Chapter 6.2 The Work of Streams

Erosion Streams are the most important way erosion takes place. They downcut or erode their channels, they transport sediments. Streams are able to erode channels by lifting lose particles by abrasion, grinding, and by dissolving soluble material. The stronger the current of a stream the quicker it erodes. Sand and gravel in a stream can cause erosion as it moves through the water. Erosion

Sediment Transport Streams transport sediment in 3 ways: In solution (dissolved load) In suspension (suspended load) Scooting or rolling along the bottom (bed load) Sediment Transport

Dissolved Load This will enter a stream through groundwater. It can also enter a stream by dissolving rock. The amount of materials dissolved depends on comate and geologic setting. The average amount of dissolved sediments is 115 to 120 ppm. Dissolved Load

This is the visible cloud of sediment suspended in water. Usually this is made of sand, silt, and clay, but during floods larger objects may transport this way because it usually speeds up the stream. Suspended Load

This is the part of the stream’s load of solid material that is made of sediment that is too large to be carried in suspension. This part only moves when the water is forceful enough to move large particles. The moving of large particles is very important in eroding the stream channel. Bed Load

Competence and Capacity Competence of a stream measures the largest particles it can transport. Competence increases with velocity. Capacity is the maximum load a stream can carry. Capacity is related to the discharge, the more water in a stream the greater the capacity. Competence and Capacity

Whenever a stream’s velocity decreases, it competence decreases and sediment starts falling out. Each particle size has a critical settling velocity. When a streamflow drops below the critical settling velocity for a certain particle size that sediment will settle out. Alluvium – the sorted material deposited by a stream. Deposition

A delta is an accumulation of sediment formed where a stream enters a lake or ocean. It occurs because the velocity of a lake or ocean decreases rapidly from the stream. As a result many channels become filled with sediment so the river will change direction, this may result in several smaller channels moving into an ocean or lake. Deltas

Natural levee is a landform that moves parallel to a stream Natural levee is a landform that moves parallel to a stream. This is where a stream has overflowed and left sediment on the bank and that sediment builds up and creates a border. Natural Levees

Stream Valleys Narrow Valleys A narrow V-shaped valley shows that the stream’s primary work has ben downcutting toward base level. These streams have rapids and waterfalls. Stream Valleys Narrow Valleys

Widening a stream happens after a narrow valley Widening a stream happens after a narrow valley. This creates erosion on the sides of the stream. This eventually produces a floodplain. This is where during a flood the river overflows its banks and floods the plain. Wide Valleys

Floods and Flood Control A flood is when the discharge of a stream becomes so great it exceeds the capacity of its channel and overflows its banks. Most floods are caused by rapid snow melt or storms that bring large amounts of rains. Flash floods usually occur with little warning and occur because of rainfall intensity and duration, surface conditions, and topography. Human interference with a stream system can worsen or cause floods. Floods and Flood Control

There are several ways to control flooding, the first is artificial levees. These are earthen mounds built up on the banks of rivers. This increases the volume (amount) of water a stream can hold. Problems with artificial levees are the stream bed rising, so it takes less water to overflow the levee. They are not usually built to withstand extreme flooding. Artificial Levees

These dams store floodwater and then let it out slowly. Some problems with building dams are they trap sediment, deltas and floodplains erode because they do not get silt to replenish them, and in some cases large dams may cause damage to the river environment. Flood-Control Dam

Many scientists think floodplains should be preserved in their natural state. That means not building in floodplains. Limiting Development

A drainage basin is the land area that contributes water to a stream A drainage basin is the land area that contributes water to a stream. A divide is an imaginary line that separates one drainage basin from another. The Mississippi River has the largest drainage basin in North America. Drainage Basins