Anthropogenic alteration of river flows, lake levels, and the duration and extent of wetland inundation – collectively, flow regulation – degrades habitats, with manifold direct and indirect effects on populations, community structure and food webs. These can arise from an overall insufficiency of water, or changes in flow that result in there being too much or too little water at particular times. Fluxes of sediments and nutrients are also affected by trapping in dams, while water released downstream may have altered temperature, flow magnitude or periodicity, and erosive power. To mitigate such effects in rivers and associated wetlands, it is essential to allocate sufficient water as an environmental flow (e-flow) in order to sustain native biodiversity and ecosystem functioning. There is general consensus that this allocation should involve sufficient water to mimic the natural flow regime. Obstruction of movement of materials and migrations by animals (mainly fishes) by dams will require different and innovative solutions, and the proliferation of hydropower dams on large rivers (Amazon, Mekong) will have serious consequences for ecology and fishery yields.