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Impacts

Man-made barriers such as dams, weirs and other impounding structures typically have the following negative effects on the environment of rivers:

•  Habitat loss: Natural dynamics and river habitats are lost upstream of dams as they are ‘drowned’ or suffer depleted flows downstream due to the alteration of water flow conditions. As a result, aquatic flora and fauna are dramatically altered (Gough et al., 2018).
• Flow regulation is one of the main adverse ecological consequences of dams and reservoirs to rivers. This is evident in downstream river ecosystems and is a result of dam operations reducing natural flows, eliminating peak flows, changing seasonal flow patterns, regulating low flows or other regulatory practices. Flow regulation may have significant negative effects on fish fauna and benthic invertebrate communities.
• Fragmentation: Rivers are transformed into a series of ponded sections; dams block migration routes for fish in both up and downstream directions and habitats are isolated through fragmentation. This transforms natural fish fauna and leads to local extinction of fish species (Gough et al., 2018).
• Sediment: Dams block transport of sediments in rivers, leading to accumulation and poor water quality in the reservoir, deprivation of sand and gravels downstream of dams, higher risk of erosion downstream of dams and in river deltas, and to a decrease in habitat quality upstream and downstream of the dam (Gough et al., 2018).
• Water quality: Storage of organic material and nutrients in reservoirs and also in backwater from smaller dams often leads to a decrease in water quality, changes in temperature and the capacity to dissolve oxygen, and sometimes to seasonal stratification (Gough et al., 2018). Ponded sections have a longer water residence time, thus enhancing eutrophication effects such as phytoplankton blooms.