We prefer linking to nonprofit sites with pictures, maps, and further links. Drowned lower valley became Chesapeake Bay . Because flow is not constant, many rivers produce floods and droughts. Flood control projects attempt to reduce variation in flow, but frequently have negative consequences. 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.