Chapter 12 Rivers .
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Meanders
What is it?
It is formed when the moving water in a river erodes the outer banks and widens its valley. A stream of any volume may assume a meandering course, alternatively eroding sediments from the outside of a bend and depositing them on the inside. The result is a snaking pattern as the stream meanders back and forth across its down-valley axis.
River Cliffs & Slip-off Slopes
River Cliff:
A river cliff or undercut slope is a steep slope produced by lateral erosion by a riveron its concave bank or outer bend. The base of the slope on the outer bend is undercut as the river meanders in the middle course and lateral erosion becomes more effective. The top part of the slope loses support as a result of basal undercutting, and collapses by the process of mass wasting. A river cliff is producedon the undercut bank or concave bank. The slope gradually retreats or recedes, together with the river cliff. a series of river cliffs is known as a river bluff. The river meander becomes more and more pronounced an gradually shifts or migrates downstream towards the lower course.
Slipoff Slope:
Cut-offs & ox-bow lakes
What is it?
1. An oxbow lake is a detached meander of a cut-off formed when the narrow neck separating the two ends of a meander is broken through by
(i)active lateral erosion at the outer bends(or, concave banks) or
(ii)flood water.
(i)active lateral erosion at the outer bends(or, concave banks) or
(ii)flood water.
The river in the lower course meanders widely across the low-lying plain. Lateral erosion and undercutting occur on the concave bank where the stream flows the fastest.
Deposition occurs on the convex bank because of the slack water. Continuous erosion of a concave bank and deposition on the convex bank of a meandering river cause the formation of a very pronounced meander with two concave banks getting closer.
The narrow neck of land between the two neighbouring concave banks is finally cut through by either lateral erosion of the two concave banks or the strong currents during a flood.
When this happens, a new straighter river channel is created and an abandoned loop, called a cut-off, is formed.
Waterfalls and Plunge pools
Waterfalls
How it is formed:
1. A waterfall forms when a river flows from a band of resistant rock to less resistant rock. The band of resistant rock can lie horizontally, vertically, or it can dip downstream. The less resistant rock that lies downstream will be eroded faster than the resistant rock.The defferential erosion causes a change in the gradient of the river.
This will result, over time, in the sudden steepening of the river bed from the resistant rock to the less resistant rock so that the water plunges down almost vertically and rapidly as a waterfall.
2. A waterfall can also be formed where faulting causes land to be displaced, with one block being uplifted relative to the adjacent block. This displacement causes a difference in height, and the water cascades down the fault scarp.
Plunge pools
A plunge pool is a pool, lake, or pond that is small in diameter, but deep.
It is also a depression at the foot of a waterfall. The depression is excavated, enlarged and deepened by the hydraulic action of the plunging water as it lands on the bottom river bed with great force. Rock debris, swirled about by the turbulent water at the base of the waterfall, erodes the depression intensively by abrasion to further enlarge and deepen it.
If you cannot see the video , please refer to this link http://www.youtube.com/v/Nhi4cf9yCPs .
Floodplains and levees.
What is it?
A flood plain is a wide and low-lying plain found on both sides of a river.
- Floodplains.
-Levees.
How it is formed:
When there is heavy rain and the amount of water in the river channel is more than the amount that can be contained, the water overflows the banks and floods the surrounding areas.
Once out of the channel, the water encounters more friction because of a larger wetted perimeter formed by the large flat low land. The increase in friction reduces the energy of the water and slows down its speed. This results in deposition.
The larger and coarser materials are deposited first because they are heavier. They are deposited at the river banks. The smaller and finer materials are carried further away because they are lighter and deposited further away from the river. The coarser materials at the river banks form raised embankments called leeves while the finer materials form the flood plain.
-Video on the formation of floodplains and levees!
Valley
Upper Course
V-Shaped Valleys
In the upper course of a river, water flows quickly through a narrow channel with a steep gradient; as it does so it cuts downwards. This vertical erosion results in a number of distinctive landforms including the steep sloping v-shaped valley through which the river flows in its upper course.
How a v-shaped valley is formed:
- Vertical erosion (in the form of abrasion, hydraulic action and solution) in the river channel results in the formation of a steep sided valley
- Over time the sides of this valley are weakened by weathering processes and continued vertical erosion at the base of the valley
- Gradually mass movement of materials occurs down the valley sides, gradually creating the distinctive v-shape.
- This material is then gradually transported away by the river when there is enough energy to do so.
As the river flows through the valley it is forced to swing from side to side around more resistant rock outcrops (spurs). As there is little energy for lateral erosion, the river continues to cut down vertically flowing between spurs of higher land creating interlocking spurs.
Middle Course
Characteristics:
- Lateral erosion widens the valley profile.
- River bends become more pronounced producing meanders.
- A flood plain begins to form on the valley floor.
- The lateral erosion of the river truncates the interlocking spurs. The floor of the valley widens as the river on concave banks deposits material.
Common landforms here are river beaches and river cliffs.
Lower Course
The lower course has the gentlest slopes - both in long profile and across the valley floor. This almost flat land is known as the flood plain.
The river may have very large meanders and ox-bow lakes. The mouth of a river is when it reaches open water - either a lake or the sea. Under certain conditions a delta can be found here.
Rapids
What is it?
Rapids are a series of very short and fast-flowing turbulent falls of water along a part of river. The water is relatively shallow.
Rapids are formed due to a sudden steepening of the stream gradient, but without a sufficient break in slope to form a waterfall, or from the river flowing over a series of thin layers of hard and soft rock.
The 3 main arrangements of the resistant rocks which may give rise to a rapid are:
1. Horizontal resistant strata or sills.
2. Resistant strata or sills gently dipping upstream.
3. Vertical resistant strata dykes.
Slave River - Rapids