Lower Course Landforms (Estuaries & Deltas)

Estuaries

Definition and Characteristics: An estuary is the wide part of a river valley where the river meets the sea. It is a coastal area where freshwater from the river mixes with saltwater from the sea. Estuaries are often broad, funnel-shaped, and have tidal influences. They are typically found in the lower course of a river.

Tidal Influence and Saltwater Mixing: Estuaries experience tides, which cause the water level to rise and fall regularly. During high tide, seawater pushes upstream, mixing with the rivers freshwater. This creates a brackish water environment with varying salinity levels. The tidal action also affects the flow velocity and sediment movement within the estuary.

Examples in the UK: The Thames Estuary is one of the most well-known estuaries in the UK. It is located in southeast England and is a major shipping route into London. Other examples include the Humber Estuary on the east coast and the Severn Estuary in the southwest. These estuaries support important ecosystems and are often sites for ports and industry.

Estuaries are important for wildlife, providing habitats for fish, birds, and other species that thrive in the mix of fresh and saltwater.

For instance, the Thames Estuarys tidal range can reach up to $7\,\mathrm{m}$, causing significant movement of water and sediment.

Deltas

Formation Process: Deltas form at the mouth of a river where it flows into a sea or lake. As the river slows down, it loses energy and deposits the sediment it has carried from upstream. Over time, these deposits build up to create a delta, which is a landform made of sediment.

Sediment Deposition at River Mouths: The sediment deposited includes sand, silt, and clay. This sediment accumulates because the rivers velocity decreases sharply when it meets the standing water of the sea or lake, reducing the rivers capacity to carry sediment. Finer sediments like silt and clay settle in calmer water because they remain suspended longer and settle when velocity decreases.

Types of Deltas:

• Arcuate Delta: Shaped like a fan or bow, with smooth, curved edges. Example: The Nile Delta.
• Birds Foot Delta: Has long, finger-like projections extending into the sea. Example: The Mississippi Delta.
• Cuspate Delta: Pointed or tooth-like shape, formed by the action of waves and tides shaping the sediment.

Deltas are rich in nutrients and support fertile farmland and diverse ecosystems. They are also often densely populated due to their flat land and access to water.

For example, the Mississippi River deposits sediment to form a birds foot delta, with distributaries extending into the Gulf of Mexico.

Processes Shaping Lower Course Landforms

Deposition Dominant in Lower Course: In the lower course of a river, the gradient is gentle and the rivers velocity decreases. This causes the river to deposit much of its sediment load, shaping landforms such as estuaries and deltas.

Role of River Velocity and Sediment Load: The velocity of the river reduces because the channel widens and the river meets tidal influences. A slower river cannot carry as much sediment, so deposition occurs. The sediment load includes fine particles like silt and clay, which settle in calm water.

Interaction with Tides and Sea Level: Tides influence the movement of water and sediment in estuaries. High tides push saltwater upstream, while low tides allow river water to flow out to sea. Sea level changes can also affect the shape and size of estuaries and deltas over time.

For example, in the Severn Estuary, tidal bores occur where the incoming tide forms a wave that travels upstream, affecting sediment movement and deposition patterns.

The balance between river flow and tidal action determines how sediment is distributed, influencing the development of mudflats, salt marshes, and deltaic landforms.

For instance, if a river carries $5000\,\mathrm{tonnes}$ of sediment per day but its velocity halves as it enters the estuary, much of this sediment will settle out, building up the estuary floor.

If a river slows from $6\,\mathrm{m\,s^{-1}}$ to $2\,\mathrm{m\,s^{-1}}$, sediment deposition increases because the rivers capacity to carry sediment decreases.