How Drainage Basins Connect Land and Rivers

A drainage basin, also called a catchment, is the area of land drained by a river and its tributaries. The outer boundary is a watershed, usually following higher ground. Rain falling on one side of that boundary flows towards one river system, while rain on the other side flows somewhere else.

Inside the basin, small streams join larger ones. The place where two channels meet is called a confluence. A river's source lies near the beginning of its network, while the mouth is where the river finally enters a lake, sea or another river.

Thinking in terms of a basin is useful because events upstream can affect places far downstream. Water, sediment and pollutants can all move through the connected network.

Not all rainfall immediately enters a river. Some is intercepted by leaves and branches. Some infiltrates into soil, where it may be stored or taken up by plants. Water can move slowly through soil and rock before reaching a channel as throughflow or groundwater flow.

When rain falls faster than the ground can absorb it, water may travel across the surface as runoff. Steep slopes can encourage rapid movement, while gentle slopes usually give water more time to infiltrate. Saturated or frozen soil can also increase runoff.

These pathways affect river discharge, which is the volume of water passing a point in a certain time. A basin that sends water rapidly into channels can produce a sharp rise in discharge after heavy rain, while slower pathways spread the response over a longer period.

Vegetation can slow water, encourage infiltration and return water to the atmosphere through transpiration. Urban surfaces such as roofs and roads are much less permeable, so rainfall often reaches drains and rivers quickly. Removing trees or compacting soil can also alter how water moves.

When studying a drainage basin, check:

• •The position of the watershed.
• •The pattern of tributaries and confluences.
• •Slope steepness across the basin.
• •Soil, rock and vegetation cover.
• •Urban surfaces, drains and other human changes.

Basin shape can matter too. In a compact basin, water from many tributaries may reach the main channel within a similar period. In a long narrow basin, arrival times can be more spread out. This can influence how quickly river discharge rises after widespread rainfall.

Geographers show a basin's response to rain with a storm hydrograph, a graph that plots river discharge over time alongside the rainfall that caused it. Rainfall is usually drawn as bars and discharge as a line. Before the storm, the line sits at a level called base flow, which is mostly supplied by groundwater. After rain begins, discharge rises along the rising limb, reaches a peak and then falls along the falling or recession limb as water drains away. The whole shape is a record of how the basin handled the storm.

The most useful measurement is lag time, the gap between the peak of rainfall and the peak of discharge. A short lag time and a tall, narrow peak suggest water reached the channel quickly, as it might in a small, steep or heavily urbanised basin. A long lag time and a low, broad peak suggest slower pathways through soil, rock and vegetation. Comparing hydrographs from the same river before and after a new housing estate, or from two neighbouring basins, lets you see the effects of land use directly in the numbers.

Most drainage basins eventually empty into an ocean, but some do not. In an endorheic basin, water flows inward to a lake or low point and leaves only by evaporation or by soaking into the ground. Several large examples exist. The Great Basin in the western United States drains to salty lakes and dry flats, and the Caspian Sea, the largest lake on Earth by area, collects rivers such as the Volga with no outlet to the ocean. The Dead Sea, fed by the Jordan River, is another.

Because evaporation removes water but leaves dissolved minerals behind, these terminal lakes tend to become salty over time. They are also very sensitive to changes upstream. The Aral Sea in Central Asia was once one of the largest lakes in the world, fed by the Amu Darya and Syr Darya rivers. From the 1960s, Soviet planners diverted much of their flow to irrigate cotton and other crops. With little river water arriving, the lake shrank dramatically and split into separate parts, leaving exposed salty seabed, collapsed fisheries and stranded fishing boats.

Be careful with the word watershed, because it means different things on either side of the Atlantic. In British geography it usually means the dividing line between basins, but in North America it often means the whole basin itself. Major divides can also meet at a single point. On Triple Divide Peak in Glacier National Park, Montana, rain falling on different slopes can end up in the Pacific Ocean, in the Gulf of Mexico by way of the Missouri and Mississippi, or in Hudson Bay in northern Canada. One summit can feed three of a continent's great drainage systems.

Divides are not fixed forever. A river with a steeper course can erode backwards towards its source, a process called headward erosion, until it cuts through the higher ground and reaches a neighbouring stream. The upper part of that stream is then diverted into the more aggressive river, an event known as river capture. The landscape usually keeps clues. The captured river often turns sharply at an elbow of capture, while downstream of the break the beheaded river is left too small for its valley. Where its old course crossed a ridge, a dry notch called a wind gap may remain.

The market town of Pickering in North Yorkshire sits beside Pickering Beck, a stream draining the moors above it, and it flooded several times between 1999 and 2007. A conventional answer would have been a high concrete wall through the town, but that was judged too costly and intrusive. Instead a partnership of agencies and local people tried to change how the whole catchment responded to rain. The idea, known as natural flood management, is to hold water back across the upper basin so that it reaches the town more slowly, rather than simply defending the channel at the bottom.

Workers built hundreds of leaky dams from logs and brushwood across small moorland streams, planted woodland, blocked old drainage channels and added low barriers of heather bales. Lower down, an earth bank was built to store floodwater temporarily and release it gradually. In hydrograph terms, the aim was a longer lag time and a lower, broader peak. During the heavy rain around Christmas 2015, when floods struck many towns elsewhere in northern England, Pickering avoided serious flooding, and analysis suggested the scheme had reduced the peak flow. Such measures work best for smaller catchments and cannot stop every extreme flood.

A drainage basin links every slope, tributary and water pathway feeding the same river system. Rain may reach channels quickly as surface runoff or more slowly through soil and groundwater. Look beyond the river channel to the whole catchment, and floods, water quality and river flow become easier to explain.