ARTICLE Human impact on rivers EXPLORE
Human beings have an impact on river ecosystems. The relationship living organisms have with each other and with their environment is extremely complex. Impacts on a species or a non-living element may have long-term consequences for a river ecosystem.
Excess nutrients contribute to river pollution
Agricultural intensification has resulted in nutrient and chemical loss to nearby rivers. This can lead to eutrophication of backwater areas of rivers.
Several key areas of human impact on river ecosystems are:
pollution
flow modifications
exotic species
harvesting.
Pollution
Pollution is difficult to control because it is often the result of human infrastructure around a river. Pollution enters the river, sometimes in small amounts, at many different locations along the length of the river. Common sources of pollution come from rural and urban areas.
The clearing of forests to produce farmland has led to on-going erosion, with large quantities of sediment deposited into rivers. Agricultural intensification (substantial increases in fertiliser application and increased stock numbers) has resulted in nutrient and chemical loss to nearby streams and rivers. Elevated nutrient concentrations (especially nitrogen and phosphorus – key components of fertilisers) can result in the eutrophication of slow-moving waterways.
Urban areas add to this pollution when contaminants (PAHs and heavy metals) are washed off hard surfaces such as roads and drain into water systems. Sulfur dioxide and nitrous oxide emitted from factories and power stations enter river systems through acid rain. Sewage and effluent are discharged into rivers in some areas.
Pollution can lower the pH of the water, affecting all organisms from algae to vertebrates. Biodiversity decreases with decreasing pH.
Farmers, industry and local authorities are working together to reduce direct pollution from entering New Zealand rivers.
Flow modifications
Dams alter the flow, temperature and sediment in river systems. Reduced flow alters aquatic habitats – reducing or removing populations of fish, invertebrates and plants that depend on the flow to bring food. Reduced flow also decreases tributary stream flow, changing habitats and altering the water table in the stream aquifer. Consequently, riverside vegetation may be affected and decline in numbers. This may affect animal biodiversity, for example, bird species may leave the area if their habitat is lost or altered
