Streamflow and other hydroclimatic reconstructions for the western US:

Colorado TreeFlow
Over 20 streamflow reconstructions in Colorado developed by Woodhouse, Lukas, et al.

Extended streamflow reconstruction for the Colorado River
Reconstruction of Lees Ferry (762-2005) developed by Meko et al. (2007)

Updated Streamflow Reconstructions for the Upper Colorado River Basin
Reconstructions of Lees Ferry (1490-1997) and 9 other UCRB gages developed by Woodhouse et al. (2006).

California TreeFlow
Several recent hydroclimatic reconstructions in California.

North American Drought reconstructions
Gridded reconstructions of summer PDSI covering most of North America. Most reconstructions in the West are very reliable back to AD1500, and extend back further with less reliability.
Additional info and visualizations for NA Drought reconstructions

Synchronous Extreme Flows in Upper Colorado and Salt-Verde-Tonto River Basins
Website for the collaborative project between the University of Arizona's Laboratory of Tree-Ring Research (LTRR) and the Salt River Project (SRP). Go to Full Final Report, Appendix 7 for data access

Reconstructing Past Climate in the Southwest
1000-year reconstructions of cool-season precipitation developed by Ni et al. (2002) for every climate division in Arizona and New Mexico.

Other Hydroclimatic Reconstructions at NOAA Paleo
Reconstructions of hydroclimatic variables, including drought indices, precipitation, and streamflow, contributed by researchers from around the world.

Tree-ring data

International Tree-Ring Data Bank (ITRDB)
Over 2500 tree-ring chronologies from all over the world are archived in the ITRDB. ITRDB can be searched by text query or map-based interface
Explanation of ITRDB data formats
Explanation of COFECHA (quality-control program)