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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.

The impacts of barriers vary according to the height and location of barriers. A major impact on a river could be caused by a single, very damaging structure or by the accumulated effects throughout the length of the river of a series of small structures, which may have only a small impact individually (EEA, 2018). The location of barriers in a catchment determines, to a large extent, their impacts on sediment fluxes, fluvial habitats such as floodplains and deltas, and on the abundance and diversity of freshwater biota. For example, barriers in lowlands can prevent or delay fish migrations, while headwater barriers can alter downstream flows and sediment transport (Jones et al., 2019).

The height of barriers also plays a major role in determining the impacts on freshwater biota and the surrounding ecosystem. High-head structures (large structures), typically higher than 8 m or 15 m, often create large impoundments, which can cause shifts in the composition of biota communities within the reservoir as well as downstream. Low-head structures (small structures) can also impact key ecological processes just as strongly. Because of their very large number, small structures are likely to cause greater cumulative impacts and a more significant loss of river connectivity than high-head structures (Jones et al., 2019).