Drainage Basins & Flooding
What Is a Drainage Basin?
A drainage basin is the area of land where every drop of rain eventually finds its way into the same river system. The boundary of a basin is the watershed — usually a high ridge of land — and inside the watershed, water moves through a series of stores and flows. The drainage basin is the basic unit geographers use to study where water comes from, where it goes, and why floods happen.
Water enters a basin as precipitation, sits temporarily in stores (vegetation, soil, groundwater, lakes), moves through flows (interception, infiltration, throughflow, overland flow) and finally leaves as river discharge at the mouth.
Stores and Flows
When rain falls on a drainage basin, the first thing it usually meets is vegetation. Trees and shrubs catch droplets on their leaves — called interception. Some evaporates straight back into the air, some drips slowly to the ground. Once on the soil, water can soak in (infiltration), move sideways through the upper soil (throughflow), or run across the surface as overland flow. Deep below, water enters cracks in the rock (groundwater flow) and feeds rivers slowly throughout the year.
A healthy basin slows water down. Forests intercept up to a third of rainfall before it reaches the soil. Grassland and well-aerated farmland soils let water infiltrate. Rivers respond gradually, peaking days after rain falls. A damaged basin — ploughed bare, compacted by livestock, or paved over — rejects water at the surface. Overland flow dominates, water reaches the river within hours, and discharge spikes sharply. The same rainfall produces a small flood in the first basin and a serious flood in the second.
Hydrographs Tell the Story
A hydrograph records the response of a river to a storm. Rainfall is shown as bars and discharge as a line. The rising limb shows water arriving in the channel, the peak discharge is the highest point, and the falling limb shows the river slowly returning to its base flow. The time gap between peak rainfall and peak discharge is the lag time.
A flashy hydrograph has a short lag time and steep rising limb — common after intense rain in urban or deforested basins. A low hydrograph has a long lag and gentle limbs — common in forested rural basins with permeable rock.
Physical Causes of UK Flooding
Rivers flood when discharge exceeds channel capacity and water spills onto the floodplain. Several physical conditions push UK rivers towards this point.
Heavy or prolonged rainfall is the obvious trigger. Saturated soil cannot absorb new rain, so almost everything runs straight into the river. Steep upland slopes — common in north-west Britain — speed water downhill. Impermeable rocks like granite stop water soaking in deep underground. Sparse vegetation cover reduces interception. When several of these line up at once, even a routine storm can flood a town downstream.
Human Causes
Land-use change tilts the balance further. Building on a floodplain replaces grass and soil with roofs, roads and concrete — a typical city centre rejects 70–90 % of rainfall as overland flow. Drains move that water to the river within minutes. Deforestation in upland catchments removes the trees that would have intercepted rain. Ploughing fields up and down the slope creates ready-made channels for surface flow. Even small changes — a new car park here, a hedge removed there — add up.
On Boxing Day 2015, Storm Eva dumped over 100 mm of rain on the upper Calder Valley in West Yorkshire in a single day. Soils were already saturated from a wet December, much of the upland moor had been drained for grouse shooting, and the steep narrow valley funnelled water rapidly downhill. The River Calder rose 2 metres in a few hours, overtopping defences in Hebden Bridge and Mytholmroyd. Around 2,000 homes and 1,000 businesses were flooded across the Calderdale district, and damage exceeded £100 million. Almost every cause — physical and human — played a role.
Annual Patterns
UK rivers also have a yearly rhythm. In winter, low evaporation and saturated soils mean even moderate rain causes high discharge. In summer, plants intercept and transpire much more water, soils have spare capacity, and discharge stays low. The annual hydrograph — a year-long record — shows clear winter peaks. Climate change is making winter rainfall more intense and summer droughts longer, sharpening both ends of the cycle.