Drainage and river erosion

Posted on March 23, 2015

Land-use changes and climatic trends have been held responsible for historical increases in river flow, such as those recorded in the USA over the last half-century. Higher flow rates are linked to increases in sediment load and stream turbidity, with consequent declines in the habitat quality and recreational value of rivers. To disentangle the factors responsible for increased flows, hydrological data collected from 215 Minnesota watersheds since 1940 were examined. Over the period in question annual rainfall increased by less than 15%. However, at the same time, in many catchments there were disproportionately higher increases (of up to 200%) in river flows, especially in agricultural areas where there had been a shift from the growth of small grains and forage species to the row-cropping of soybean. Conversion to soybean was associated with reduced rates of evaporation and crop transpiration, and a consequent increase in the amount of rain water entering rivers, but this change led to only modest increases in stream flow. In overall terms, changes in rainfall and crop evapotranspiration explained less than one-half of the observed increase in average flow. The remainder was attributed to the introduction of ditches and subsurface drains that channelled water into rivers from wetlands and low-lying depressions, and these increases in stream flow were strongly correlated with expansions (of 10-40%) in stream channel width. This finding supports other research that suggested that most of the eroded sediment entering rivers came from the stream channel rather than from agricultural fields. Therefore, crop conversions that require artificial drainage pose a risk to riverine water quality, and agricultural managers need to take into account the under-recognised connection between drainage systems and stream erosion.

Reference: Schottler, S.P. et al. 2014. Twentieth century agricultural drainage creates more erosive rivers. Hydrological Processes 28,
1951–1961. http://onlinelibrary.wiley.com/doi/10.1002/hyp.9738/pdf