Rivers and Watersheds

About this topic

A watershed (drainage basin) is the land area that drains into a river and its tributaries; its edge is the divide. Rivers erode, transport and deposit sediment. In the upper course they cut steep V-shaped valleys with waterfalls; in the middle course they widen; in the lower course they meander across flood plains and form deltas at the mouth.

Rivers provide water, transport, power and fertile land. Floods are natural but worsened by deforestation and urbanisation. Dams regulate flow but alter ecosystems.

A river's behaviour changes along its journey. Near the source, steep slopes and fast water cut down into the bed and carry big boulders. Downstream, the gradient flattens, the river carries finer sand and silt, and it swings side to side, eroding the outside bend and depositing on the inside, which forms meanders. Eventually a meander may be cut off to leave an oxbow lake. When the river slows at its mouth, it drops its load as a delta, such as the Nile or Mississippi deltas.

The Amazon watershed drains about 7 million square km, around 40 percent of South America. Rivers provide drinking water, irrigation, fish, transport and hydropower, and ancient civilisations like Mesopotamia and Egypt grew along them. Flood plains are fertile because floods add silt, but they are also risky for settlements. Hydrologists measure discharge in cubic metres per second to predict floods. Natural flood management, such as planting trees and restoring wetlands, slows runoff. A common misconception is that rivers always flow north to south; they flow downhill. Also, what you pour into a street drain can end up in the local river.