WaterWatch is a US Geological Survey online product that gives consolidated river level and flood stage information across the US. The graphic is from 12 June 2008 showing the floods across the midwestern US.
A similar set of data (not precisely the same, but overlapping) is kept by the National Weather Service, as part of their Advanced Hydrologic Prediction Service. This has a whole set of comprehensive rainfall measures, which are as useful in drought condition monitoring as they are in flood monitoring.
This precipitation map image, for instance, shows the last 2 weeks of rain as a percentage of normal – with the extreme rains in Iowa, Wisconsin, and southwestern Michigan sharply contrasted with the dry western Upper Peninsula.