A watershed, also known as a drainage basin or catchment area, is defined as all the land that channels rainfall and snowmelt to a common outlet, such as a river, lake, reservoir, or ocean. It is a continuous area of land where all water flows downward to a single discharge point. This collection system includes surface water bodies like streams and wetlands, but also all the underlying groundwater. The watershed effectively acts as a funnel, gathering precipitation from the entire land area it covers and directing it toward its outlet.
Defining Boundaries and Scale
The physical structure that determines a watershed is the “watershed divide,” or drainage divide, which is a topographic boundary separating adjacent drainage basins. This boundary is made up of the highest points of elevation, such as hills, ridges, or mountain ranges. Any precipitation that falls on one side of this divide will flow into one watershed, and water falling on the other side will flow into a different one.
Topography is the main factor determining a watershed’s boundaries, allowing watersheds to exist on a vast range of scales. A watershed can be as small as the land draining into a local creek or large enough to encompass massive river basins. For example, the Mississippi River Watershed is the largest in the United States, draining 1.15 million square miles and covering all or parts of 31 states. Almost all large watersheds are composed of many smaller, interconnected sub-watersheds that flow into a main river system in a hierarchical pattern.
How Water Moves Through the System
Water movement through a watershed is governed by the hydrologic cycle, beginning with precipitation that falls onto the land. This water then follows three main pathways on its journey to the common outlet. The first path is surface runoff, or overland flow, which occurs when water flows across the land surface when the ground is saturated or when precipitation falls faster than the soil can absorb it. This water travels quickly over the surface and enters streams and rivers.
The second major route is infiltration, where water soaks into the ground. The rate of infiltration is highly dependent on the soil type, with dense clay soils allowing less water to pass through than porous sandy soils. Water that infiltrates the soil may be absorbed by plants, evaporate, or continue its downward movement through the soil layers.
The third path is groundwater flow, which connects surface water to underground reservoirs called aquifers. Infiltrated water that percolates deeper into the ground becomes groundwater, which moves slowly through the subsurface. This groundwater eventually seeps into the nearest stream or river, providing the sustained “base flow” that keeps streams flowing even during dry periods.
Ecological Importance and Water Quality
Watersheds provide numerous ecological functions and natural benefits. They support a high diversity of aquatic and terrestrial life by creating unique habitat zones. The surrounding vegetation, particularly in transitional areas known as riparian zones along the edges of water bodies, is important.
Riparian zones help maintain water quality by acting as natural buffers. Deep-rooted native vegetation stabilizes stream banks, which prevents erosion and reduces the amount of sediment that enters the water. This vegetation also filters upland sediments and pollutants, such as nutrients and chemicals, before they can reach the stream. The trees and plants in these areas provide shade, which regulates water temperature, creating suitable conditions for fish and other cold-water species.
Managing Human Impact
Human activities across the landscape can significantly impact a watershed’s function and the quality of its water resources. The main threat comes from non-point source pollution, which is widely dispersed and does not originate from a single pipe or discharge point. This type of pollution is carried by rainfall or snowmelt runoff as it moves over and through the ground.
Common non-point sources include excess fertilizers and pesticides from agricultural lands, sediment from construction sites, and oil, grease, and chemicals from urban stormwater runoff. These pollutants travel through the hydrological pathways of the watershed, ultimately depositing into lakes, rivers, and groundwater. To mitigate these impacts, watershed management focuses on conservation strategies like land-use planning and implementing best management practices (BMPs). These actions can include creating vegetated buffer strips to absorb runoff, implementing conservation tillage on farms, and controlling erosion at construction sites.