The River Great Stour (East Kent) - an excellent case study with photos, aerial photos, a virtual walk and geology/topography maps. Useful for river management, human impact & studies of flooding.
The U.K. Environment Agency Flood Report - the Report of the Easter 1998 Floods which caused much damage in southern England.
The Los Angeles River - a virtual tour of the Los Angeles River from source to mouth
River Systems of the World - not a very attractive site, but it has links to articles on dozens of major rivers.
Red River, N.Dakota, U.S.A. - a large site dedicated to flood information on the Red River.
US Geological Survey - realtime water data, fascinating stuff...
International Rivers Network - an article on the impact of dams.