River channel landforms
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River channel landforms
In the upper part of a river, vertical erosion is common creating a steep sided river valley. Interlocking spurs form as the river has to take a winding course due to the highland areas that jut out.
The rock a river flows over is not uniform and waterfall and result after a river has flowed over hard rock and meets a band of soft rock. The velocity increases as the water nears the edge of the fall, because of a decrease in friction (the water is no longer in contact with the river bed).
The soft rock is worn away and over time, the hard rock is undercut and may collapse. Plunge pools are common at the base of waterfalls.
Have a look at the diagram below:
If the gradient of the river increases quickly or flows over gently dipping areas of harder rock rapids will result.
At its most basic level this is land more likely to experience flooding. If a river floods, silt is deposited on the land increasing its fertility, and the height of the flood plain increases. The flood plain can be made wider by the lateral erosion of meanders. The edge of a flood plain is quite often clearly marked by a clear slope known as a bluff line.
If a river floods, material is deposited on the banks, the material that is dropped first is the coarsest, and creates a natural embankment. When the amount of water in the river is low deposition takes place, the bed of the river rises and flooding is again likely. In some countries, artificial levees are put in place to reduce the flood risk.
At some times of the year, due to snowmelt, rivers may carry a very highload in comparison to their velocity. The channel can become full of sediment.
These are made of fine sediment and occur when a river has a decrease in energy as it enters a lake or the sea.
Pools, riffles, and meanders
These features occur due to both erosion and deposition. Pools are areas of deeper water and riffles are areas of shallower water. A pool has more erosion than a riffle, and is caused by turbulence.
The reasons for the development of meanders remain unclear, but they appear to develop first in times of flood and are related to the occurrence of sandbars. A river is meandering when its sinuosity is above 1.5.
Sinuosity = actual channel length / straight-line distance.
The development of a meander through time is shown below:
Point Bars and Ox-Bow Lakes
Where material may be deposited on the inside convex bend of a river. Material that is largest is found on the upstream side of the river. If a river meander becomes increasingly tight over time, leading to a narrow neck, in times of flood the neck is broken through. Eventually this is the route the river takes leaving the old meander separate to the river as an Ox-Bow lake.
The main features are shown in the diagram below: