The critical situation facing the health of America's water resources and aquatic ecosystems is not the result of a single activity on or near a lake, river, or stream. Instead, it is the combined and cumulative result of many individual activities throughout a waterbody's entire natural drainage area, or watershed.
Water from falling rains and melting snows generally drains into ditches, streams, wetlands, groundwater supplies, lakes, or coastal waters. A watershed includes the area of land over which water drains into these waterbodies. A watershed may be larger or small; the Mississippi River, for example, drains a one-million-square-mile watershed made up of thousands of smaller watersheds, such as the drainage basins of the creeks that flow into tributaries of the Mississippi. In smaller watersheds, a few acres of land may drain into small streams, which flow into larger streams or rivers; the lands drained by these streams or rivers make up a larger watershed.
You are always in a watershed, no matter where you are on the land surface of the planet. Even in the middle of the driest of terrains, the land surrounding you at any given time drains to a stream, river, lake, aquifer, reservoir, or directly to coastal waters. Your watershed may be covered with forest or farmland, or be almost entirely developed.
Watershed boundaries are defined by the topographic features that dictate natural drainage patterns within an area. They are not defined by any international, state, or county borders, or by the public or private ownership boundaries that they may cross.
Watersheds provide useful geographic units for resource management aimed at protecting the health of aquatic ecosystems. The types of aquatic ecosystems supported within a particular watershed are determined by a number of natural features:
These same factors also determine potential land uses within the watershed.
At the watershed's highest points, underground springs, falling rain, and melting snows feed headwaters of streams. By force of gravity, all water runs downhill to the streams and rivers within the watershed, and is eventually carried to the oceans. This freshwater network is the very lifeblood of all living communities, including human society. Rivers provide drinking water for many cities, as well as water for other domestic and industrial uses; they also serve as transportation corridors and provide food, recreation, and scenic beauty. the water's downhill pathway is also integral to the health of aquatic and terrestrial resources throughout the basin. For example, the timing and quantity of stream flow and water storage in aquifers and wetlands result in a complex pattern of fast and slow currents, eddies, and backwater pools, creating a rich variety of aquatic habitats that harbor diverse communities of plants and animals. Even periodic natural flooding plays an important role in structuring habitat and providing conditions for various plant and animal species throughout much of a river system. Flooding changes existing vegetation and reconfigures landforms in active river channels, and the silt-laden floodwaters provide the rich soils commonly found in river floodplains.
When the natural variation in stream flow or water table level (aquifer depth) is altered by dams, canals, or channelization, many natural characteristics of the varied wetland and riverine habitats and their diverse flora and fauna disappear. for example, the Everglades ecosystem in South Florida had been extensively altered by the diversion of fresh water for agriculture and other uses. Many plant and animal species that once thrived throughout the Everglades cannot survive in the increasingly saline aquatic environment, and have disappeared from the area.
Problems also arise as a result of the use or rivers and their tributaries as waste streams. Historically, streams and rivers have served as dumping sites for the refuse, waste water, and runoff from towns and cities across the country. Coastal waters are the ultimate receptacle for these wastes as they flow downstream. Especially vulnerable are estuaries, valuable nursery and feeding grounds for a host of fish, waterfowl, and other wildlife species. Over two-thirds of all commercially important fish and shellfish species, such as flounder, shrimp, oysters, and clams, depend on these shallow, protective coastal waters for part or all of their lives. Migrating birds find essential feeding grounds and resting stops in the salt marshes and mudflats of the Nation's estuaries. Pollution and destruction of wetlands and marshes have significant negative impacts on the fish and wildlife that depend on these normally productive areas.
"Soil is the most fundamental resource, and its loss the most serious of all losses....The day will come when the ownership of land will carry with it the obligation to so use and protect it with respect to erosion that it is not a menace to other landowners and the public." - Aldo Leopold
"To protect your rivers, protect your mountains." - Emperor Yu of China, 1600 B.C.
Depending on the amount and type of vegetation, the soil characteristics, the slope of the basin, and the amount and velocity of water drainage, significant amounts of topsoil, sediments, and other particles may be carried by rivers and streams from the headwaters to lower portions of the watershed. Streamside and riverside vegetation can play an important role in anchoring soil, catching silt, filtering out pollutants, and absorbing nitrogen and phosphorus, which in over-abundance, promote harmful algal blooms. the biotic complexity provided by both wetland and riparian vegetation is essential to the ecological health of watersheds. The diversity of habitat required to support a rich community of organisms is supplied by variation in the river's velocity and temperature, patches of rock and gravel of different sizes, piles of branches and leaves, small dams formed by dead trees, and a multitude of other distinct niches created by the complex interplay between water movement, geologic events, and riparian vegetation. Maintaining a green belt of vegetation along our waterways also helps prevent erosion and the silting of streams, keeps the water cool for aquatic species, and provides shelter, hiding places, and nesting sites for countless species of fish and wildlife.
The occurrence of certain natural disturbances to stream flow and vegetation is also essential to the health of the watershed. For example, landslides occur at a magnitude and frequency characteristic of a specific landscape and geologic setting. Landslides deliver new sediment and gravel to river channels where they are sorted by flowing waters. These areas serve as spawning beds for salmon, as well as habitats for aquatic insects. Landslides at natural intervals are thus integral to the health of certain fish populations. However, increases in landslide frequency and magnitude within a watershed as a result of poor land use management, such as road-building on steep, unstable slopes, result in harmful downstream impacts on the riparian vegetation, on fish populations, and on an array of other organisms using the riverine corridor.
When forest fires, logging, road-building, or any residential, industrial, or agricultural development activities destroy the trees that anchor the soil along the water's edge, silt washes into the stream, altering streambed habitat as is settles out and depriving fish and aquatic insects of oxygen, often smothering them. Similarly, unchecked water that runs off rangeland or farmland into streams and rivers cut gullies and carries away topsoil. The displaced topsoil, with other debris, can enter streams and then settle out in lakes and reservoirs where it can reduce their holding capacity, increase the costs of water filtration, interfere with hydroelectric plants, and spoil recreational uses of the waterbodies, in addition to disrupting the aquatic ecosystem.
Go to Section II: How Do We Affect Watersheds and Their Resources?
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