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Reach on the Ninilchik River.
Photo by R. T. Ourso, U.S. Geological Survey
Costello Creek in the northern Susitna River Basin.
Photo by T. P. Brabets, U.S. Geological Survey
Yellowstone Lake in Yellowstone National Park.
Photo by G. K. Boughton, U.S. Geological Survey

Rivers, streams, lakes, and reservoirs—these are all features in the landscape that contain water flowing on the earth's surface. Rivers and streams carry flowing water whereas as lakes, wetlands, and reservoirs hold or store water. Regardless of their role in the landscape, these features are all surface-water resources that are replenished by precipitation. Rain falling on the land surface that does not seep into the ground becomes runoff, which flows into rivers, streams, or lakes. The land area that drains water to a particular river, stream, or lake is called a watershed, which can be identified on a map by tracing a line along the highest elevations, generally a ridge, between two areas on a map.
Cypress Knees.
Photo by D. K. Demcheck, U.S. Geological SurveyLarge watersheds, like the Mississippi River basin, contain thousands of smaller watersheds that drain small creeks or streams, all of which drain to the Mississippi River.

About 80 percent of the total freshwater that we use daily comes from surface water. In 2000, the estimated amount of surface-water withdrawal in the United States was 323 billion gallons per day. Most of this water was used for irrigation and public supplies. Because surface water plays such a vital role in our lives, it is important to have accurate measures of streamflow to determine stream stage and discharge over time. The U.S. Geological Survey collects and analyzes streamflow data for thousands of streams across the Nation to provide accurate and timely information about the availability and variability of surface-water flow.