Rivers and Flooding

River features from source to mouth

A river begins at its source, usually in upland areas where precipitation collects or a spring emerges. From here it flows downhill through three main courses. In the upper course the river is narrow, shallow and fast-flowing, cutting a steep V-shaped valley through vertical erosion. Interlocking spurs form as the river winds around areas of harder rock. Waterfalls develop where the river flows over a band of hard rock overlying softer rock; the softer rock is eroded more quickly, creating an overhang that eventually collapses, and the waterfall retreats upstream, leaving a gorge.

In the middle course the valley widens and the river begins to meander. Meanders form because the fastest flow of water, the thalweg, swings to the outside of each bend, eroding the outer bank by hydraulic action and abrasion to form a river cliff. On the inside of the bend, where the flow is slower, material is deposited, forming a slip-off slope. Over time, meanders migrate and grow larger.

If the neck of a meander becomes very narrow, the river may break through during a flood, taking the shorter, straighter route. The old meander loop is cut off and forms an oxbow lake, which gradually silts up. In the lower course the river is wide and deep, flowing across a broad, flat floodplain. When the river floods, it deposits sediment across the floodplain, building up fertile alluvium. Where the river meets the sea, it may form a delta if it deposits sediment faster than the tides can remove it.

Types of erosion

Rivers erode their channels in four main ways. Hydraulic action is the force of the water itself hitting the bed and banks, compressing air into cracks and breaking rock apart. Abrasion (also called corrasion) occurs when the river picks up stones and scrapes them along the bed and banks, wearing them away like sandpaper. Attrition is when rocks carried by the river collide with each other, becoming smaller and more rounded over time. Solution (also called corrosion) is a chemical process in which slightly acidic river water dissolves soluble rocks such as limestone and chalk.

Causes of flooding

Flooding occurs when a river's discharge exceeds the capacity of its channel. Physical causes include prolonged or intense rainfall, rapid snowmelt, saturated or frozen ground that cannot absorb water, and impermeable rock types such as clay or granite. Human causes include deforestation, which removes trees that would intercept and absorb rainfall; urbanisation, which increases impermeable surfaces like concrete and tarmac; and poor land management. Climate change is increasing the frequency and intensity of extreme rainfall events, making floods more likely.

Flood management

Hard engineering

Hard engineering involves building artificial structures to control flooding. Dams and reservoirs store excess water upstream and release it slowly. Embankments (levees) are raised banks built along the river to increase channel capacity. Channel straightening removes meanders to speed up flow and move water downstream more quickly. Flood relief channels divert excess water away from urban areas. These methods are effective but expensive, can look unnatural, and may simply move the flood risk further downstream.

Soft engineering

Soft engineering works with natural processes and is generally cheaper and more sustainable. Flood warnings give people time to prepare and evacuate. Floodplain zoning restricts building on areas most at risk. Afforestation means planting trees in the upper catchment to intercept rainfall and slow the flow of water into the river. Managed flooding allows certain areas to flood naturally, reducing the risk to settlements. River restoration returns straightened channels to their natural meandering course, slowing flow and creating wildlife habitats.