Few useful links to download GIS and hydrologic data for USA

1) Elevation, soil, land-cover:  http://datagateway.nrcs.usda.gov/

2) Elevation and land-use:       http://nationalmap.gov/viewer.html

3) Elevation:                             http://srtm.csi.cgiar.org/

4) Soil:                                      http://websoilsurvey.sc.egov.usda.gov/App/HomePage.htm

5) Rainfall, temperature:           http://www.ncdc.noaa.gov/cdo-web/