Dam failures can create more disastrous flooding than would otherwise be possible. Most rivers carry dissolved minerals and organic compounds. Even a slow river can carry small grains of clay. As the strength of flow increases, sand, gravel, and even boulders can be dislodged and moved downstream. Centuries of erosion can leave dramatic results, including deep canyons and waterfalls.
Minerals drop out of the river when the flow slows.
Alluvial deposits can form deltas where the river flows into a lake or ocean. Farms and cities have been developed on large deltas. New Orleans sits on the delta formed where the Mississippi River flows into the Gulf of Mexico. Other heavily-populated deltas include those of the Nile in Egypt and the Mekong in Vietnam. Some communities depend on the fish that live in or travel along rivers.