Informative and well-organised: one of the better UK government sites. Includes sections on: the state of the environment (for flood modelling, river habitats, river gauging stations and flow data); flood and flood warning information; news releases; current environmental issues; articles on water demand management and much more.
http://www.environment-agency.gov.uk/envinfo/index.htm

Newfoundland and Labrador site:

A good case study from Canada: check under "Natural environment" for drainage basins and flood-risk zones, together with landscape information and climate statistics.
http://www.heritage.nf.ca/sitemap.html

Yellow River, China, home page:

A personal site that provides a tremendous amount of information on the Yellow River Basin in text, pictures and maps; lots on loess, gullying, villages and dam sites; some links to other sites. Needs updating, but still very useful.
http://hydrogen.cis.umassd.edu/~gleung/

River Mississippi:

An easy-learning site - just click on everything and learn about the history and geography of the upper Mississippi!
http://www.oldmanriver.com/

Minnesota River Basin home page:

This page provides access to a comprehensive account (by the University of Minnesota’s Department of Soil, Water and Climate) of the physical geography of the river basin, its water quality; its ‘agroecoregions’, landscapes and land uses; and its planning and management zones. Lots of maps and statistics.
http://www.soils.agri.umn.edu/research/mn-river/

Flood Tracking Chart for the Red River Basin:

This is a US Geological Survey site on the water resources of North Dakota. It is useful to anyone interested in seeing how big floods are tracked and described in the US. There are links to other US flood pages and other related US government information.
http://srv1dndbmk.cr.usgs.gov/public/rrflood/rrflood.html

International Rivers Network (IRN):

A tremendous source of links to information on all sorts of ‘rivers’ subjects, US and worldwide.Particularly good for rivers-related news items. A politically aware site, campaigning for the sustainable use of water resources, and against large destructive projects.
http://www.irn.org/basics.shtml