Rivers: Lower Course Processes and Landforms

By the time a river reaches the lower stretches of its long profile – and gets closer to base level (usually sea level, though possibly a lake) – the channel can experience an increase in deposition. This can happen for a combination of reasons:

• A decrease in energy (for example, on the inside of a meander bend)
• An increase in capacity (total load)

Both of these situations will result in a decrease in competence (maximum size or weight of load transported), and so, to the deposition of sediment.

Floodplains

Land either side of the channel is susceptible to flooding. As a river exceeds bankfullcapacity, it floods beyond its channel onto the floodplain. Each time this happens, a layer of silt or alluvium is deposited and comes from two main sources of sediment:

• Overbank deposits (fine sands, silts and clays)
• Channel deposits from slip-off slopes (gravels and coarser deposits)

As a river spills over the side of the channel there is a sudden increase in wetted perimeter (the part of the channel in contact with water) and so a fall in hydraulic radius (hydraulic radius is a measure of channel efficiency and measures the relationship between wetted perimeter and cross-sectional area – the higher the hydraulic radius, the more efficient flow is as less energy is lost through friction). This means there is more friction and therefore a decrease in velocity. Sediment transported in suspension in the water is therefore deposited on the floodplain.

Levées

The area of floodplain immediately adjacent to the channel is often raised higher than the surrounding floodplain. It consists of coarser sediment and marks the zone of greatest velocity change. When a river spills over the banks onto the floodplain the sudden reduction in competence leads to the largest particles being deposited first. As the water spreads out over the floodplain sediment is deposited in order of size so that it is the fine silts and clays which are deposited furthest from the channel. This process is called sorting.

In times of prolonged precipitation, the floodplain may become saturated and subsequent flooding will result in water remaining on the surface for extended periods of time. Levées prevent water from returning to the main channel and this means that the floodwater stagnates on the floodplain. It either evaporates into the atmosphere or percolates into the floodplain below.Sediment held in suspension will settle out onto the floodplain creating rich alluvial soils.

In periods of low discharge deposition will occur in the main channel itself. This causes the river bed to be raised and increases the risk of flooding.

Deltas

Deltas are formed when a river enters a standing or low-energybody of water (a lake, lagoon or sea). Velocity drops partly because of the lack of tidal currents (which would transport sediment away), but also because of the shallower gradient at the mouth of a river. This leads to a reduction in energy, and therefore competence, and river material is deposited – often in the form of a delta.The depositional form a delta takes depends on the relative densities of river water and sea water:

• If densities of the two bodies of water are similar, they mix freely and consistent deposition occurs. (Homopycnal flow)
• If river water is less dense (if it has a relatively warmer temperature or is entering saline water) than the other body of water, a plume of river water extends out from the mouth above a wedge of cold/saline water.Coarse sediment occursclose to the mouth but deposition if finer suspended sediment spreads out further into the sea. (Hypopycnal flow)

• If river water is denser (if it is colder water or a heavier load), water may sink rapidly and cause offshore deposition that may produce a stunted delta. (Hyperpycnal flow)

Deposition is further determined by the process of flocculation. In fresh water clay particles repel each other and remain separate and too small to settle out. When fresh water meets sodium chloride in salt water, however, electrolytic action charges the clay particles and they cluster together to form larger particles (termed floccules), which then settle on the sea bed.

On deposition, sediment is sorted both vertically and laterally. The largest, coarsest material is deposited first in horizontal layers (termed topset beds), then medium-sized sediment (foreset beds), and finally the finest material furthest away from the land (bottom-set beds).

In order for a delta to form, certain conditions must be met:

• River discharge must be high enough to overcome high energy waves or high tidal ranges
• A largematerial load is needed – this is often provided by a rapidly downcutting river or a river which erodes through unconsolidated sediment such as boulder clay
• It is more favourable to have a gentle offshore gradient than a steeply shelving shoreline for deposition to occur

If there is a large amount of sediment being deposited close to the mouth it may, over time, inhibit the riverdirectly reaching the sea so it divides into numerous small channels called distributaries in order to do so.Distributary networks are very dense and rarely stable (deltas are a dynamic landform, meaning they change form frequently), branching out and joining together frequently. Protruding distributaries line their channels with sand while waves and currents move finer silts and clays into calmer water between the fingers of coarser deposits sometimes forming what is termed a birds-foot delta such as the Mississippi. Mud is often trapped and provides perfect conditions for plant colonisation and the development of marshes.