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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).