Watersheds

A watershed, or drainage basin, is a region drained by a stream, lake, or other type of watercourse. The land is divided into millions of watersheds of varying sizes and shapes, all of which collect runoff from higher to lower elevations. Most watersheds join other larger watersheds and eventually flow into the oceans.

Characteristics

The term “watershed” has several meanings. The term is derived from the German term Wasserscheide, which means “water parting,” or the line or ridge of higher ground that separates two adjoining drainage basins. This definition is also used in Great Britain, where “watershed” refers to a drainage divide between adjacent drainage basins. The term, used in the United States and by several international agencies, has been modified to refer to the land area drained by or contributes water to a particular stream, lake, or other body of water. For this discussion, the term will refer to the region that serves as the collecting system for all the water moving downslope from higher to lower elevations on its eventual path to the ocean. In this physical context, which is governed by topography, the terms “watershed” and “drainage basin” are synonymous.

The relationship of watersheds to topography was recognized many years ago. In 1752, French geographer Philippe Buache presented a memoir to the French Academy of Sciences in which he outlined the concept of the topographic unity of a watershed. This concept was followed by European cartographers of the late eighteenth and early nineteenth centuries, who prepared maps showing each country's major drainage basins. Although these early cartographers would often exaggerate the height of the divides between watersheds, the basic concept was to show how the land was divided into various drainage basins that acted as efficient collection systems for runoff resulting from precipitation.

Watersheds transport water from upland areas to lower elevations in a variety of pathways. The most obvious path is by perennial streams that flow in channels. This form of surface runoff includes overland flow, which is the water moving over the ground surface, and base flow, which comes from that portion of the precipitation that has infiltrated through the soil into the underlying groundwater that enters the stream at some downgradient point. Thus, surface runoff from a watershed is a mix of “stormflow” or “quick flow,” which occurs right after a precipitation event, and base flow from groundwater, which takes more time to join the surface water. The rates at which surface water and groundwater move through a watershed depend on factors such as precipitation amounts and intensity, geology, soils, topography, and vegetation.

Stream Channels

Stream channels vary enormously in length and width, from a minor erosion channel that can easily be stepped over to rivers such as the Mississippi, which grows as wide as 1.5 kilometers before it empties into the Gulf of Mexico, and the Amazon, which becomes several kilometers wide before entering the Atlantic Ocean. Watersheds also have enormous range in length, area, and discharge. By far, the largest watershed in the world is the Amazon River basin, with a drainage area of about 7,045,000 square kilometers, approximately one-third of the entire area of South America. The second and third-largest watersheds in the world are the Congo River basin in Africa (3,820,000 square kilometers) and the Mississippi River watershed in the United States (3,270,000 square kilometers). Thus, the Mississippi watershed, which includes the Missouri River, drains an astonishing 40.5 percent of the entire area of the contiguous United States.

The Amazon is also the largest river in the world in terms of discharge, averaging 6,300 cubic kilometers per year. The second and third-largest dischargers in the world are the Congo River and the Orinoco River in Venezuela (1,250 and 1,100 cubic kilometers per year, respectively). The longest rivers in the world are the Nile in Africa (6,671 kilometers), the Amazon (6,300 kilometers), the Yangtze in China (6,276 kilometers), and the Mississippi (6,019 kilometers). At the other end of the spectrum are innumerable small streams in the headwaters of their watersheds near the divides, some with lengths of only a few meters.

Although there is obvious variation in shape from watershed to watershed, most tend to be pear-shaped. This shape is the most probable, as ground slopes and branching stream networks naturally evolve over time to dispose of the runoff efficiently and the sediment load in the water. Departures from the usual pear shape are attributed to structural control by underlying and exposed bedrock formations. For example, some basins are elongated in shape when they occupy long, narrow valleys, as are often found in the Appalachian region of the eastern United States, where long, resistant ridges of sandstone and quartzite run approximately parallel with less resistant valleys underlain by shale and limestone.

Most of the runoff in the humid land areas of the world eventually flows into the oceans via a series of hydrologically connected watersheds. Thus, the waters and sediment load of the Missouri and Ohio Rivers join the Mississippi River at St. Louis, Missouri, and Cairo, Illinois, respectively, and eventually flow into the Gulf of Mexico below New Orleans. Another large-scale example is the watersheds for the Great Lakes, furnishing the water for the St. Lawrence River, which flows eastward through Canada into the Gulf of St. Lawrence and the Atlantic Ocean. However, there are areas of the world where runoff flows into interior basins surrounded by high mountains that do not allow the stream to get to the ocean. This type of drainage system, called interior drainage, is common in semiarid and arid climates. Major examples of watersheds with interior drainage include the Caspian Sea in Asia (3,626,000 square kilometers), the Aral Sea in Kazakhstan and Uzbekistan (1,618,750 square kilometers), Lake Eyre in Australia (1,424,500 square kilometers), and the Great Basin in Utah, Nevada, and eastern California (500,000 square kilometers).

Watershed Divides

The divides that separate watersheds vary from sharply defined ridges in mountainous terrain to poorly defined boundaries in glaciated landscapes, regions of low relief, and areas of limited topographic expression. For example, the highest land in the Everglades (12,950 square kilometers) in Florida is only 2.1 meters above sea level, which means that natural runoff (excluding canals) flows in directions sometimes governed more by wind than by topography. Another prominent instance of a poorly defined divide occurs in southern Wyoming, where the Continental Divide, which separates the waters that flow into the Pacific Ocean from those that flow into the Gulf of Mexico, splits into two divides surrounding the Great Divide Basin. This unusual situation means that anyone who drives along Interstate 80 in Wyoming, for example, can cross the Continental Divide twice in an east-west direction.

Watershed divides, especially in mountainous areas, have often been used as political boundaries. Examples include the Andes between Argentina and Chile, the Pyrenees between France and Spain, and the Bitterroot Range between Idaho and Montana. Watershed divides also often serve as starting points for major cities. For example, Atlanta developed as a rail center in the nineteenth century because it was on the divide between the streams that flowed into the Gulf of Mexico (the Chattahoochee and Flint Rivers) and those that flowed into the Atlantic Ocean along the east coast of Georgia (the Ocmulgee River).

The drainage pattern or network of stream channels that develops within a watershed is related to local geologic and geomorphic factors. The most common drainage pattern that develops on horizontal and homogeneous bedrock or on crystalline rock that offers uniform resistance to erosion is called “dendritic” since it resembles the branching pattern of trees. All other types of drainage patterns reflect some form of structural control, such as the trellis pattern associated with the elongated watersheds in the Ridge and Valley region of the Appalachians of the eastern United States. Rectangular patterns can develop in faulted areas where the drainage paths follow the lines of least resistance that develop along the fault lines.

Watershed size and flow can change either naturally or by anthropogenic means. Major natural examples include the deflection by continental glaciation of the upper Missouri River from Hudson Bay in eastern Canada to its present-day confluence with the Mississippi River and the geologic subsidence and tilting that diverted the drainage of the Nyanza area in East Africa from the Congo River, which flows into the Atlantic Ocean, to Lake Victoria, which drains into the Nile and the Mediterranean Sea. The flow through the Florida Everglades has been substantially altered by drainage activities and canal building for agricultural purposes, which started in the late nineteenth century. Water that used to flow into the Everglades and Florida Bay from Lake Okeechobee was diverted to the canalized Caloosahatchee River, which empties into the Gulf of Mexico, and the Miami, North New River, Hillsboro, West Palm Beach, and St. Lucie Canals, which are connected with the Atlantic Ocean on the east coast of Florida. Another instance of anthropogenic intervention with watershed flow is illustrated by the diversion of water from Lake Michigan, which is part of the Great Lakes and St. Lawrence River system, to the Chicago Ship and Sanitary Canal, which is connected with the Illinois River, which flows into the Mississippi River. The purpose of the canal was to transport sewage from the Chicago metropolitan area away from Lake Michigan, which is used as a water source.

Watersheds remain of crucial importance. Not only do they supply water for drinking and agricultural and industrial use, but they also have several critical ecological functions. They are essential to water filtration, mitigate erosion, control floods, promote carbon storage, cycle nutrients, and encourage biodiversity. These environmental benefits are also crucial to halting global climate change. The Environmental Protection Agency has stated that the United States watershed areas contribute $450 billion to the national economy regarding food, tourism, and their assistance in producing manufactured goods. Finally, watersheds produce beautiful portraits of nature for people to enjoy and are the means of many forms of recreation. 

Principal Terms

base flow: that portion of stream flow that is derived from groundwater

discharge: the volume of water per unit of time that flows past a given point on a stream

drainage divide: the ridge of land that marks the boundary between adjacent watersheds

interior drainage: watersheds in arid areas where the runoff does not flow into the oceans

perennial stream: a stream that has water flowing in it throughout the year

relief: the difference in elevation between the highest and lowest points of land in a particular region

runoff: that portion of precipitation in a watershed that appears directly in surface streams without having first entered into the groundwater

stream: water flowing in a relatively narrow but clearly defined channel from higher to lower elevations under the influence of gravity

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