US EPA - excerpts from Water: Rivers and Streams

Small streams, headwaters and streams that flow only part of the year protect against floods, filter pollutants, and provide food and habitat for many types of fish.

Small streams, including those that don’t flow all of the time, make up the majority of the country’s waters. They could be a drizzle of snowmelt that runs down a mountainside crease, a small spring-fed pond, or a depression in the ground that fills with water after every rain and overflows into the creek below. These water sources, which scientists refer to as headwater streams, are often unnamed and rarely appear on maps. Yet the health of small streams is critical to the health of the entire river network and downstream communities.

Importance of Streams

Streams, headwaters and streams that flow only part of the year provide many upstream and downstream benefits. They protect against floods, filter pollutants, recycle potentially-harmful nutrients, and provide food and habitat for many types of fish. These streams also play a critical role in maintaining the quality and supply of our drinking water, ensure a continual flow of water to surface waters, and help recharge underground aquifers.

Clean drinking water:

Streams play a critical role in the quality and supply of our drinking water by ensuring a continuous flow of clean water to surface waters and helping recharge underground aquifers. In the continental United States, 357,000 miles of streams provide water for public drinking water systems. Of that total, 58 percent (more than 207,000 miles) are headwater streams. Approximately 117 million people– over one-third of the total U.S. population – get some or all of their drinking water from public drinking water systems that rely in part on these streams.
See: geographic analysis of surface drinking water provided by headwater streams.

Flood and erosion protection:

Headwaters, seasonal streams and rain-dependent streams absorb significant amounts of rainwater, runoff and snowmelt before flooding. These streams have significant storage ability and play a critical role in protecting downstream communities by moderating flooding during heavy flow and by maintaining flow during dry weather.  Over the last 30 years, freshwater flooding has cost an average of $7.8 billion in direct damage to property and crops each year.1

Groundwater recharge:

Streams are also vital for recharging the nation’s groundwater supply. Water enters the groundwater through the stream bed. Even during dry periods, groundwater replenishes flow in the stream to feed downstream waterways. In arid regions, water from rain-dependent and seasonal streams supports springs, wetlands and plants far from the recharge areas. A major source of water in rivers in the Southwest is from groundwater released into streams that only flow part of the year.

Pollution reduction:

Streams can reduce the pollution that flows to downstream rivers, lakes, bays, and coastal waters. They are able to retain sediments and excess nutrients, such as nitrogen and phosphorus, and prevent these pollutants from traveling further downstream where they could cause algal blooms or dead zones. 

Wildlife habitat:

Streams that only flow for part of the year are unique and diverse habitats that can support thousands of species, including plants, fish, amphibians, birds and mammals. These streams are important as spawning and nursery habitats, seasonal feeding areas, refuge from predators and competitors, shelter from extreme weather and travel corridors. Many stream species, including fish, snails, crayfish, insects and salamanders, are now in danger of extinction as a result of human actions. A few dozen species are already listed under the U.S. Endangered Species Act; hundreds of others are rare enough to be considered for listing.

Streams that flow for only part of the year provide crucial habitat, food and water for plants and wildlife.  In the arid West, vegetation and wildlife near these streams – which often have water flowing just below the surface even when the surface looks dry – is significantly higher than in the surrounding uplands.