Lower Course Landforms (Floodplains & Levees)

Floodplains

Definition and formation: A floodplain is a wide, flat area of land alongside a river, found mainly in the lower course. It is formed by the river depositing fine sediment (silt and clay) during floods when the river overflows its banks. Over time, repeated flooding builds up layers of fertile soil.

Location in the lower course: Floodplains are typically found in the lower course of a river where the gradient is gentle, and the river channel is wide and deep. The river flows more slowly here, allowing sediment to settle and be deposited.

Role in flooding: Floodplains act as natural storage areas for floodwater. When the rivers discharge is high, water spills over the banks and spreads across the floodplain. This reduces the speed and volume of water downstream, lowering flood risk temporarily.

Soil fertility: The sediment deposited on floodplains is rich in nutrients, making the soil very fertile. This makes floodplains ideal for agriculture, especially for growing crops like wheat and vegetables.

For instance, the floodplain of the River Severn supports rich farmland due to regular deposition of nutrient-rich silt during floods.

Floodplains also provide important habitats for wildlife, supporting diverse plants and animals that thrive in these wetland environments.

Levees

Natural embankments: Levees are raised banks found alongside river channels, especially in the lower course. They are natural embankments made of sediment deposited during floods.

Formation process: During a flood, the river overflows its banks and water slows down as it spreads onto the floodplain. Heavier, coarser sediment (sand and gravel) is deposited closest to the river channel, building up the levees. Finer sediment travels further onto the floodplain.

Function in flood events: Levees act as natural barriers that help contain the river within its channel during normal flows and minor floods. They reduce the chance of flooding by raising the riverbanks.

Types: natural vs artificial: Natural levees form naturally over time by sediment deposition. Artificial levees (or embankments) are man-made structures built to increase flood protection, but these are part of flood management and not covered here.

For example, natural levees are common along the lower River Thames, where sediment has built up over many years.

Processes Shaping Lower Course Landforms

Deposition of sediment: In the lower course, the rivers velocity decreases due to a gentler gradient and wider channel. This causes sediment carried from upstream to be deposited, especially during floods when the river overflows its banks.

Overbank flooding: When the rivers discharge exceeds the channel capacity, water spills onto the floodplain. This is called overbank flooding. It deposits layers of sediment, gradually building up the floodplain and levees.

River channel changes: The river channel in the lower course is wide and deep, with a gentle gradient. Sediment deposition can cause the channel to shift position slightly over time, creating features like meanders (covered in other notes). The channel may also become shallower as sediment builds up.

Sediment size and sorting: Sediment deposited in the lower course is usually fine (silt and clay) because larger particles have been deposited earlier upstream. Sorting occurs because heavier particles settle first near the riverbanks (forming levees), while finer particles travel further onto the floodplain.

For example, after a flood on the River Trent, fine silt is deposited across the floodplain, while sand and gravel form natural levees close to the channel.