Damming is known to be one of the most important anthropogenic impacts on freshwater ecosystems. Dams fragment river systems and potentially block migration routes, leading to the loss of megafauna. They change the downstream flooding patterns and sediment deposition leading to the loss of floodplains, riparian zones and wetlands. It is important to note that both surface water losses and gains can be detrimental to biodiversity and ecosystems. 

Time period: months to years.