Fluvial Processes and Landforms

Learning Objectives

Describe the three types of sediment load carried by a river.

Explain how river velocity affects erosion and deposition.

Identify landforms created by river erosion (e.g., V-shaped valleys, meanders) and deposition (e.g., deltas, alluvial fans).

Rivers Shaping the Land

Fluvial processes are those associated with rivers and streams and the deposits and landforms created by them. Water is the most important agent of erosion on Earth.

How Rivers Carry Sediment

Rivers transport weathered material, known as sediment load, in three ways:

1.Dissolved Load: Soluble minerals are dissolved in the water and carried in solution.

2.Suspended Load: Fine-grained particles (silt and clay) are held up and carried along by the turbulence of the water flow. This is what makes most rivers look muddy.

3.Bed Load: Coarser, heavier particles (sand and gravel) are rolled, bounced, or dragged along the bottom of the riverbed.

Erosion and Deposition

The ability of a river to erode or deposit sediment is primarily controlled by its velocity.

High Velocity (fast-moving water): Has high energy and can carry a larger sediment load. This leads to erosion. This is typical of a river's upper course in steep, mountainous terrain.

Low Velocity (slow-moving water): Has low energy and cannot carry as much sediment. This leads to deposition. This is typical of a river's lower course as it approaches a lake or ocean.

Fluvial Landforms

Erosional Landforms:

V-shaped Valley: In their upper course, fast-flowing rivers cut downward into the landscape, creating steep-sided, V-shaped valleys.

Meanders: As a river reaches flatter ground, it begins to flow in broad, looping bends called meanders. The water flows fastest on the outside of the bend, causing erosion (a cut bank), and slowest on the inside of the bend, causing deposition (a point bar).

Oxbow Lake: A U-shaped body of water that forms when a wide meander from a river is cut off, creating a freestanding body of water.

Depositional Landforms:

Delta: A fan-shaped landform created by the deposition of sediment as a river flows into a slower-moving or standing body of water, such as a lake or an ocean.

Alluvial Fan: A fan-shaped deposit of sediment formed where a fast-flowing stream emerges from a narrow canyon onto a flat plain, causing it to slow down and drop its sediment.

Key Terms

Fluvial: Of or found in a river.
Sediment Load: The total amount of sediment being transported by a river, including dissolved, suspended, and bed loads.
Meander: One of a series of regular sinuous curves, bends, loops, turns, or windings in the channel of a river, stream, or other watercourse.
Delta: A landform created by deposition of sediment that is carried by a river as the flow leaves its mouth and enters slower-moving or standing water.
V-shaped Valley: A valley with a V-shaped cross-section, formed by the downcutting erosion of a river in its upper course.