Flow regulation is the alteration of the natural flow regime via construction of dams or diversions. Dams interrupt the downstream movement of water – and anything moving with the water, such as sediment – by creating an obstruction and an upstream reservoir. Depending on the details of dam design and operation, the downstream flow regime may become rapidly varied on a predictable schedule (dams used to generate hydroelectric power) or the peak flows may be reduced and the base flows increased (water storage or flood control dams). Regardless of the specifics, a dam alters the magnitude, duration, and timing of downstream water movement in a manner that commonly stresses or kills native aquatic and riparian species that are evolutionarily adapted to a natural flow regime. The altered flow regime typically also alters downstream sediment movement in a manner that results in channel and floodplain erosion and simplification. The dam itself usually blocks up- and downstream movements by aquatic organisms such as fish passage, even if fish passage structures are included in dam design.

An extensive scientific and popular literature describes the many detrimental effects of dams on river corridors. Although hydroelectric power is sometimes regarded as a clean, renewable source of energy, the increased emissions of methane and carbon dioxide from artificial reservoirs, and the massive destruction of river ecosystems, suggest that dams are not a clean or environmentally friendly source of power. Nonetheless, only an estimated 2% of the total lengths of rivers in the contiguous United States are not affected by dams.

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Although less well-documented than dams, diversions that transfer water from a river into adjacent lands or move water from one river to another are widespread in the contiguous United States. By removing water from the source river, diversions contribute to drying, sediment accumulation, and longitudinal disconnectivity in the river corridor. The receiving river may have unnaturally large peak flows that cause erosion and may be entered by contaminants and exotic organisms that harm the ecosystem of the receiving river.

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