Geoscience Reference
In-Depth Information
erosion, in some cases by several orders of magnitude. Much of that soil finds its way in-
to a river, causing associated changes in channel form and ecology. These types of altera-
tion to rivers have been recorded all over the world, first occurring thousands of years ago
in agricultural areas of the Mediterranean and China, and more recently elsewhere. Other
forms of food production can also increase runoff and erosion. Grazing and trampling by
livestock reduces vegetation cover and causes the compaction of soil, which reduces its in-
filtration capacity.
As rainwater passes over or through the soil in areas of intensive agriculture, it picks up
residues from pesticides and fertilizers and transports them to rivers. In this way, agricul-
ture has become a leading source of river pollution in certain parts of the world. Concentra-
tions of nitrates and phosphates, derived from fertilizers, have risen notably in many rivers
in Europe and North America since the 1950s and have led to a range of environmental, so-
cial, and economic problems encompassed under the term 'eutrophication' - the raising of
biological productivity caused by nutrient enrichment. The growth of algae is the primary
concern, leading to human health problems - and hence additional costs of water treatment
for drinking - and effects on other river species. In slow-moving rivers, for example, the
growth of algae reduces light penetration and depletes the oxygen content of the water,
sometimes causing fish kills.
Of course, many of these effects can be controlled by conscious efforts to conserve soil and
water on agricultural land. These sorts of measures are undertaken for all sorts of reasons,
not least because losing soil and water from fields has an adverse effect on crop yields.
Numerous studies undertaken in the Yellow River Basin in China have demonstrated the
benefits of soil and water conservation measures, including tree planting and the construc-
tion of terraces, introduced in this area primarily to reduce sedimentation in the river's
reservoirs. Discontinuing a land use that exacerbates runoff or sediment production is also
likely to reduce these effects if the previous vegetation cover is re-established, but this does
not always occur. Investigations in the central Andes of Peru found that where agricultural
terraces had been abandoned, the rates of soil erosion increased because the environment
was too dry for plants to grow on the terraces without attention from farmers.
Another form of land use that has similar effects is mining. In western Siberia, the sediment
load of the Kolyma River more than doubled during the 1970s and 1980s due to widespread
gold mining in the catchment disturbing vegetation and increasing erosion. Interestingly,
records of the Kolyma's discharge over the same period showed no significant trend, in-
dicating that runoff had remained the same. Many mining operations have also caused con-
tamination in rivers. Waste rock and 'tailings' - the impurities left after a mineral is extrac-
ted from its ore - typically still contain metals which can be leached into soils and water-
