All materials for constructing the knowledge representing water (surface and subsurface) features; i.e., anything to do with water and water networks (hydrology).
Aquifers, wells, springs, etc. These generally come from individual states. For example, Maine has the Maine Geological Survey (MGS).
Waterbodies, like lakes, rivers, and wetlands. This information comes primarily from USGS NHDPlus data (NHDWaterbody).
How surface waterbodies connect so water can flow. This is modeled from USGS NHDPlus data (NHDFlowline).