Drainage Basins, Watersheds, and River Courses

A drainage basin (or watershed) is the area of land drained by a river and its tributaries, bounded by a ridge or divide, and a river’s course is divided into the upper, middle and lower course, each with typical channel features and processes. In brief: a river begins at a source, is joined by tributaries at confluences and ends at a mouth, where it enters the sea, a lake or another river. The land gets steeper and the river narrower and faster in the upper course, and it becomes wider, deeper and slower toward the lower course. The long profile of a river is concave: steep near the source and gentle near the mouth.

Understanding river systems is essential for managing water, floods and land use. This guide defines the parts of a river system, drainage basins and watersheds, the three river courses, compares them in a table, works through calculations of gradient and drainage density, outlines the history of river studies and corrects common mistakes.

What are the parts of a river system?

• Source: where a river starts, often a spring, a lake, melting ice or boggy upland where many small streams join.
• Tributary: a smaller stream or river that flows into a larger (main) river.
• Confluence: the point where two rivers or streams meet.
• Channel: the path in which the water flows, between the banks and over the bed.
• Mouth: where the river ends in a sea, lake or larger river, often in an estuary or delta.
• Distributary: a branch that flows away from the main river, typically in a delta.

What is a drainage basin?

A drainage basin is the entire area of land drained by a river and its tributaries. The boundary is the watershed (also called a divide), an imaginary line along the ridges of high ground, where rain falling on one side flows into one river and on the other side into another. (In British English “watershed” means the dividing line, but in American English it is often used for the whole basin.) The water entering a basin is the precipitation; it leaves by the river mouth, by evaporation and by transpiration (the loss from plants), and some is stored in the soil and groundwater. Basins can be small (a stream draining a few square kilometers) or huge (the Amazon basin is over 6 million km²). The pattern of the tributaries is the drainage pattern: dendritic (tree-like) on uniform rock, trellis (right-angled) where there are alternate hard and soft rock ridges, radial on a volcano. Drainage density is the total length of streams divided by the basin area, and it is higher where the rock is impermeable and rainfall heavy, which causes quick runoff and flooding.

What are the upper, middle and lower courses?

The upper course is in the highlands near the source: a steep gradient, a narrow, shallow channel, a rough bed with large angular rocks, and a V-shaped valley. The river has a lot of energy and cuts down (vertical erosion). The middle course is where the gradient lessens, the valley and channel widen, the river picks up more tributaries and volume, and it begins to swing from side to side (lateral erosion), forming meanders. The lower course is near the mouth: the gradient is very gentle, the channel is wide, deep and smooth with fine sediment, and the valley floor is a broad floodplain with large meanders. Deposition dominates here. Although the water speed is often thought to decrease downstream, in fact the average velocity usually increases, because the bed is smoother and the channel more efficient, and the discharge increases as tributaries join. The long profile is a graph of the height of the river against distance from the source; the cross profile is the shape of the valley and channel at a point.

River courses compared

Worked example: gradient and drainage density

1. Gradient of each course (hypothetical river): upper course, 1,500 m down to 300 m over 60 km: (1,500 − 300) ÷ 60 = 20 m per km. Middle course, 300 m to 100 m over 140 km: 200 ÷ 140 = 1.43 m per km. Lower course, 100 m to 0 m over 300 km: 100 ÷ 300 = 0.33 m per km.
2. Overall gradient: the total length is 60 + 140 + 300 = 500 km and the total fall is 1,500 m, so the mean gradient is 1,500 ÷ 500 = 3 m per km. The upper course is about 20 ÷ 0.33 = 60 times steeper than the lower course, which gives the concave long profile.
3. Drainage density: a basin of 120 km² has a total stream length of 360 km. The drainage density is 360 ÷ 120 = 3 km per km².
4. Stream order (Strahler method): streams with no tributaries are of order 1; when two streams of the same order join, the order increases by one. A basin with 64 first-order streams, 16 second-order, 4 third-order and 1 fourth-order stream has a bifurcation ratio (the ratio of the number of streams in successive orders) of 64 ÷ 16 = 4, 16 ÷ 4 = 4 and 4 ÷ 1 = 4, a constant value of 4.
5. Runoff: a basin of 500 km² receives 900 mm of rain a year, a volume of 500 × 10⁶ m² × 0.9 m = 450 × 10⁶ m³. If 35 percent leaves as river flow, the volume of runoff is 157.5 × 10⁶ m³, a mean discharge of 157.5 × 10⁶ ÷ 31.5 × 10⁶ s = 5 m³/s.

How did the study of rivers develop?

Early civilizations measured river levels, such as the Nile gauges (nilometers) in Egypt, to predict floods and harvests. Pierre Perrault and Edme Mariotte showed in the 17th century that rain is enough to explain river flow. In 1945, Robert Horton proposed the laws of stream numbers and lengths, which Arthur Strahler later simplified in 1952 with the stream ordering system that is still used. William Morris Davis proposed the idea of the “cycle of erosion” (1899), with the stages of youth, maturity and old age of a river valley, which is now regarded as too simple. Modern studies use gauging stations, satellite data and GIS.

Which mistakes are common?

Mistake 1: Thinking that rivers always flow faster near the source

The gradient is steepest there, but the rough bed and shallow channel slow the water. Velocity often increases downstream as the channel becomes smoother and deeper.

Mistake 2: Confusing a watershed with a river

In the British usage, a watershed is the dividing line between basins. In the American usage it is often the whole basin.

Mistake 3: Mixing up tributary and distributary

A tributary flows into the main river. A distributary flows away from it.

Mistake 4: Confusing the long profile with the cross profile

The long profile shows height along the length of the river, and the cross profile shows the shape across the valley.