At high elevations, streams are just beginning streams that have small channels and steep gradients. This means that the stream will have a high velocity and will do lots of work eroding its stream bed. The higher the elevation, the farther the stream is from where it eventually meets the sea.
Base level is the term for where a stream meets sea level or standing water, like a lake or the ocean. Streams will work to downcut their stream beds until they reach base level. As a stream moves out of high mountainous areas into lower areas closer to sea level, the stream is closer to its base level and does more work eroding the edges of its banks than downcutting into its stream bed.
At some point in most streams, there are curves or bends in the stream channel called meanders Figure The stream erodes material along its outer banks and deposits material along the inside curves of a meander as it flows to the ocean Figure This causes these meanders to migrate laterally over time.
Once a stream nears the ocean, it is very close to its base level and now deposits more materials than it erodes. As you just learned, one place where a river deposits material is along the inside edges of meanders. If you ever decide to pan for gold or look for artifacts from an older town or civilization, you will sift through these deposits.
Gold is one of the densest elements on Earth. Streams are lazy and never want to carry more materials than absolutely necessary. It will drop off the heaviest and largest particles first, that is why you might find gold in a stream deposit. Imagine that you had to carry all that you would need for a week as you walked many kilometers. At first you might not mind the weight of what you are carrying at all, but as you get tired, you will look to drop off the heaviest things you are carrying first!
When a river floods or overflows its channel, the area where the stream flows is suddenly much broader and shallower than it was when it was in its channel. The farmers who use the floodplain areas around the Nile River rely on these deposits to supply nutrients to their fields each year as the river floods its banks.
At flood stage, a river will also build natural levees as the largest size particles build a higher area around the edges of the stream channel Figure When a river meets either standing water or nearly flat lying ground, it will deposit its load.
If this happens in water, a river may form a delta. From its headwaters in the mountains, along a journey of many kilometers, rivers carry the eroded materials that form their stream load.
Suddenly the river slows down tremendously in velocity, and drops the tremendous load of sediments it has been carrying. Deltas are relatively flat topped, often triangular shaped deposits of sediments that form where a large river meets the ocean. The name delta comes from the capital Greek letter delta, which is a triangle, even though not all deltas have this shape.
A triangular shaped delta forms as the main stream channel splits into many smaller distributaries. As the channel shifts back and forth dropping off sediments and moving to a new channel location a wide triangular deposit forms. There are three types of beds that make up a delta Figure The first particles to be dropped off are the coarsest sediments and these form sloped layers called foreset beds that make up the front edge of the delta.
Further out into calmer water, lighter, more fine grained sediments form thin, horizontal layers. These are called bottomset beds.
During floodstage, the whole delta can be covered by finer sediments which will overlie the existing delta. These are called topset beds. These form last and lie on top of the rest of the delta. There are three distinct stages: the upper, middle, and lower course. The upper course, commonly known as the source, has a small width, is shallow, and has few small tributaries.
In the middle phase, the river morphology changes in width and depth. As the river gathers momentum downstream towards the lower course phase, the width reaches its widest point.
The following factors contribute to the widening of a river downstream. The volume of water increases downstream due to increasing inlets from tributaries and rainwater. The flow downstream is characteristically turbulent, chaotic, and erratic.
The larger discharge requires a bigger space due to the added volume of water. To contain the water, the river enlarges along the banks, leading to an enlarged width. As a river flows downstream, its velocity increases. The speed increases due to the fact that more water is added from tributaries along the course of the river. Additionally, less water is in contact with the river bed, which results in less energy needed overcome friction.
The larger mass of water causes wider and deeper water channels in order to allow water in the river to flow more freely. The slope of a river decreases as it flows progressively downstream. Gravitational force is stronger uphill, but gentle in the lower course. The processes of river erosion operate here.
Hydraulic action - where the sheer force of the water erodes the stones, bed and banks of the river. Corrasion - where stone sin transport are thrown into the bed and the banks eroding them. Corrosion - where weak acids within the water react with the rocks and bed and bank of the river. Attrition - where stones in transport are thrown into one another. The diagram on below shows the major changes downstream. In the source area the drainage basin an area of land drained by a river and its tributaries contains V shaped valleys and waterfalls, and the dominant processes are erosion.
Erosion tends to be vertical straight down into the land. In the middle section of the drainage basin the river starts to erode laterally. This section contains meanders and Ox bow lakes, and the river creates a flood plain often with Levees.
Here, Material is deposited and erosion can also occur. In the lower drainage basin deposition dominates as a river enters a sea or lake, the valley is at its widest and deltas and estuaries are major landforms and habitats.
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