Water Resources of the United States: http://water.usgs.gov/waterwatch 

American Whitewater Gauges - USA: http://www.americanwhitewater.org/gauges/

AWA River Search: http://www.americanwhitewater.org/search/

River Flow Info Phone Numbers [old, some may no longer work]:

AMC - Boston River Line (617) 433-7108
Androscoggin & Kennebeck - Maine (800) 557-3569
Connecticut River (Hartland Rapids, Wilder Dam) 888-356-3663
Deerfield River - New England Power (413)625-8414, 800-452-1737
Kennebec River - Central Maine Power (Harris Station) (207)672-3715
Merrimack - PSNH in Manchester,NH (603)634-3569
New England River Forcast Center.... 800-376-7120
Otter Creek Dam (603) 352-4130 (Keene NH)
Pemigewasset - PSNH in Manchester,NH (603)634-3569
Rapid River - Maine (207)784-4501
Swift River (Saco Bound) 603-447-2177 or 3801

New England

Army Corps of Engineers Realtime Data: https://rsgis.crrel.usace.army.mil/nae/pls/nae/nae_web.nae_webmenu.displaymenu?menu=main
AMC and  New England F.L.O.W. http://amcbcc.cnc.net/release/
Androscoggin, Pontook Dam, NH http://www.americanwhitewater.org/rivers/id/3686/ 
Blackwater River, NH http://www.americanwhitewater.org/rivers/id/1163/  TBD - April   cfs Blackwater Slalom

Dead River, ME: http://www.americanwhitewater.org/rivers/id/787/ 

Deerfield River, MA 

Dryway-Monroe Bridge/#5 Station Dam: http://www.americanwhitewater.org/rivers/id/681/ 

Zoar Gap/Fife Brook Dam: http://www.americanwhitewater.org/rivers/id/682/ 

Phone Before You Drive: 1-800-452-1737. 
Dryway - Monroe Bridge code: 255122.
Fife Brook Dam code: 255123.
The  flow phone system provides real time data as well as forecasts for the next 24 hours. The information at this number is for both the Connecticut and Deerfield Rivers. The call is free, sponsored by PG&E National Energy Group. The update usually happens around 5 P.M.

Esopus Creek, NY: http://www.dec.state.ny.us/website/dow/esopus.html 

Housatonic River - Bulls Bridge/Falls Village, CT  - ?

Kennebec River, ME: http://www.americanwhitewater.org/rivers/id/438/ Guaranteed water on weekends. Normally you can expect 4800 cfs from 10 am - 2 pm. (800) 557-3569  Normal release levels are 4800 to 6000 cfs from 10am till 2pm but can vary. Call 207-672-4848 for release information.