the mouth of which sand and gravel are being washed into the sea. To the north, pisolitic ironstone sands and fine gravels have been carried down from coastal slopes to form a depositional terrace prograded on to the fringing coral reef that lines the coast up to Paraouyé (plate 27). Locally, the beach of dark sand has been cemented into layers of beach rock by calcium carbonate precipitation. The delivery of sand to this coast is evidently the outcome of prospecting and mining activities in the coast range during the past century, so that the beach rock must be of very recent origin.
The Houaïlou and its tributaries, notably the Neaoua, drain an extensive basin (combined catchment: 539 sq km) north-east of the Col des Roussettes in the Central Ranges. In its head-waters the ultrabasic massif of Me Maoya has been subject to opencast nickel mining, and some waste material has been carried downstream. Torrential runoff during Cyclone Gyan in December 1981 widened the river channel and carried masses of sand and gravel, including peridotite boulders, further downstream, with sediment of diminishing calibre extending still further. At the river mouth sandy material washed down during floods has been built by wave action into a chain of spits and barrier islands. In this deltaic region the Houaïlou is joined by the Néaoua River from the south, draining valleys cut into ultrabasic massifs that have been extensively mined for nickel. Red clay, silt, sand, and gravel are being carried by this river down to its confluence with the Houaïlou, so that coastal deposition has been modified in recent decades by the effects of hinterland mining activity. Between 1954 and 1976 the delta shoreline was changed by the erosion of a wooded depositional island 250 metres long and 30 to 50 metres wide, and by the redistribution of derived sandy sediment by longshore drifting to prograde the coast to the north-east.
West of the ultrabasic plateau that forms the peninsula of Cap Bocage, the lower basin of the Ho River (catchment: 23 sq km) contains some opencast nickel mines, and pisolitic ironstone sands derived from the ferruginous lateritic crusts have been carried downstream and built by wave action into beach ridges bordering the river mouth in Baie Ugué. The effects of hinterland mining activity are only slight at the river mouth at this stage, but in the north-east of the Baie Ugué the fringing coral reef has been severely damaged by accumulation of waste of varying calibre from the Pounehoa mine on the Cap Bocage plateau, swept directly down the steep coastal slope into the sea.
Néavin and Monéo
The Néavin and Monéo rivers, with a combined catchment of 202 sq km, mark the northern limit of the ultrabasic Grand Massif du Sud on the east coast. The Néavin valley drains an area that has received some mining waste from hilltop excavations to the south, but the rest of the catchment is in strongly folded Mesozoic sedimentary and volcanic rocks which have not been mined. The impact of mining activities on this river system has therefore been minor. At and below the confluence of the Néavin and Monéo is a deltaic area with mangroves on shoals and a seaward fringe of spits and beach ridges built of grey sand and gravel, much of which has come from laterized ultrabasic hillsides in the coastal fringe. As with the Thio, the Kouaoua, and the Houailou, there have been marked changes in the last two decades in the configuration of spits bordering the river mouth, especially since these were washed through by floodwaters during Cyclones Alison and Gyan.
The North-Eastern Rivers
North of the Monéo the rivers of north-eastern New Caledonia drain catchments that do not contain large ultrabasic massifs and have only small outliers of overthrust peridotite. In sequence, with catchment areas in parentheses, the major rivers are the Mou, or Oué Pouémaeu (58 sq km), the Nimbaye (321 sq km), the Tchamba (187 sq km), the Amoa (182 sq km), the Hienghène (155 sq km), and the Ouaième (338 sq km). In their middle and lower reaches these rivers carry loads of generally grey sand and gravel, but at their mouths they are typically estuarine inlets or deep rias, in most cases with variable sand spits at the coast. The air photograph taken in 1976 of the Ouaième ria shows its mouth blocked by a sand barrier, but this was swept away by floods following subsequent cyclones, especially Cyclone Gyan in 1981 (which raised the river level 14.4 metres at the head of the inlet), and has subsequently been rebuilt as a cuspate foreland south of the river mouth.
The small Massif de Grandié, on the watershed between the Amoa and the Tiwaka, has not been subject to mining activity, and these rivers have not been affected by downwashed waste. The Massif de Tchingou, in the Tiwaka basin, has been prospected and mined on a small scale at two locations, and the small amount of mining waste has slightly affected some of the headwaters of this river. The upper basin of the Tipindjé includes two small peridotite overthrusts: the Oua Tilou massif, where mining activity has been very slight, and the small Massif de Poindas, subject recently to more intensive mining, which has so far led to waste pollution only in a few head-streams and in the upper reaches of the Tipindjé.
This review of the major river systems of the Grande Terre of New Caledonia has indicated the extent of impact of opencast mining on ultrabasic massifs in terms of changes on hillslopes, in valley floors, along river channels, on deltas, and in adjacent coastal areas. Figure 21 indicates the pattern of impact at river mouths around New Caledonia. This impact has been severe on the Oué Bouameu, the Tinip, the Poué Koué, the Népoui; the Coulée, and the Rivière des Pirogues, on the west coast, and on a number of east coast rivers, notably the Ouango, the Karoipa, the Goua, and the Néaoua flowing into the Houailou. On a number of other rivers, notably the Pouembout, the Ouha, the Moindah, the Rivière du Cap, the Ouenghi, the Tontouta, the Dumbéa, and on the east coast the Ouinné, the Ngoye, the Thio, the Nakéty, the Canala, the Kouaoua, the Koua, and the Houaïlou, the effects are so far slight or moderate but may become more severe as waste from existing mines is added to the river system and as existing augmented channel loads move downstream. It is evident that opencast hilltop mining in the ultrabasic massifs of New Caledonia has profoundly modified upland areas and has had a spreading influence on the features of river channels, river mouths, and adjacent coastal environments.
The discharge of gravels to the mouth of the Koumac River, which drains a catchment unaffected by opencast mining, is an indication that a coarse-grained sediment yield is not necessarily the outcome of such mining. The rapid growth of the Comboui delta, on the east coast, is partly due to ravine erosion of hillslopes and partly to disturbance by mineral prospecting, yielding eroded sand and silt to the river mouth. Arrival of red clay and pisolitic ironstone sands at river mouths is probably the most reliable indicator of modifications due to opencast hilltop mining of the laterized mantles of ultrabasic massifs. While the effects of such mining are prominent in the landscapes of New Caledonia, it is likely that road-making and mineral prospecting have greatly augmented fluvial sediment yields, both in catchments affected by opencast mining and in those where no mining has taken place. Other factors such as clearing, burning, and grazing of the natural vegetation cover as well as cultivation in hilly country have accelerated runoff and increased the yields of sediment to New Caledonian rivers. Mention has been made of the existence, particularly on hill country inland from the central west coast, of abandoned cultivation terraces (tarodières), built by indigenous people in the pre-colonial era. The yields from such land management systems would have been mainly fine-grained (sand, silt, and clay), in contrast with the wider range of grain sizes (up to gravel, cobbles, and boulders) in the waste materials generated by opencast hilltop mining on deeply weathered ultrabasic formations during the past century.