Photo taken at the Bio Bio region (Chile), V. Vanacker, March 2013
Over the last century, human activities in North America have resulted in sediment movement equivalent to 3,000 years of natural erosion processes. A large-scale study published in Nature Communications reveals valuable results for conservation and water and soil restoration programmes.
Reducing the impact of human activity on the environment is today’s main challenge. We hear a lot about CO2 emissions and their effects on climate, but we hear less about soil erosion. Yet our activities (agriculture, timber production, urbanisation) have a big impact on accelerating soil erosion and thus on the transport and accumulation of sediment in lakes and reservoirs. This results in decreased natural fertility of soils, increased mudflows, and degraded watercourses. These consequences entail direct costs but also indirect costs such as dredging, soil and watershed restoration, and water treatment.
Veerle Vanacker, a professor at the UCLouvain Earth and Life Institute, and her colleagues David Kemp of the China University of Geosciences, in Wuhan, China, and Peter Sadler of the University of California, Riverside, in the US, have analysed the rates of sediment accumulation in the floodplains of North America over the last 40,000 years. The results show that since the arrival of Europeans and the development of agriculture, erosion and the accumulation of alluvium – deposits of pebbles, mud, and sand carried by flowing water – occurred ten times faster than when these processes were the work of Mother Nature alone. In order to plant crops and establish farms, settlers had to clear large areas, which, subsequently devoid of trees, became highly susceptible to soil erosion.
As a result, sediment, a product of soil erosion, settled in rivers and eventually accumulated on the surrounding banks and floodplains. One consequence was the Dust Bowl in the central United States. In the 1930s, the farmland of the Southern Great Plains was subjected to severe drought. This bare, wind-exposed land became the site of incredible dust storms, forcing millions of farmers to move to the cities via the famous Route 66. These years of ecological and agricultural disaster are also known as the Dirty Thirties.
‘Knowing the natural values of soil erosion versus the values accelerated by human activity is essential to increasing the effectiveness of water and soil conservation programmes,’ explains Prof. Vanacker. ‘The good news is that in recent decades the rate of erosion has decreased in some areas through effective agri-environmental measures.’ Hence the major soil and water conservation programmes developed in the 1960s, 1970s and 1980s have begun to take effect. This is a valuable observation for better identifying anthropogenic activities that can lead to a reduction in sediment accumulation rates, and for implementing programmes in soil degradation hotspots where the rate of deforestation remains rapid.
Photo taken in the Andes (Ecuador), V. Vanacker, February 2012.
Measures to limit the impact of human activities on soil erosion and sedimentation rates include minimum ploughing, protecting areas near river flood plains, and restoring wetlands. The last two measures make it possible to create buffer zones where sediment can accumulate.