Damming of rivers can have an impact on water in a number of ways. Water flowing out of dams has reduced suspended material as a large amount settles on the bottom of dams. Water is depleted of nutrients and is often more saline with detrimental effects on downstream agriculture and fisheries.
An example of a small dam.
The Katse Dam in Lesotho forms part of the Lesotho Highlands Water Project (LHWP). The project was identified as a possible means to supplement the water supply to South Africa. Water from the dam first travels through a 45 km, 4 m diameter tunnel, exiting at a hydroelectric station near Muela. The dam's high elevation allows a gravity flow delivery system to South Africa, in addition to hydroelectric power to Lesotho, and was a prime reason behind the choice of site.