The Drainage Basin

(a) Source — the starting point of a river, often a spring, glacier or marsh in highland.

(b) Watershed — the boundary (usually a ridge of higher land) separating one drainage basin from another.

(c) Tributary — a smaller river or stream that flows into a larger river.

(d) Mouth — the point where a river ends, normally entering a sea, lake or larger river.

Input: Precipitation (rain, snow, sleet or hail).

Outputs:

• River discharge — water leaving the basin via the river to the sea / lake / larger river.
• Evapotranspiration — water leaving as vapour through evaporation from surfaces and transpiration from plants.

A drainage basin is an open system within the wider hydrological cycle. Its single input is precipitation: rain, snow, sleet or hail falling within the area enclosed by the watershed.

Water is then held in several stores: vegetation (interception), the soil (soil moisture), saturated rock (groundwater in aquifers) and the river channel itself. It moves between these stores through flows — surface runoff (fastest, across the land), infiltration (into soil), percolation (deeper into rock), throughflow (sideways through soil) and groundwater flow / baseflow (slow, through aquifers).

Water finally leaves the basin via two outputs: river discharge at the mouth, and evapotranspiration back to the atmosphere from surfaces and plants.

A drainage basin is an open system because water continuously enters from outside via precipitation (rain, snow, sleet, hail) and continuously leaves via river discharge (water flowing out at the mouth) and evapotranspiration (water rising back to the atmosphere). Matter crosses the basin boundary in both directions, which is the defining feature of an open system.

The global hydrological cycle, by contrast, is closed. At the scale of the whole planet, water cannot escape to space and is not added from outside. The total volume of water on Earth is therefore constant — only its form (ice, liquid, vapour) and location (oceans, atmosphere, land) change.

A heavy storm sharply increases the basin's single input — precipitation, triggering a cascade of changes through stores and flows.

Stores fill up. Interception by vegetation increases, but quickly reaches a ceiling. Soil moisture then rises until the soil becomes saturated; from this point no more water can infiltrate. As percolation continues, groundwater in the underlying rock also rises, lifting the water table.

Flows accelerate. Once the soil is saturated, infiltration stops and surface runoff — the fastest flow — increases rapidly, sending water overland to streams. Throughflow through saturated soil also speeds up, contributing additional water to the river network.

Outputs rise sharply. River discharge climbs to its peak: on a storm hydrograph the rising limb is steep and lag time is short. If discharge exceeds bankfull capacity, water spills onto the floodplain — flooding. Once the storm ends, surface runoff decays first; baseflow (groundwater) sustains the river through the falling limb back to normal flow.

The closed-system idea matters here: nothing has been added to the planet, but the basin's local distribution has been pushed temporarily out of balance.

Flood risk in a drainage basin is fundamentally controlled by how quickly and in what quantity precipitation reaches the river channel — and several physical characteristics of the basin set this response.

Basin shape. A round / circular basin delivers water from all parts of the catchment to the main river at similar times → short lag time and a sharp peak discharge. An elongated basin staggers tributary inputs → longer lag time and a lower, flatter peak. Round basins are therefore more flood-prone for the same rainfall.

Basin size. A small basin responds quickly to a storm but produces a smaller discharge peak. A large basin (Ganges, Mississippi) integrates rainfall over a huge area and can produce massive sustained flood waves.

Relief (slope steepness). Steep upland sides accelerate surface runoff under gravity → short lag time, flashy hydrograph, higher flood risk. Gentle gradient slows runoff and gives more time for infiltration.

Geology. Impermeable rock (granite, clay) prevents infiltration → high surface runoff → flashy basin. Permeable rock (chalk, sandstone) allows infiltration → slower throughflow and groundwater flow → subdued response.

Vegetation cover. Dense forest intercepts rainfall and root systems aid infiltration → longer lag time. Deforested or bare basins have minimal interception → faster runoff and higher flood risk.

Named basin — Bangladesh's Ganges-Brahmaputra-Meghna delta. Bangladesh sits where three massive rivers converge in a delta that is mostly less than 12 m above sea level. The catchment area is enormous, integrating rainfall across the Himalayas, Tibet, Nepal, India and Bangladesh. The monsoon climate delivers ~80% of annual rainfall in summer, Himalayan snowmelt adds to summer discharge, and deforestation in the upper catchment (Nepal, Bhutan) reduces interception. The combination of: large catchment area + convergence of three rivers + saturated monsoon soils + low-relief flat country (so water cannot drain away) makes Bangladesh one of the most flood-prone countries on Earth. More than 60% of the country was flooded in 1998, and severe flooding remains an annual concern.

Assessment. No single physical characteristic determines flood risk in isolation. Bangladesh shows how multiple factors COMPOUND: a large round basin + impermeable saturated soils + low relief + deforested upper catchment + monsoon climate all combine. The watershed and channel network define the basin; relief, geology and vegetation determine how RAPIDLY rainfall becomes discharge; size and shape determine the SCALE of that discharge. Physical features set the basin's intrinsic flood susceptibility, on which human activity then layers further pressure.

A drainage basin can be described either as a list of features — source, watershed, tributaries, mouth — or as an OPEN SYSTEM with inputs, outputs, stores and flows. Both descriptions are valid, but the integrated-system view captures something the feature-list cannot: how changes in one part propagate through the whole basin.

The case for the integrated-system view. A drainage basin has ONE input (precipitation), TWO outputs (river discharge + evapotranspiration), several STORES (interception, soil moisture, groundwater, channel) and several FLOWS (surface runoff, infiltration, percolation, throughflow, groundwater flow). These elements are CAUSALLY LINKED:

• Rising precipitation FILLS stores → flows accelerate → discharge rises.
• Saturated soil STOPS infiltration → forces water into faster runoff → shortens lag time.
• Removing vegetation REDUCES interception and infiltration → speeds runoff.
• Building dams TRAPS water in stores → flattens downstream discharge curves.

None of these can be understood by looking at a single feature in isolation. A reservoir on a tributary affects flood risk at the mouth; deforestation in the upper catchment changes the regime of the lower river; abstraction for irrigation downstream changes how much water is available for ecosystems further on.

Named basin — the Mississippi (USA). The Mississippi drains ~40% of the conterminous United States, integrating rainfall and snowmelt from 31 states and two Canadian provinces.

• Inputs: precipitation (heavy along the Mississippi corridor; large snowmelt from the upper Midwest each spring).
• Stores: thousands of natural lakes, wetlands, alluvial aquifers, reservoirs behind dams.
• Flows: vast tributary network (Missouri, Ohio, Arkansas, Red rivers all join); runoff from the High Plains; baseflow from the Ozark aquifers.
• Outputs: discharge into the Gulf of Mexico (~16,800 m³/s mean); evapotranspiration from forests, farms and wetlands.
• Human alteration of the system: ~5,600 km of levees built and managed by the Mississippi River Commission to confine the river; the Bonnet Carré Spillway diverts flood water into Lake Pontchartrain to protect New Orleans; multiple reservoirs (Lake Itasca, Lake Sakakawea, Truman Reservoir) store water; vast agricultural drainage from the Corn Belt accelerates runoff.

When Hurricane Katrina (2005) hit New Orleans, the levee system failed — but the disaster was not just an engineering failure. Wetland loss (~25% loss in the Mississippi delta since 1930) had reduced the natural storm-surge buffer; dredging the Mississippi River-Gulf Outlet had created a "funnel" for surge; sediment trapped behind upstream dams had starved the delta of fresh material so it was sinking even as the sea was rising. ~1,800 people died and ~$160 billion in damage was caused. This was a SYSTEM failure, not a feature failure.

The case for the feature-list view. For early learning and basic mapping, separating source, watershed, tributaries and mouth is useful. It is also the foundation of fieldwork: you need to identify features on a map before you can measure them. The labelled-diagram exercise is genuinely valuable.

Judgement. The feature-list view is a USEFUL STARTING POINT but quickly becomes inadequate. Real drainage basins are deeply integrated systems, where physical processes, human management and ecological functions all interact. Modern flood management (the UK's "natural flood management" approach; integrated catchment management) explicitly works with the system rather than treating features in isolation. The Mississippi shows that even the most heavily engineered basins are systems whose components cannot be managed independently. The integrated-system view is therefore both more analytically powerful and a better guide to action than the feature-list view.

The watershed has a strong claim to being the most important feature because it defines the basin itself. Without a watershed, there is no boundary, so no basin: every drop of precipitation falling inside the watershed becomes input to one specific river's network; precipitation falling on the other side of the ridge drains into a different basin entirely.

The watershed's relief and geology also shape the basin's behaviour. Steep, impermeable upland produces fast surface runoff, short lag times and flashy hydrographs (and therefore higher flood risk). Gentle, permeable upland slows the response and reduces peak discharge.

However, other features matter just as much. The river channel network (tributaries and confluences) determines the routing speed and the size of contributing area at each point. The mouth controls how water leaves the basin and where sediment is deposited. The internal stores (soil, groundwater, vegetation) regulate how much precipitation actually reaches the river. In a flat, permeable basin, soil and groundwater stores can be more decisive than the watershed itself.

Judgement. The watershed is the most important defining feature — it makes a drainage basin a discrete system — but in terms of how the basin behaves, the entire system (watershed plus channel network plus stores) matters together.