The U.S. Geological Survey provides a variety of hydrologic and water-resource information. Their official site for maps and graphs on stream flow, with links to NWS and other related data, is "Waterwatch":
water.usgs.gov/waterwatch/

For real-time hydrologic (stream-gaging) data for many states and links to historical streamflow data see
water.usgs.gov/public/realtime.html

The 1998 USGS report to Congress on the decline of streamgaging stations is at
water.usgs.gov/streamgaging

Discussion of the deliberate flooding of the Grand Canyon is found at
water.usgs.gov/public/wid/FS_089-96/FS_089-96.html

with a related followup study, "Effects of flooding on plant production downstream from Glen Canyon Dam", at
pubs.usgs.gov/fs/FS-060-99/

Many flood-related USGS Fact Sheets, including those in the reference list, are available online, often in both PDF and HTML format; see
water.usgs.gov/wid/indexlist.html

Some of special note include "Large floods in the United States: Where they happen and why" (USGS Circular 1245, 2003)
pubs.usgs.gov/circ/2003/circ1245/pdf/circ1245.pdf

"The world's largest floods, past and present: Their causes and magnitudes" (USGS Circular 1254, 2004)
pubs.usgs.gov/circ/2004/circ1254/pdf/circ1254.pdf

"Flood hazards—A national threat" (USGS Fact Sheet 2006-3026, January 2006)
Pubs.usgs.gov/fs/2006/3026

"StreamStats: A U.S. Geological Survey Web application for stream information" (USGS Fact Sheet FS 2004-3115, 2005)
md.water.usgs.gov/publications/fs-2004-3115/

The National Water Conditions site offers streamflow data (current) over the United States and southern Canada; it is a joint effort of USGS and Environment Canada's Climate Information Branch, at
water.usgs.gov/nwc

Images and information on the Scientific Assessment and Strategy Team (SAST), formed to assess the 1993 Mississippi River basin flooding and to develop a long-term floodplain management plan, are found in USGS Fact Sheet 10399 at
erg.usgs.gov/isb/pubs/factsheets/fs10399.html

and links to a variety of other information/sites on that flood are found at
ghrc.msfc.nasa.gov/

The U.S. Army Corps of Engineers maintains a page with links to much information about the 1993 Mississippi flooding
pubs.usgs.gov/gip/ocoee2/

For flash-flood safety information:
www.nws.noaa.gov/om/brochures/ffbro.htm

Information about the 1997 Red River flooding is available in USGS Open-File Report 97-575 at
nd.water.usgs.gov/pubs/ofr/ofr97575/index.html

and photographs at
nd.water.usgs.gov/index/flood.html

The Great Salt Lake, which has no outlet streams, flooded badly in the 1980s; see
edcwww.cr.usgs.gov/earthshots/slow/GreatSaltLake/GreatSaltLake

The Dartmouth Flood Observatory at Dartmouth College has created a site through which information on and high resolution images of floods since 1 January 1994 can be accessed at
http://www.dartmouth.edu/artsci/geog/floods/index.html