STREAM EROSION AND DEPOSITION

EROSION AND DEPOSITION

These two agents: erosion and deposition are the most important agents that affect weathered materials.

Erosion involves the physical removal of weathered material from its source.

Weathered material is called sediment and can be either solid particles (varying in size from clay to boulder size particles) or dissolved materials (e.g. salt)

GATHERING

There are three general methods by which streams can acquire weathered products.

Hydraulic action is the action of running water on stream beds and banks. The flow of water provides enough energy to dislodge sediment particles from the bed and banks and transport them. This is the dominant method of erosion along stream banks lined with soil.

Abrasion occurs when exposed rock along the stream bed and/or bank is worn down by the impact of solid particles within running water.

Dissolution is the final process of erosion. Flowing water dissolves ions from either sediments or rock along the stream bed and banks.

TRANSPORTATION

Once the weathered products are within the stream, they are transported downstream by one of three ways.

Running water contains a dissolved load- ions dissolved by chemical weathering.

Solid particles can be transported either by suspended load or bed load

The suspended load includes small particles like clay and silt that are transported within the water column because of their low density relative to water. An example would be muddy water in stream that you have seen, which is the transportation of clay or silts by suspension.

Bed load consists of coarser particles such as sand and gravel. These larger particles cannot remain in suspension because of their high density relative to water. As a result, they experience erratic transport along the stream bed, with much bouncing, skipping (saltation), rolling and sliding.

Competence is the maximum size of particles that can be transported by a stream. A faster flowing stream has more energy than a slower moving stream and therefore has a higher competence.

Capacity is the total sediment load a stream can carry. Capacity is affected by discharge, not velocity. The larger the discharge of a river, the more sediment it can carry and the greater its capacity.

Deposition of suspended load and bed load can occur quite far from the source and in various environments, from the headwaters all the way to the mouth.

At the headwater little deposition of sediment takes place due to the high velocity of the water

Suspended load or bed load deposition occurs when a stream’s velocity slows and competence decreases. This generally occurs closer to the mouth of the stream.

The dissolved load will be deposited into the larger body of water into which the stream flows, e.g. ocean.

STREAM DEPOSITION

Stream velocity tends to be higher in the upper center of a stream. As a result, deposition tends to occur along the banks or bed of a stream. This type of deposit is termed a bar, and occurs along the length of a stream.

Deposits termed alluvial fans and deltas tend to form near the mouth of a stream

Braided streams are characterized by an intricate network of dividing and re-joining channels.

Meandering streams are characterized by a single sinuous channel with curves called meanders.

Meandering streams have semicircular channels.

The outer bank is referred to as a cut bank

The inner bank is characterized by a deposition of sediment known as a point bar.

With flooding these types of streams can create Oxbow lakes.

Deltas are formed when a stream flows into another body of water.

The velocity of the stream decreases rapidly and the capacity of the stream decreases.

Deposition of the sediment load occurs and the shoreline builds up.

Alluvial fans are sediment deposits on lad. They generally form on lowlands adjacent to

highlands where fast-flowing streams carry material from the highlands to the lowland.

BASE LEVEL

Streams can only erode to their base level,. Base level is the elevation at which water is

horizontal (level) and will no longer flow. Streams have temporary base levels, like lakes,

waterfalls, and dams.