The USGS currently operates a nation-wide network of about 8,000 streamgages, where streamflow is monitored continuously. Historically, the USGS also has operated more than 12,000 streamgages that are not currently operated. In addition, the USGS operates thousands of partial-record stations where streamflow is measured periodically. Data collected at the continuous streamgages and partial-record stations are used to calculate the streamflow statistics for the stations. These data may be accessed through the NWIS-Web database. The statistics are calculated for streamgages by using software that implements standard procedures prescribed by the USGS Office of Surface Water. 

StreamStats provides estimates of streamflow statistics for user-selected sites by solving regression equations. These equations were developed through a process known as regionalization, in which regression analysis is used to relate streamflow statistics computed for a group of selected streamgages (usually within a State) to basin characteristics measured for the streamgages. Basin characteristics measured for ungaged sites can be entered into the resulting equations to obtain estimates of the streamflow statistics. Regression equations usually are applicable to an area that is coincident with the borders of a state, or to subregions within a state. That is because studies to develop the equations most commonly have been done through a cooperative funding mechanism whereby individual state agencies partner with the USGS to fund at least half of the cost for the study.

Each state has an introductory page that identifies the streamflow statistics that can be estimated by use of regression equations, provides citations for the reports that contain the regression equations and special notes, if any, about use of StreamStats for that state, and identities of the agencies that cooperated with the USGS to implement the state. You should read the cited reports to understand how StreamStats provides estimates of flows for data-collection stations and ungaged sites in the state. When you select a state in the StreamStats version 4 user interface a black circle with an "i" will appear beside the name of the state. Click on the circle to see the introductory page for that state. The introductory pages also can be accessed in version 4 by clicking on the About link above the map and then on the page that will subsequently appear, click on the State/Regional Info tab.