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Home›World geography›Rivers and mountains

Drainage basins and divides

Rain that falls on 41 percent of the 48 contiguous United States ends up in one river, the Mississippi.

Loading the 3D model

Drag to turn. Scroll or pinch to zoom.

Click anywhere to drop rain. The shaded areas are drainage basins, and the dashed purple lines between them are divides. Drops on either side of a divide end up in different places.

Where does the rain go?

Rain that reaches the ground has three choices. Some soaks in, some evaporates or is taken up by plants, and the rest runs downhill over the surface into rills, streams and rivers. The area of land that sends its water to one common outlet is called a watershed, or drainage basin. The USGS defines it exactly that way: an area of land that drains all the streams and rainfall to a common outlet, such as the outflow of a reservoir, the mouth of a bay, or any point along a stream channel. The words watershed, drainage basin and catchment are used almost interchangeably.

1 a ridge that forms a divide, 2 the outlet of the middle basin, 3 a tributary joining the main stream, 4 a stream in the neighbouring basin. Rain on each side of the dashed line drains to a different outlet.

Divides

Ridges and hills that separate two watersheds are called drainage divides. A raindrop that lands exactly on the crest has to choose, and two drops a step apart can end up in different oceans. In the scene, switch on Basins and drop rain on both sides of a dashed purple line. The drops run away from each other. Then drop rain in the middle of one coloured area and watch it join the stream that drains that basin. The readout names the basin and the edge where the water leaves the map.

The model uses the rule a raindrop follows: always take the steepest way down. That is also why rivers sit in valleys and why contour lines bend into Vs where they cross a stream. The V points upstream because the valley floor is lower than the slopes on either side, so the line of equal height has to travel up the valley before it can cross.

Basins inside basins

Watersheds nest. A creek has its own small basin, which is part of the basin of the river it joins, which is part of a larger basin again. The USGS divides the United States this way into 22 regions, 245 subregions, 405 basins, about 2,400 subbasins, about 19,000 watersheds and about 105,000 subwatersheds, each with its own code. The biggest of them all in the country is the Mississippi. The US Army Corps of Engineers calls it the third largest drainage basin in the world, after the Amazon and the Congo, covering more than 1,245,000 square miles in all or part of 31 states and two Canadian provinces.

BackgroundWaters from as far east as New York and as far west as Montana reach the lower Mississippi, the Corps of Engineers notes, and the basin is shaped like a funnel with its spout at the Gulf. The EPA adds that nearly 30 percent of Americans live inside it. A drop of rain in Montana and a drop in Pennsylvania, more than 2,500 kilometres apart, can pass New Orleans on the same day.
Pollution travels the same way. Anything washed off the land in a basin can end up at its outlet, which is why the Mississippi basin is managed as one system.
Key points
  • A drainage basin (watershed) is all the land that drains to one outlet.
  • Ridges between basins are divides.
  • Small basins nest inside larger ones.
  • The Mississippi basin covers 41 percent of the contiguous US.
  1. What is a drainage basin?
    • All the land that drains to one outlet
    • The bottom of a river
    • A lake behind a dam
  2. What separates two drainage basins?
    • A divide (ridge)
    • A delta
    • A tributary
  3. What percent of the 48 contiguous US states does the Mississippi drain?
  4. Which two basins are larger than the Mississippi?
    • Amazon and Congo
    • Nile and Yangtze
    • Ob and Yenisey
  5. The Mississippi basin covers about 1,245,000 square miles. About how many million km² is that? (1 sq mi = 2.59 km²; one decimal place.)
  6. How many states drain at least partly into the Mississippi?
  7. In the scene, two drops land a short step apart on either side of a divide. What happens?
    • They flow to different outlets
    • They meet at once
    • They stay put
  8. Why do contour lines make a V where they cross a stream?
    • The valley floor is lower than the slopes beside it
    • Rivers are drawn first
    • It is a printing convention only
  9. How many hydrologic regions does the USGS divide the US into (the 2-digit level)?
  10. What share of Americans live in the Mississippi/Atchafalaya basin, according to the EPA?
    • Nearly 30 percent
    • About 5 percent
    • About 70 percent
Sources for this course (24)
  • USGS Water Science School, Watersheds and Drainage Basins (usgs.gov/water-science-school/science/watersheds-and-drainage-basins: common outlet, drainage divide, watershed = drainage basin = catchment, larger watersheds contain smaller ones)
  • USGS Nonindigenous Aquatic Species, Hydrologic Unit Codes explained (nas.er.usgs.gov/hucs.aspx: 22 regions, 245 subregions, 405 basins, about 2,400 subbasins, 19,000 watersheds, 105,000 subwatersheds)
  • US Army Corps of Engineers New Orleans District, The Mississippi Drainage Basin (mvn.usace.army.mil: third largest basin after the Amazon and Congo, 41 percent of the 48 contiguous states, more than 1,245,000 square miles, 31 states and two Canadian provinces)
  • US EPA, The Mississippi/Atchafalaya River Basin (epa.gov/ms-htf: 2,350 miles, 15 percent of North America, borders of 10 states, nearly 30 percent of Americans live in the basin)
  • National Geographic Education encyclopedia: Source (resource/source: headwaters, the tributary farthest from the end; Gangotri Glacier, Breg spring for the Danube, Lake Itasca for the Mississippi)
  • Oxbow Lake (resource/oxbow-lake: concave banks erode, convex banks collect sediment, Mississippi meander cut-offs, billabong)
  • Delta (resource/delta: name from the Greek capital letter delta)
  • River (resource/river)
  • Anatomy of the Nile (resource/anatomy-nile: White Nile from Lake Victoria, Blue Nile from Lake Tana, Nile fan about 70,000 km2)
  • Fold Mountain (resource/fold-mountain: anticline and syncline; Appalachians once taller than the Himalayas)
  • USGS This Dynamic Earth, The Himalayas: Two continents collide (pubs.usgs.gov/gip/dynamic/himalaya.html: 2,900 km, collision 40 to 50 million years ago, India 6,400 km south of Asia 80 million years ago moving about 9 m a century, rising more than 1 cm a year)
  • Geological Society of London, Continental/Continental: The Himalayas (crust about 75 km thick)
  • Kathmandu Post, 8 Dec 2020 (Everest 8,848.86 m announced jointly by Nepal and China; Peak XV given as 8,778 m in 1847)
  • 국토지리정보원 대한민국 국가지도집 청소년판 2022 (nationalatlas.ngii.go.kr): 산 (page_3151: K2 8,611 m, 안나푸르나 8,091 m, 안데스 가장 긴 산맥, 국토 약 70% 산지, 백두산 2,744 m 천지 직경 3,550 m, 한라산 1,947 m, 오름 약 370여 개)
  • 강 (page_3152: 강물은 지구 물의 0.0002%, 아마존 전 세계 강물의 20%, 오비강 약 200일 결빙, 압록강 한반도에서 가장 길고 유역이 넓음)
  • 산지 지형 (page_3175: 평균 고도 약 448 m, 동아시아 910 m, 백두대간과 산경표, 분수계)
  • 산지 지형 also: 한라산 식생의 수직적 분포 (page_3151)
  • 고위 평탄면 대관령 고랭지 채소·목축 (page_3175)
  • 하천 지형 (page_3176: 동해 쪽 하천 짧고 급함, 남한에서 유역 가장 넓은 강 한강, 가장 긴 강 낙동강, 하상계수 섬진강 270, 나일강 30, 댐 이전 한강 390)
  • 국가지도집 하천 (page_180: 한강 494.4 km, 유역 25,953 km2, 북한 포함 35,770 km2; 낙동강 510.3 km, 23,384 km2; 금강 397.8 km, 9,912 km2)
  • 한국민족문화대백과사전 압록강 (encykorea.aks.ac.kr E0035176: 803.3 km, basin 31,226 km2)
  • 낙동강 (E0011565: 521.5 km)
  • 지리산 국립공원 (E0054158: 천왕봉 1,915 m)
  • 국가지도집 통계 26p 주요 강의 길이 (압록강 803 km).

Checked: 2026-10-18