This article includes a list of references, related reading or external links, but its sources remain unclear because it lacks inline citations. (February 2019) Watersheds of North America are large drainage basins which drain to separate oceans, seas, gulfs, or endorheic basins. There are six generally recognized hydrological continental divides which divide the continent into seven principal drainage basins spanning three oceans (Arctic, Atlantic and Pacific) and one endorheic basin. The basins are the Atlantic Seaboard basin, the Gulf of Mexico basin, the Great Lakes-St. Lawrence basin, the Pacific basin, the Arctic basin, the Hudson Bay basin, and the Great Basin. Together, the principal basins span the continent with the exception of numerous smaller endorheic basins.
Watersheds of North America LocationGreenland, Canada, United States, MexicoThe Atlantic Seaboard basin in eastern North America drains to the Atlantic Ocean; the Great Lakes-St. Lawrence basin in central and eastern North America drains to the Gulf of St. Lawrence on the Atlantic Ocean or to the Labrador Sea; the Gulf of Mexico basin in the southern United States drains to the Gulf of Mexico, a basin of the Atlantic Ocean; the Pacific basin in western North America drains to the Pacific Ocean or the Gulf of California on the Pacific Ocean; the Arctic basin in northwestern North America drains to the Beaufort Sea of the Arctic Ocean or directly to the Arctic Ocean; the Hudson Bay basin in central northern North America drains to Hudson Bay on the Labrador Sea (which may be considered part of either the Arctic Ocean or Atlantic Ocean), or to the Arctic Sea via Foxe Basin and Fury and Hecla Strait; the Great Basin in western United States is an endorheic basin which does not drain to any ocean. The Atlantic Seaboard basin is bounded by the Atlantic Ocean to the east, the Eastern Continental Divide to the west, the Great Lakes-St. Lawrence Divide to the north, and the Lake Okeechobee endorheic basin to the south. The Gulf of Mexico basin is bounded by the Gulf of Mexico to the south, the Eastern Continental Divide to the east, the Great Lakes-St. Lawrence Divide to the northeast, the Laurentian Divide to the North, and the Continental Divide to the west. The Pacific Basin is bounded by the Continental divide to the east and Pacific Ocean to the west; the basin excludes the endorheic Great Basin in the west. The Great Basin has a closed loop boundary encompassing substantially all of Nevada, the western half of Utah and parts of Oregon, California, Idaho, and Wyoming. Most lakes are not actually endorheic, but endorheic basins may not have standing water, or have water only seasonally. The most significant endorheic basins are these:
The western boundary (Eastern Continental Divide) of the Atlantic Seaboard basin and eastern boundary of the Gulf of Mexico basin is formed by the Appalachian Mountains to the North, the Piedmont Plateau and lowland ridges of the Atlantic Coastal plain to the south. The eastern boundary of the Pacific basin and western boundary of the Gulf of Mexico basin (Continental Divide) is the Rocky Mountains. A principal river of a basin is a river that drains directly to the incident ocean, sea or gulf, or into an endorheic basin. Great Lakes-St. Lawrence basin
Atlantic Seaboard basin
Gulf of Mexico basin
Arctic basin
Hudson Bay basin
Pacific basin
Great Basin
A drainage basin is an area of land where all flowing surface water converges to a single point, such as a river mouth, or flows into another body of water, such as a lake or ocean. A basin is separated from adjacent basins by a perimeter, the drainage divide,[1] made up of a succession of elevated features, such as ridges and hills. A basin may consist of smaller basins that merge at river confluences, forming a hierarchical pattern.[2]  Other terms for a drainage basin are catchment area, catchment basin, drainage area, river basin, water basin,[3][4] and impluvium.[5][6][7] In North America, they are commonly called a watershed, though in other English-speaking places, watershed is used only in its original sense, that of a drainage divide. In a closed drainage basin, or endorheic basin, the water converges to a single point inside the basin, known as a sink, which may be a permanent lake, a dry lake, or a point where surface water is lost underground.[8] Drainage basins are similar but not identical to hydrologic units, which are drainage areas delineated so as to nest into a multi-level hierarchical drainage system. Hydrologic units are defined to allow multiple inlets, outlets, or sinks. In a strict sense, all drainage basins are hydrologic units but not all hydrologic units are drainage basins.[8] Major continental divides, showing how terrestrial drainage basins drain into the oceans. Grey areas are endorheic basins that do not drain to the oceans Ocean basinsThe following is a list of the major ocean basins:
Largest river basinsThe five largest river basins (by area), from largest to smallest, are the basins of the Amazon (7M km2), the Congo (4M km2), the Nile (3.4M km2), the Mississippi (3.22M km2), and the Río de la Plata (3.17M km2). The three rivers that drain the most water, from most to least, are the Amazon, Ganga, and Congo rivers.[11] Endorheic drainage basins
Endorheic drainage basins are inland basins that do not drain to an ocean. Around 18% of all land drains to endorheic lakes or seas or sinks. The largest of these consists of much of the interior of Asia, which drains into the Caspian Sea, the Aral Sea, and numerous smaller lakes. Other endorheic regions include the Great Basin in the United States, much of the Sahara Desert, the drainage basin of the Okavango River (Kalahari Basin), highlands near the African Great Lakes, the interiors of Australia and the Arabian Peninsula, and parts in Mexico and the Andes. Some of these, such as the Great Basin, are not single drainage basins but collections of separate, adjacent closed basins. In endorheic bodies of standing water where evaporation is the primary means of water loss, the water is typically more saline than the oceans. An extreme example of this is the Dead Sea. Drainage basins have been historically important for determining territorial boundaries, particularly in regions where trade by water has been important. For example, the English crown gave the Hudson's Bay Company a monopoly on the fur trade in the entire Hudson Bay basin, an area called Rupert's Land. Bioregional political organization today includes agreements of states (e.g., international treaties and, within the US, interstate compacts) or other political entities in a particular drainage basin to manage the body or bodies of water into which it drains. Examples of such interstate compacts are the Great Lakes Commission and the Tahoe Regional Planning Agency. HydrologyDrainage basin of the Ohio River, part of the Mississippi River drainage basin In hydrology, the drainage basin is a logical unit of focus for studying the movement of water within the hydrological cycle, because the majority of water that discharges from the basin outlet originated as precipitation falling on the basin.[12] A portion of the water that enters the groundwater system beneath the drainage basin may flow towards the outlet of another drainage basin because groundwater flow directions do not always match those of their overlying drainage network. Measurement of the discharge of water from a basin may be made by a stream gauge located at the basin's outlet. Depending on the conditions of the drainage basin, as rainfall occurs some of it seeps directly into the ground. This water will either remain underground, slowly making its way downhill and eventually reaching the basin, or it will permeate deeper into the soil and consolidate into groundwater aquifers.[13] As water flows through the basin, it can form tributaries that change the structure of the land. There are three different main types, which are affected by the rocks and ground underneath. Rock that is quick to erode forms dendritic patterns, and these are seen most often. The two other types of patterns that form are trellis patterns and rectangular patterns.[14] Rain gauge data is used to measure total precipitation over a drainage basin, and there are different ways to interpret that data. If the gauges are many and evenly distributed over an area of uniform precipitation, using the arithmetic mean method will give good results. In the Thiessen polygon method, the drainage basin is divided into polygons with the rain gauge in the middle of each polygon assumed to be representative for the rainfall on the area of land included in its polygon. These polygons are made by drawing lines between gauges, then making perpendicular bisectors of those lines form the polygons. The isohyetal method involves contours of equal precipitation are drawn over the gauges on a map. Calculating the area between these curves and adding up the volume of water is time-consuming. Isochrone maps can be used to show the time taken for runoff water within a drainage basin to reach a lake, reservoir or outlet, assuming constant and uniform effective rainfall.[15][16][17][18] GeomorphologyDrainage basins are the principal hydrologic unit considered in fluvial geomorphology. A drainage basin is the source for water and sediment that moves from higher elevation through the river system to lower elevations as they reshape the channel forms. EcologyThe Mississippi River drains the largest area of any U.S. river, much of it agricultural regions. Agricultural runoff and other water pollution that flows to the outlet is the cause of the hypoxic, or dead zone in the Gulf of Mexico. Drainage basins are important in ecology. As water flows over the ground and along rivers it can pick up nutrients, sediment, and pollutants. With the water, they are transported towards the outlet of the basin, and can affect the ecological processes along the way as well as in the receiving water source. Modern use of artificial fertilizers, containing nitrogen, phosphorus, and potassium, has affected the mouths of drainage basins. The minerals are carried by the drainage basin to the mouth, and may accumulate there, disturbing the natural mineral balance. This can cause eutrophication where plant growth is accelerated by the additional material. Resource managementBecause drainage basins are coherent entities in a hydrological sense, it has become common to manage water resources on the basis of individual basins. In the U.S. state of Minnesota, governmental entities that perform this function are called "watershed districts".[19] In New Zealand, they are called catchment boards. Comparable community groups based in Ontario, Canada, are called conservation authorities. In North America, this function is referred to as "watershed management". In Brazil, the National Policy of Water Resources, regulated by Act n° 9.433 of 1997, establishes the drainage basin as the territorial division of Brazilian water management. When a river basin crosses at least one political border, either a border within a nation or an international boundary, it is identified as a transboundary river. Management of such basins becomes the responsibility of the countries sharing it. Nile Basin Initiative, OMVS for Senegal River, Mekong River Commission are a few examples of arrangements involving management of shared river basins. Management of shared drainage basins is also seen as a way to build lasting peaceful relationships among countries.[20] The catchment is the most significant factor determining the amount or likelihood of flooding. Catchment factors are: topography, shape, size, soil type, and land use (paved or roofed areas). Catchment topography and shape determine the time taken for rain to reach the river, while catchment size, soil type, and development determine the amount of water to reach the river. TopographyGenerally, topography plays a big part in how fast runoff will reach a river. Rain that falls in steep mountainous areas will reach the primary river in the drainage basin faster than flat or lightly sloping areas (e.g., > 1% gradient). ShapeShape will contribute to the speed with which the runoff reaches a river. A long thin catchment will take longer to drain than a circular catchment. SizeSize will help determine the amount of water reaching the river, as the larger the catchment the greater the potential for flooding. It is also determined on the basis of length and width of the drainage basin. Soil typeSoil type will help determine how much water reaches the river. The runoff from the drainage area is dependent on the soil type. Certain soil types such as sandy soils are very free-draining, and rainfall on sandy soil is likely to be absorbed by the ground. However, soils containing clay can be almost impermeable and therefore rainfall on clay soils will run off and contribute to flood volumes. After prolonged rainfall even free-draining soils can become saturated, meaning that any further rainfall will reach the river rather than being absorbed by the ground. If the surface is impermeable the precipitation will create surface run-off which will lead to higher risk of flooding; if the ground is permeable, the precipitation will infiltrate the soil.[5] Land useLand use can contribute to the volume of water reaching the river, in a similar way to clay soils. For example, rainfall on roofs, pavements, and roads will be collected by rivers with almost no absorption into the groundwater. A drainage basin is an area of land where all flowing surface water converges to a single point, such as a river mouth, or flows into another body of water, such as a lake or ocean.
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What is the lowest point that most watersheds drain into
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