What is surface water and what affects its availability?

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Alaska Mountain Range, Credit: USGS/Photo by John J. Mosesso

Material adapted from: Vandas, S.J., Winter, T.C., and Battaglin, W.A. 2002. Water and the Environment, p. 20-23. Published by the American Geosciences Institute Environmental Awareness Series. Click here to download the full handbook.

Surface waters include streams, rivers, lakes, reservoirs, and wetlands. The term stream is used here to represent all flowing surface water, from brooks to large rivers. Surface waters and their associated ecosystems provide habitat to many plant and animal species. Because surface waters are on the land surface, they are easily developed for use and provide about 78 percent of the United States’s total off-stream water use.

Stream flow varies in response to climatic factors and human activities. Some streams have a small annual discharge for the large size of their drainage area, such as the Colorado River, and some have a greater demand for their water than they can supply without reservoir storage. Because of their importance as a water source, flow rates for selected streams are continuously monitored by stream gages. Discharge is the amount of water moving down a stream per unit of time. Discharge is the product of the average velocity of flowing water and the cross-sectional area at a selected site on a stream. Average velocity is determined by measuring flowing water at many locations and depths across the selected measurement site. The cross-sectional area and the average velocity at each of these measured locations are multiplied to calculate discharge at that point. The discharges for all locations are added to obtain the total discharge of the stream.

Streams are a dynamic part of the environment and are good indicators of what is happening in a watershed. Stream flow in a watershed includes all water contributed from headwater areas, stream banks, channels, flood plains, terraces, connected lakes, ponds, wetlands, and groundwater. Because watersheds are complex systems, each tends to respond differently to natural or human activities.

The physical characteristics of a watershed (land use, soil type, geology, vegetation, slope, and aspect) and climate control the quantity and quality of water that flows from them. Changes to any of these characteristics can affect water quantity and quality. For example, the removal of vegetation by natural causes such as fire can change the water storage and infiltration characteristics of a watershed. Because burned areas contain less vegetation to slow runoff and hold soil in place, the rate and quantity of water that runs off the surface to streams increases, and so does erosion. During heavy rains, the increased runoff and erosion can result in increased chance of flooding, mudslides, and impaired water quality.

Water seeks the path of least resistance. As water flows through a watershed, it picks up and deposits sediments, soil and rock particles, creating stream corridors. These corridors, which consist of stream channels, banks, and flood plains, are affected by natural and human activities that occur within watersheds. The physical processes of sediment transport and deposition are critical to the formation of the stream corridor.

The transport of sediment within and from a watershed is one of the major processes that help shape the surface of the Earth. Sediment particles are classified by size, with smallest being clay and the largest being boulders. Smaller particles are usually carried in suspension while the larger materials are moved along the channel bottom by rolling, sliding, or bouncing.

One of the major activities of a stream is to transport materials within and out of a watershed. Sediment transport rates of a stream are a function of stream power, which is a measure of the combined effect of the slope at the streambed (higher slopes generate higher stream velocities) and discharge (volume of water). Where stream power is reduced, a stream’s sediment carrying power is also reduced, and a portion of the sediment is deposited. For example, sediment is deposited following the peak, or highest, discharge of a flood. Sediments can be deposited in channels for short periods of time and moved again or remain stationary as in alluvial fans or in large reservoirs. Stream channels and their flood plains are constantly adjusting to changing water quantities and sediment supplied by their watersheds. Long-term changes in runoff and sediment load may lead to long-term changes in channel characteristics.

Learn More

  • USGS Surface Water Information Pages (Website), U.S. Geological Survey
    Links to basic information on surface water, real-time surface water data and information, surface water publications, and information on USGS surface water projects and activities.
  • Water as One Resource: How interactions between groundwater and surface water impact water availability (Webinar), American Geosciences Institute
    Webinar on the links between groundwater and surface water, and implications for water management and policy.
  • National Water-Quality Assessment (NAWQA) Program (Website), U.S. Geological Survey
    Links to national water quality studies and publications, including regional and national assessments of water-quality status and trends for surface water and groundwater, as well as national synthesis assessments on pesticides, volatile organic compounds, nutrients, trace elements, and aquatic ecology.
  • Surface Water Treatment Rules (Website), Environmental Protection Agency
    Website describing a variety of existing surface water treatment rules designed to reduce microbial contamination of drinking water.
  • Streamer (Interactive Map), U.S. Geological Survey
    Interactive map that allows users to locate and trace the sources and sinks of all rivers and major streams in the United States.