Changing river environments

The drainage basin and the water cycle

A drainage basin is the area of land drained by a river and its tributaries. Its boundary is the watershed, the ridge of high land that separates it from the next basin. Inside the basin, water collects in the channel, the route the river follows. Small streams called tributaries join the main river at a confluence, and the river begins at its source and ends at its mouth where it meets the sea or a lake.

A drainage basin works as an open system. Precipitation is the input, water is stored in the soil, in the rock and in lakes, and water leaves as river discharge at the mouth and as water vapour in the air. Everything in between is a flow, and the exam question you will meet most often is the one that asks how a change in one flow changes the flood risk.

The drainage basin sits inside the water cycle, which is a closed system: the same water is recycled endlessly between the sea, the air and the land. Over the sea, evaporation turns liquid water into water vapour. Over the land, transpiration releases water vapour through the leaves of plants, and evapotranspiration is the total of evaporation from surfaces plus transpiration from plants. The vapour rises, cools and condenses into clouds, and falls back as precipitation.

Once rain reaches the ground, its route decides how quickly the river responds. Water can be held on leaves by interception, soak into the soil by infiltration, sink deeper into the rock by percolation, creep sideways through the soil as throughflow, travel slowly through permeable rock as groundwater flow, or rush straight across the surface as overland flow. Overland flow reaches the channel in minutes or hours; groundwater flow can take years. The faster the water arrives, the shorter the lag time between the rainfall and the peak of the flood, and the greater the flood risk.

River characteristics, the long profile and the Bradshaw model

The long profile is the gradient of the river from its source to its mouth, drawn as a line. It is concave: steep in the upper course where the river cuts through the hills, and almost flat in the lower course where it crosses the lowland. The cross profile shows the same valley cut across, and it changes just as clearly. In the upper course the valley is narrow and steep-sided, a V-shaped valley with a shallow channel; by the lower course the valley is wide and flat-floored, with a deep channel and a broad floodplain either side.

You must be able to use the characteristics in the syllabus precisely. Width is measured across the water surface, depth is measured from the surface to the bed, and velocity is the speed of flow, measured in metres per second. Discharge is the volume of water passing a point in a given time, measured in cubic metres per second (cumecs), and it is calculated as cross-sectional area x velocity. The wetted perimeter is the length of the channel bed and banks that is in contact with the water: the greater it is, the more friction there is and the more energy the river loses.

The Bradshaw model predicts how these characteristics change from source to mouth in a river with no major tributaries joining it. It is a model rather than a rule, so it is most useful when you use it to explain what the river is doing and then say where reality does not fit. A river that is joined by a large tributary, or that has a dam or a water abstraction on it, or that flows through a town, will not match the model at that point.

Processes within a river: erosion, transportation and deposition

A river erodes in four ways, and the syllabus names all four. Hydraulic action is the force of the water itself: air and water are forced into cracks in the bank or bed, and as the pressure is released the trapped air expands and the rock is broken away. Abrasion (also called corrasion) is the scraping and rubbing of the load against the bed and banks, rather like sandpaper. Attrition is the knocking together of the load itself, which breaks the material into smaller and rounder pieces and is the reason load particle size falls downstream. Solution (also called corrosion) is the chemical dissolving of soluble rock such as limestone and chalk by acids in the water.

Transportation happens in four ways too. Traction is the rolling and dragging of large boulders along the bed. Saltation is the bouncing of smaller pebbles in short hops. Suspension is fine silt and clay carried within the water, which is what makes a river look brown after heavy rain. Solution is dissolved material carried invisibly in the water. Which process dominates depends on velocity and on the size of the material, so in the upper course traction and saltation carry the rough load, while in the lower course most of the load is in suspension.

Deposition is the dropping of material when the river no longer has enough energy to carry it. It happens wherever velocity falls: on the inside of a meander bend, where the channel widens or the gradient flattens, behind an obstacle, and where the river meets still water at a lake or the sea. The largest and heaviest material is dropped first, which is why boulders are left in the upper course while fine silt is carried as far as the sea and dropped last.

• Erosion: hydraulic action (force of the water), abrasion (load scraping the bed), attrition (load hitting load), solution (chemical dissolving).
• Transportation: traction (rolling), saltation (bouncing), suspension (carried in the water), solution (dissolved).
• Deposition happens when velocity falls, and the heaviest load is dropped first.
• Attrition is the process that makes the load smaller and rounder downstream, so it links directly to the Bradshaw model.

Landforms of erosion: waterfalls, rapids, gorges, V-shaped valleys, interlocking spurs and potholes

A waterfall forms where a band of hard rock lies over softer rock. The softer rock is eroded more quickly by hydraulic action and abrasion, cutting a step and hollowing out a plunge pool beneath. The hard rock is left overhanging, unsupported; it eventually collapses under its own weight, and the waterfall moves upstream. The steep-sided gorge below the fall is the route the waterfall has cut back through, so a waterfall and its gorge are formed together and retreat together. Rapids are the same idea on a smaller scale: alternating bands of harder and softer rock along the bed create a stretch of broken, turbulent water without a single large drop.

A V-shaped valley is cut in the upper course, where the river has a steep gradient and most of its energy goes into vertical erosion of the bed. The valley sides are steepened by weathering and mass movement, and the loose material that falls in is carried away by traction and saltation, so the river is not choked. The valley stays narrow and steep-sided because the river has little sideways erosion to widen it.

Interlocking spurs are the ridges of land that jut out alternately from each side of a V-shaped valley. In the upper course the river has little power to cut through a resistant outcrop, so it winds around it and the spurs appear to interlock when you look down the valley. Potholes are rounded hollows bored into the river bed. Pebbles carried in turbulent water are spun round in a small dip in the rock, and abrasion deepens the hollow into a pothole; several potholes close together can widen and deepen a channel.

Landforms of deposition: meanders, ox-bow lakes, levees, floodplains and deltas

A meander is a bend in the river, and it grows because erosion and deposition work on opposite sides of the same bend. The water swings across the channel in a corkscrew movement, so the fastest flow is thrown against the outer bank. There, hydraulic action and abrasion undercut the bank to make a steep river cliff, and the bend becomes sharper. On the inner bank the water is slower and shallower, so material is deposited to form a gentle slip-off slope. Because the bend keeps eroding outwards and depositing inwards, the meander migrates across the floodplain.

An ox-bow lake is an old meander that has been cut off. Erosion on the outer banks of two adjacent bends brings the narrow neck of land between them closer together, and in a flood the river breaks straight across the neck because that is the shortest route. Deposition then seals the ends of the abandoned bend with sand and gravel, leaving a curved lake that slowly silts up and is colonised by plants.

Levees are natural embankments along the banks of a river in its lower course. When a river floods, it spreads out over the floodplain and its velocity falls immediately, so it drops its load. The coarsest material is deposited first, right beside the channel, and each flood adds another layer. Over many floods these layers build up the raised banks called levees, which make the channel sit above the level of the surrounding land. The floodplain itself is the wide, flat area of land on either side of the river, built up from the fine alluvium deposited by successive floods; it is flat because the sediment fills the hollows, and it is fertile because the silt is rich in nutrients.

A delta forms where a river enters a lake or the sea. The river's velocity falls sharply in the still water, so it deposits its load faster than waves and tides can remove it. The sediment builds out into the water, blocks the channel and forces the river to split into several distributaries, which deposit more material in turn. The shape of the delta depends on the balance between how much sediment the river brings and how much the waves and tides take away: where deposition wins the delta grows seawards, and where wave action is strong the sediment is spread along the coast instead.

Rivers as opportunities and hazards: the causes and impacts of flooding

Rivers offer a great deal. Their water is used for drinking, industry and irrigation, the floodplain silt makes some of the most fertile farmland in the world, and the flat land beside the river is easy to build on. Rivers carry goods and people, supply water for cooling in power stations and for hydroelectric power, and support fishing and tourism. Against that, the same rivers flood, erode their banks, and carry the waste that people put into them.

The natural causes of flooding are mostly about how quickly water reaches the channel. Heavy or prolonged rainfall, or a sudden storm, can put more water into the basin than the channel can carry. Snowmelt in spring releases a large volume of water at once. Steep slopes make water run off quickly instead of soaking in. Impermeable rock such as granite or clay, and soil that is already saturated from earlier rain, stop infiltration, so overland flow takes over. A small drainage basin with a dense network of tributaries delivers water to the main channel in a short time. Low-lying, flat land floods easily because there is nowhere for the water to drain, and a storm surge or high tide at the mouth can hold back the river and raise levels upstream.

The human causes are mostly changes people make to the basin. Urbanisation covers the ground with tarmac and concrete and adds drains that carry water straight to the river, so less water infiltrates and the flood peak arrives faster. Deforestation removes interception and the roots that hold soil open, so overland flow increases. Farming on steep slopes, ploughing up and down the slope and overgrazing compact the soil and leave it bare. Building on the floodplain puts property in the way of the water, and bridges, embankments and straightened channels can slow or speed flow in ways that raise levels elsewhere.

The impacts of flooding are social, economic and environmental, and they vary with the scale of the event. Socially, floods cause injuries and deaths, force people out of their homes, close schools and workplaces, spread water-borne disease and leave long-term stress and anxiety. Economically, they damage houses, businesses, farms and roads, interrupt transport and trade, push up insurance costs and cost a great deal to repair; the cost of flood defences is also an economic impact. Environmentally, floods erode soil, wash sewage, oil and chemicals into the river, damage habitats and spread pollution downstream. There can also be positive impacts: the silt left behind renews the fertility of farmland and recharges groundwater supplies.

Managing river flooding and river pollution

Hard engineering for flooding uses built structures to control the river. Dams and reservoirs store water and release it slowly, but they are very expensive, flood land upstream and trap the sediment that used to build the delta and the beaches downstream. Artificial levees and flood walls raise the banks so the channel can hold more water, but if they are overtopped or breached the flood is deeper and faster, and they cut the floodplain off from the river. Channel straightening and widening speed the water through a town but push the flood peak downstream faster. Flood relief channels, flood storage areas and pumping stations all reduce the level at a chosen point, and all cost money to build and maintain.

Soft engineering works with the river instead of fighting it, and this is what makes it more sustainable. Afforestation and planting along the banks increase interception and infiltration, so less water reaches the channel quickly. Land-use zoning and planning controls stop new building on the floodplain, and existing buildings can be made flood-resistant. Floodplain restoration and wetland creation give the river room to spread out and store water, and permeable paving and sustainable drainage systems let water soak away in towns. Flood warnings, evacuation plans and insurance help people to prepare. Soft schemes are usually cheaper to run, provide habitats and amenity, and benefit several groups at once, but they take years to take effect, need land that someone else wants to use, and may not be enough on their own to protect a town in a very large flood.

River pollution has mostly human causes. Industry discharges chemicals and hot cooling water; sewage works release untreated or partly treated waste when they are overloaded; farms spread fertiliser and slurry that is washed off the fields; abandoned mines drain metals and acid into streams; and towns contribute oil, litter and plastic from roads and drains. The impacts follow from the chemistry: fertiliser and sewage cause eutrophication, in which algae bloom, die and are decomposed by bacteria that use up the dissolved oxygen, so fish suffocate. Toxic metals and chemicals poison organisms directly, sediment smothers spawning grounds, and contaminated water cannot be used for drinking or irrigation, which raises the cost of treatment and can spread disease.

Managing pollution means stopping it at the source before treating it. Sewage is treated in stages, industrial discharges are licensed and monitored so that only agreed amounts may be released, and buffer strips of vegetation beside fields trap fertiliser and soil before they reach the water. Farms can reduce fertiliser use and store slurry in sealed tanks, mines can treat their drainage, and roads can be designed so that run-off passes through a settling pond. Restoring a river means more than cleaning the water: reoxygenating weirs, replanting banks, constructed wetlands and restocking fish all help, and monitoring keeps the pressure on. The judgement to make is that prevention is cheaper and more effective than treatment, but it needs regulation, investment and cooperation between everyone in the basin, including the countries upstream.