Meanders are formed by erosion and deposition
In the middle and lower courses, rivers develop large bends called meanders.
1. The current is faster on the outside of the bend because the river channel is deeper (there's less friction to slow the water down).
2. So more erosion takes place on the outside of the bend, forming river cliffs.
3. The current is slower on the inside of the bend because the river channel is shallower (there's more friction to slow the water down).
4. So eroded material is deposited on the inside of the bend, forming slip-off slopes.
Ox-Bow Lakes are formed from Meanders
Meanders get larger over time - they can eventually turn into an ox-bow lake:
Erosion causes the outside bends to get closer until there is only a small bit of land left between the bends (called the neck).
The river breaks through this land, usually during a flood and the river flows along the shortest course.
Deposition eventually cuts off the meander forming an ox-bow lake.
Waterfalls and Gorges are formed by Erosion
Waterfalls form where a river flows over an area of hard rock followed by an area of softer rock.
The softer rock is eroded more than the hard rock, creating a "step" in the river.
As water goes over the step it erodes more and more of the softer rock.
A steep drop is eventually created, which is called a waterfall.
The hard rock is eventually undercut by erosion. It becomes unsupported and collapses.
The collapsed rocks are swirled around at the foot of the waterfall where they erode the softer rock by abrasion. This creates a deep plunge pool.
Over time, more undercutting causes more collapses. The waterfall will retreat (move back up the channel), leaving behind a steep-sided gorge.
Flood Plains are formed by Deposition
The flood plain...
This is the wide valley floor on either side of a river which occassionally gets flooded.
When a river floods onto the flood plain, the water slows down and deposits the eroded material that it's transporting. This builds up the flood plain (makes it higher).
Meanders migrate (move) across the flood plain, making it wider.
The deposition that happens on the slip-off slopes of meanders also builds up the flood plain.
Levees are formed by Deposition
These are natural embankments (raised bits) along the edges of a river channel.
During a flood, eroded material is deposited over the whole flood plain.
The heaviest material is deposited closest to the river channel, because it gets dropped first when the river slows down.
Over time, the deposited material builds up, creating levees along the edges of the channel, e.g. along the Yellow River in China.