ES U4 Chapter 13.2 Stream Erosion and Deposition

Delta

A fan-shaped deposit that forms when a river flows into a quiet or large body of water, such as a lake, an ocean, or an inland sea.

Alluvial fan

A fan-shaped deposit which may form when a steep mountain stream meets dry, level land at the base of a mountain. When it reaches the land, the stream's velocity decreases greatly. As a result, the stream drops a large part of its sediment load.

Capacity

A measure of the total amount of sediment a stream can carry.

Competence

A measure that describes the maximum size of the particles a stream can carry.

When will a river deposit its load of sediment?

A river will deposit its load of sediment when either its velocity or its discharge decreases.

When may a river's discharge decrease?

A river's discharge may decrease when people divert water for irrigation of for city water supplies. Discharge may also decrease if the river passes through an arid region where it loses water by evaporation into the air and seepage into the ground. In humid regions, however, river's usually grow larger as they approach a sea and are fed by new tributaries.

When may a rivers velocity decrease for deposition to occur?

A river's velocity may decrease of its channel widens or the river meets an obstruction in the form of a curving bank or a rock outcrop. For example, when a river rounds a bend, it's velocity increases on the outside of the curve. Its velocity slows on the inside part of the curve, so sediments tend to be deposited there. However, the greatest decrease in a river's velocity occurs when it empties into a sea or a lake. At this point most of the river's remaining sediment is deposited.

Differences between a delta and an alluvial fan

Alluvial fans are formed on land, whereas deltas are formed in water. The sediments of an alluvial fan are coarse sands and gravels rather than fine silt and clay. Alluvial fans also have a sloping surface, not flat like that of a delta.

In what three ways is it possible for a river to transport, or carry, its load?

In solution, in suspension, and in its bedrock.

Suspension

One way that rock material is carried by a river. Suspended material includes clay, silt, and fine sand. Although these suspended materials are heavier than water, the turbulence of the stream flow stirs them up and keeps them from sinking. Turbulence includes swirls and eddies that form in water as a result of friction between the stream and its channel. The faster a stream flows, the more turbulent and muddy it becomes. A rough or irregular channel also increases turbulence.

Pothole

Oval or circular basins formed in the bedrock of a river when its water develops small whirlpools. As sand, pebbles, and small boulders swirl around in the whirlpools, they grind pebbles in the bedrock. The cutting tools that have formed a pothole are often found at its bottom.

Bed load

The bed load consists of sand, pebbles, and boulders that are too heavy to be carried in suspension. These heavier materials are moved along the streambed, especially during floods. Boulders and pebbles roll or slide along the river bed. Large sand grains are pushed along the bottom in a series of jumps and bounces.

What factors affect the competence and capacity of a stream?

The competence and capacity of a stream depend on the stream's velocity and discharge. Because the velocity and discharge of a given stream are not constant, the competence and capacity of a stream are not constant. Competence and capacity vary along a stream and change throughout the year.

Load

The eroded rock and soil materials transported downstream by a river.

Relationship between stream velocity and discharge and stream competence and capacity

The greater the velocity and discharge of a stream, the greater the streams competence and capacity.

Deposition

The process by which materials are deposited. Deposition occurs when a river or stream no longer has enough energy to transport materials.

How do deltas form?

Water velocity slows almost to a standstill when a river meets a large body of water. Because of the decrease in velocity, sediments are deposited. Over time, sediments build up, forming a delta.