Formation of meandersWatch
Reading the markscheme for the new specimen paper, it states that the answer needs to relate to the development of riffles and pools, and this is the part that is confusing me most.
If someone could help me out here with a model answer I would be very grateful. I think I get the general jist of how they are formed, its just I'm finding it difficult to put it into into a succint sequence that is easy to revise.
It will be a 15 marker so I guess a good amount of detail is need.
Go to question 1b. There's a good but blurry diagram which might help.
in the new textbook look on pages 19, 21-22.
1. Meanders form when areas of alternating pools (deep water) and riffles(shallow water) develop at equally spaced intervals along a stretch of river. The distance between pools is usually 5-6 times the width of the river bed.
2. Because water is deeper in pools, the river is more efficient when passing over them. The energy and erosive power is therefore increased when passing over these areas. On the other hand, the river is less efficient when passing over riffles as there is more friction causing the river to lose energy.
3. This combination of the river gaining and losing efficiency at different intervals causes the river’s flow to become uneven, and maximum flow is concentrated on one side of the river.
4. As the water speeds up, turbulence increases in and around pools. This leads to corkscrew-like currents in the river called helicoidal flow. These helicoidal currents spiral from bank to bank, causing more lateral erosion (abrasion and hydraulic action) and deepening of the pools – river cliff. This leads to the increased amount of eroded material being deposited on the inside of the next bend where the river loses energy – slip off slope.
5. Combination of erosion and deposition exaggerates bend until large meanders are formed. These combined processes also give the meander’s asymmetric cross-section.
-In a river there are alternationg POOLS and RIFFLES
- POOLS are DEEPER stretches of SLOWER moving water
- Riffles are SHALLOWER, but FASTER sections of water
- The POOLS eventually MIGRATE to OPPOSITE SIDES of the channel
-The RIFFLES are found in the STRAIGHTER sections of the channel
- This causes the river to become SINUOUS (twist from side to side- like a snake)
-On the OUTER BEND of the MEANDER the current is flowing fast causing HYDRAULIC ACTION, CORRASION AND CORROSION to ERODE the OUTER BEND
-This produces a RIVER CLIFF
- This eroded material is transported by a HELICOIDAL (corkscrew) movement
-It is DEPOSITED on the INNER BEND
- This creates a POINT BAR (river beach)
-The meanders MIGRATE DOWNSTREAM and WIDENS the valled by LATERAL EROSION (sideways erosion as apposed to vertical erosion)
Thar we go. xD
1 meanders are spinous bends in the river which form as a result of riffle and pools.
2 even in a straight line channel, the river will deposit load at times of low flow. These areas are called riffles and between them are depressions called pools.
3 once riffles form, friction increases, hydraulic radius decreases and flow is inefficient. The river therefore tries to flow around the riffles.
4 this causes the thalweg (line of maximum river depth and velocity) to be propelled towards the outer bank.
5 energy is greatest at the outer bank so is undercut. This creates a concave shape or a river cliff.
6 on the inside, energy is low so deposition builds up creating a convex shape- a point bar.
7 once they've been created, helicoidal flow transports material from river cliff to point bar. Helicoidal flow is a cork screw like action.
8 as erosion continues, migration occurs both laterally and downstream. However, the river doesn't get any wider- it's sinuosity is increased. (Bendiness)
Don't know if the question wants you to continue and talk about oxbow lakes that are formed from meanders...
9 in addition a swan neck meander is formed from continuous deposition on the convex inner bank and erosion of the outer bank.
10 the neck of the land between the 2 concave banks is cut through by lateral erosion or strong currents of a flood
11 when this happens a new straighter river channel forms and an abandoned meander loop called a cut off is created.
12 eventually, ongoing deposition of the channel on the edges seals off the cut off from the river channel creating an ox bow lake. Over time, it will become vegetated and form a scar.
Hope this helps, my teacher said you can always draw diagrams for 15 mark questions! And example is also quite good to include if you know one but I don't sorry!!