What is a Drainage Basin?

Rain falling on a hillside does not choose its river at random. A single ridge can decide the outcome completely: water landing on one side of it will end up flowing towards one river system, while water landing on the other side, sometimes only a short distance away, will flow towards a completely different one, possibly reaching a different sea altogether.

The whole stretch of land whose water all ends up feeding the same river is called a drainage basin. Every field, hillside, road and rooftop within that stretch eventually sends its water into that one river, whether directly or through smaller streams first. Britain is not covered by a single drainage basin but by hundreds of them, sitting side by side, each one draining into its own river.

The boundary separating one drainage basin from the next is called a watershed. Because water flows downhill, a watershed almost always follows the highest ground available, since that is the point where the land starts sloping away in two different directions at once. The watershed running along the Pennines, for example, is significant enough to split rainfall across a large part of northern England: water landing on its western side flows towards the Irish Sea, while water landing on its eastern side flows towards the North Sea.

The point where a river begins is called its source. Sources are typically found in hilly or mountainous areas, since a river needs a slope to start moving down. Many sources are springs, where groundwater that has travelled underground is forced back up to the surface. A common misunderstanding is that a river has only one such starting point. In fact, a single drainage basin can contain a large number of separate sources, since every small trickle running off a hillside counts as one. The actual source of a well-known river is often unremarkable to look at, sometimes little more than a damp, uneven patch of ground with water slowly emerging from it, rather than anything dramatic.

As these small streams flow downhill, they begin joining together. A smaller river that flows into a larger one is called a tributary, and the exact spot at which two rivers join is known as a confluence. At a confluence it is sometimes possible to see a visible line in the water, because the two rivers can be carrying different amounts of sediment, so one runs clearer and the other runs browner, and for a while the two flows resist mixing completely. The River Thames illustrates how much this adds up over distance: by the time it reaches London, it has been joined by around twenty named tributaries, meaning it is carrying water that originally fell as rain across four different counties.

Eventually, a river reaches the sea at a point called its mouth. Where the fresh water of the river meets the salt water of the sea, an estuary can form. Estuaries are unusual environments: the river slows down considerably as it widens out, and slower-moving water is less able to carry the material it has been transporting, so it drops much of it there, often creating mudflats and salt marsh. Because the water in an estuary is never fully fresh or fully salty, and because it changes constantly with the tide, only certain specially adapted plants and animals can survive in these conditions. This makes estuaries some of the richest wildlife habitats found anywhere in the country.

All of these features can also be identified on an Ordnance Survey map once the pattern is known. Rivers appear as blue lines, so a source is simply the point where a blue line begins, usually somewhere high up, often surrounded by contour lines that are packed closely together. A confluence shows up as a spot where two blue lines come together to form a single one, and a mouth is where a blue line meets the coastline, where the map's colouring changes from green to blue. A watershed is harder to spot, since it is never actually drawn on the map. Finding one means looking at the contour lines to identify the ridge of highest ground lying between two river systems, then checking that the blue lines on either side of that ridge flow away from it in opposite directions.

In short: a drainage basin is the whole stretch of land feeding one river, and a watershed is the ridge of high ground that separates it from its neighbours. Within a basin, water begins at a source, often a modest spring rather than anything dramatic, and a single basin can contain many sources rather than just one. As the water moves downhill, tributaries join the main river at confluences, before the river finally arrives at the sea at its mouth, sometimes forming a rich, ever-changing estuary. Every one of these features leaves a recognisable mark on an Ordnance Survey map, once a reader knows to look for blue lines, their junctions, and the shape of the contour lines around them.