Human impact on rivers

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.

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.

Changes in water temperature due to flow modification can affect insect development by not allowing them to complete their life cycle.

Rivers are connected systems, and barriers such as dams, culverts and floodgates disconnect one area from another. They prevent species such as eels from migrating – isolating previously connected populations.

Water taken from rivers for irrigation can lower river flows (a concern in Canterbury).

Exotic species

Exotic species have been introduced to river systems sometimes intentionally (for example, for fishing purposes or as food for other species) and sometimes unintentionally (for example, species come in on the bottom of boats or on fishing gear or they escape from pond areas during flooding, such as koi carp).

These organisms can affect native species. They may compete with them for prey and habitat. They may prey on native species, alter habitats, breed with native species to produce another species or they may introduce harmful diseases and parasites. Once established, these species can be difficult to control or eradicate, particularly because of the connectivity of the flowing river. They can easily migrate to many areas affecting native species.

Harvesting

Excessive fishing in river ecosystems can drastically reduce numbers of species. For example, numbers of eels and whitebait in the Waikato River have reduced since the 1970s. Commercial eeling began in the 1960s and peaked in the 1970s with an annual average catch of 2000 tonnes. In the early 1980s, 400–450 tonnes per annum were harvested, with less than 200 tonnes per annum harvested since 2000.

Whitebait tonnage has also drastically reduced from an average of 46 tonnes per annum in the 1950s to 3 tonnes in 2000. Reducing stocks of a particular species can have an effect on other species such as birds that feed off river fish. The birds leave the area when river fish decline. Find out more about whitebaiting.