Aggravation of Floods in the Amazon River as a Consequence of Deforestation?

ABSTRACT

It has been claimed, on the basis of relatively shortterm gage observations and the perceptions of riverbank dwellers, that height and duration of Amazon floods are increasing. The paper discusses these assertions and some of the processes that might lead to the predicated trends. Two such processes are: an increase in peak discharge and a decrease in channel cross-section. Both could be triggered or reinforced by human-induced changes in the environment. By altering the hydrologic relations of plant, soil and water, deforestation in the headwaters can enhance runoff. It can also increase the sediment load, which, if the carrying capacity of the river is exceeded, may be deposited in and aggrade the channel. That deforestation in the Amazon drainage, now affecting small affluents, will eventually bear upon the regime of major tributaries and of the main stem itself can hardly be disputed. However, given the vastness of the basin, and the proportionately small and concentrated areas impacted by settlement, reports that such effects are already perceptible thousands of kilometers from the cleared areas require careful scrutiny.

A detailed statistical analysis, combining gage readings and adequate precipitation data, may, of course, reveal the existence, in one or more sections of the river, of long-term changes in flow that cannot be explained in terms of rainfall variability. Even then, one should not exclude the possibility that such trends might be elucidated without invoking human intervention. They might, indeed, be caused by stream adjustment to neotectonic influences.

In relation to the testimony of riverine populations, major inundations are likely to polarize concerns and dim the memory of moderate floods.