Unit 5: River System (Fluvial Geomorphology)

This unit focuses on understanding the features and processes of river systems, which are a key part of interpreting landscapes in the field.

Meandering, Bank Erosion, and Ox-bow Lake Formation

Meandering River

A meandering river is a river with a single, sinuous (winding) channel. These are common in low-gradient (flat) areas. The key to meandering is the flow dynamics within the channel.

• Thalweg: The path of deepest and fastest-flowing water in the river. In a meander, the thalweg swings to the outside of the bend.
• Bank Erosion (Cut Bank): The fast-flowing water (thalweg) on the outer bank of a meander causes active erosion, forming a steep, unstable bank called a cut bank.
• Deposition (Point Bar): The slower-moving water on the inner bank of a meander does not have enough energy to carry sediment, so it deposits sand and gravel, forming a gently sloping feature called a uploads (point bar).

This combined process of erosion on the outside and deposition on the inside causes the meander to migrate (move) and become more sinuous over time.

Ox-bow Lake Formation

An ox-bow lake is a U-shaped lake that is formed when a wide meander of a river is cut off.

The Process:

1. High Sinuosity: A meander becomes very curved, and the two cut-banks (the "neck" of the meander) get closer and closer together.
2. Cut-off: During a flood, the river has high energy and takes the shortest path, breaking through the narrow neck. This is a neck cut-off.
3. Abandonment: The river now flows along this new, straighter, high-energy path. Sediment (silt and clay) is deposited at the "entrances" to the old, abandoned loop.
4. Ox-bow Lake: This sediment eventually plugs the old loop, separating it from the river and forming an independent, crescent-shaped lake.

Floodplains

A floodplain is the flat, low-lying land on either side of a river channel that is inundated (covered) by water during a flood.

Floodplain Features:

• Formation: Floodplains are built up over thousands of years by the vertical accretion (build-up) of fine-grained sediment (silt and clay) that is deposited as muddy floodwaters spread out and slow down.
• Natural Levees: These are low, natural ridges of coarser sediment (sand) built up right next to the river banks. When the river floods, it dumps its heaviest sediment first (right at the bank), forming these levees.
• Backswamps: The low-lying, poorly-drained areas of the floodplain behind the natural levees. This is where the finest clay settles out.

Paleo-channel

A paleo-channel (or "fossil channel") is the remnant of an old, abandoned river channel. It represents the former path of a river.

Identifying Paleo-channels:

These are key features to look for in the field or on satellite images/topographic maps.

• Topography: They often appear as long, shallow, winding depressions in the landscape.
• Soil/Vegetation: The sediment inside the old channel (often sand and gravel) is different from the surrounding floodplain (silt and clay). This leads to different soil and different vegetation patterns (e.g., a line of trees growing in an otherwise grassy field).
• Examples: An ox-bow lake is a type of paleo-channel. Over time, it will fill with sediment and become an ox-bow scar.

Identifying paleo-channels is important for finding groundwater resources and understanding how a river's course has changed over time (river avulsion).

Drainage Pattern

A drainage pattern is the geometric arrangement of a river and its tributaries (the smaller streams that feed it) as seen from above. The pattern is a direct reflection of the underlying geology.

Common Drainage Patterns: