Watersheds, also called drainage or catchment basins, are areas of land where precipitation drains into a common body of water such as a lake, river, or ocean. This includes precipitation from clouds like rain or snow, groundwater, and other bodies of water within the basin. Watersheds are powerful components of the natural landscape, and it is important to understand the factors that impact their condition.
The size and shape of a drainage basin is determined by many features of its landscape. Often, the first that comes to mind is an area’s topography. The steepness of hills and mountains, along with the distance between a precipitation source and bodies of water, also determine how quickly it reaches its destination. Additionally, different soil types impact water movement, with some types (like sand) much more permeable than others (like clay). If the surface is too impermeable for precipitation to reach the soil in the first place, which is the case in developed areas covered by roofs and pavement, it forms runoff and reaches bodies of water without spending time as groundwater. Extremely large drainage areas are made of a number of tributary basins, which collect precipitation in streams and then deliver water to the major rivers. Watersheds can be made of any number of smaller drainage basins, which is called a river system.
The elevated boundary between areas drained by different basins is called a divide, and a continental divide completely separates large river systems to different regions of a continent. In North and South America, the Great Continental Divide runs along the peaks of the Rocky Mountains and Andes, with water to the west running into the Pacific Ocean and to the east into the Atlantic. Another continental divide exists along the Himalayan Mountains in South Asia and continues along the coast of the Arabian Peninsula and eastern Africa, directing precipitation into the Indian Ocean. On the other side of this divide, to the north of the Himalayas, exists a feature called an endorheic basin—in these regions, precipitation never reaches an ocean, but is retained in a smaller body of water like a lake or inland sea.
Knowing the extent of watersheds is important for both natural and sociopolitical reasons. Scientists interested in hydrology and ecology often study entire drainage basins because the majority of the precipitation, sediments, nutrients, and pollutants flowing through a watershed originated there, too. Many conservation efforts protect watersheds as holistic units as well, called watershed management, and some countries and states even have governing bodies for basins in their territory. In the field of geopolitics, the study of how international relations are influenced by geographical factors, watersheds can be the cause of conflict or of harmony through mutual governance and accountability.
This map layer was created using a model that predicts water flow with elevation data. It separates one watershed into two, creating a divide, at every location where:
- Two river branches meet at a single location
- Each branch has an individual upstream area of at least 100 square kilometers (about 38.61 square miles)
Each time a divide is created, the model makes a new “level”—this map layer shows levels two and four. You can click the map to see the land area each basin covers and compare the sizes of basins from different levels. This is just one way to define a watershed’s boundaries on a map, a process called watershed delineation.
Watershed conservation is a very important part of keeping water clean and safe. The Nature Conservancy explains that there are a lot of ways to help protect your watersheds, like conserving water, disposing of waste and chemicals safely, or choosing to walk or bike instead of drive. Check out the Protected Areas layer to find the areas of your watershed that need special care.
