HydroSHED

HydroSHED provides not only the location of rivers (see this post) and water bodies (see this post) but also the direction of water flow.

Developed by the Conservation Science Program of World Wildlife Fund (WWF), in partnership with the U.S. Geological Survey (USGS), the International Centre for Tropical Agriculture (CIAT), The Nature Conservancy (TNC), and the Center for Environmental Systems Research (CESR) of the University of Kassel, Germany.

The documentation and the data can be downloaded at USGS's website.