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.
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.