Construction of dams or other regulators changes the river flow regime and natural sediment transport and affects environmental flows, i.e. the flow of water within rivers and groundwater systems to maintain downstream ecosystems and their benefits. It is not only a matter of reduced flows but also shifting in seasonal variations and reducing seasonal variability (e.g. increasing low-flow and reducing high-flow levels). This will affect seasonally inundated wetlands and species whose life cycles respond to the seasonal variation, e.g. fish spawning. Trapping of sediments behind dams and the effects this has on sediment and nutrient transport downstream – and eventually the impacts on river deltas and coastal waters – have been documented for many large rivers including the Nile, the Indus and the Mississippi. Sediments are deposited in the deltas and their loss means that the coastline will recede, such as in the case of the Mississippi delta. The nutrients attached to the sediment feed the flood recession agriculture along the rivers as well as the coastal fisheries at the delta. Trapping of these nutrients means the collapse of coastal fisheries, as was experienced in the Mediterranean Sea outside the Nile delta, the Mexican Gulf outside the Mississippi delta and the Arabian Sea at the Indus delta.
Reduction in environmental flows is frequently experienced in river basins where water extraction for agriculture, industry, energy and household consumption, as well as constructed storages, reduces the flow, particularly in the low-flow part of the season. An example is the Yellow River, where the environmental flow was used as one of the key parameters considered in restoration and protection efforts.
Another parameter used to assess and monitor environmental impacts on species and ecosystems is connectivity or habitat fragmentation. This is used in the Danube and has been used to try to predict the possible impacts of dam development in the Mekong River, where planned dams would drastically reduce ecosystem connectivity in the Lower Mekong Basin. A key issue in relation to ecosystem connectivity is the barrier effect to up- and downstream fish migration. The impacts of dam developments in the Columbia and Mississippi rivers on the fish stocks clearly document the damage of habitat fragmentation on migratory fish.