Geomorphic processes and human interventions: summing up

We have tried to demonstrate in this chapter that mountain geomorphology faces many problems in its attempts to identify and to rank specific slope processes in terms of their relative importance or absolute amounts of 'work' accomplished. These problems confound both determination of the manner of evolution, over geological time, of the mountain landscape as we see it today. They also limit understanding of what is currently happening in natural, or nearnatural, situations. The crux of this issue is the representativeness (or lack of) of collected data in both space and time. Especially difficult is assessment of the relative and absolute importance of high magnitude events with very long recurrence intervals compared with the continuous slow-moving downslope material transfer. Regardless of these problems a broad and extremely valuable understanding of denudation rates, orogenesis, and sediment transfer on a regional scale and within a geological time-frame has emerged.

We conclude from the foregoing discussion that natural processes in this dynamic region virtually obliterate the effects of human intervention, in so far as they can be gauged from the available data. Human intervention takes many forms; we have emphasized, within the Himalayan context, deforestation and general changes in land use as influenced primarily by the needs of a rapidly growing subsistence mountain farming population to sustain themselves. Other interventions have been introduced, including road construction and the installation of hydroelectricity facilities.

In taking our discussion from a rather academic review of geomorphology as a science into considerations of soil erosion and landslides that are widely presumed to be caused by bad land-use practices, we have inferred that the two sets of processes, geophysical and human, are probably several orders of magnitude apart. Furthermore, we believe we have demonstrated that the adherents of a theory stipulating that human intervention is the primary culprit of the large-scale environmental damage (as perceived from a human point of view) have failed to prove their case. Landslides and gullies, bare soil, floods, and silted reservoirs and irrigation works are visually graphic. At a local, site specific, scale they are serious and need to be corrected or reduced. However, we have found no reliable data to indicate that a specific mountain road, for instance, however badly constructed, is contributing a single grain of silt to the Ganges-Brahmaputra delta. We believe that, until there is very strong evidence to the contrary, the geophysical and climatic processes of our region, as reflected in some of the most rapid uplift estimates (orogenesis) and high rates of mountain denudation and concomitant accumulation of vast thicknesses of sediments, in the form of the Ganges and Brahmaputra plains, give adequate explanation for the workings of this extremely active landscape. The next major issue - large-scale processes on the plains - is the focus of Chapter Six.