The National Water Information System (NWIS) provides access to water-related data at over 1.5 million sites in the United States and its territories. The sites are grouped into five categories: surface water, groundwater, springs, atmospheric, and other. At any given site, one or more types of data may be available. Where data collection is continuous, data may be available in real time, as individual observations, or as daily summaries. Some sites may have data from water quality samples. At stream sites, data may be available for flood peaks. For surface water and groundwater sites, data may be available for individual field measurements. For many active or recently active sites, there may also be an online annual summary report.

Site information can be exported in several different formats, including text (txt), Microsoft Excel (xls), comma separated (csv), tab separated (rdb), keyhold markup language (kml) used for Google Earth or compatible map software, and Ersi shapeFile (.shp) for ArcGIS or compatible GIS software. The Table of Sites is displayed directly in the web browser, and all other formats are exported as a zipped data packages.