Landforms

Geography: KS3, topic 8.1.6

River Landforms Caused by Erosion

When a river flows over hard rock that overlays softer rock, a waterfall can form.
The less resistant rock begins to erode by abrasion and hydraulic action.
A step or notch forms in the softer rock, creating a river rapid.
Over time, the softer rock is eroded more and more, whereas the harder rock is left sticking out.
The height of the drop increases as the water pounds the less resistant rock at the base. Eventually a waterfall forms.

The less resistant rock starts to be eroded underneath the hard rock. This process is called undercutting.
The overhanging harder rock collapses because it's unsupported.
The waterfall now has harder rock to swirl around its base. The resulting abrasion creates a plunge pool.
Hydraulic action keeps undercutting behind the waterfall.
If the process repeats over a long time, a gorge may form.

In the upper courses of rivers, valleys are steep and V-shaped because most of the erosion that happens is in a vertical direction.
Because the rivers don't have enough energy to erode sideways, they are restricted to winding around the hills on the valley sides.
These interlocking hillsides are the interlocking spurs.

Erosion and deposition in parts of rivers that have deep and shallow sections lead to meanders forming.
On the outside of bends, the deep channel means the current is fast.
River cliffs form here because the high energy water has more erosional power.
On the inside of bends, the shallow channel means the current is slow.
Slip-off slopes form here because sediment is deposited by the low-velocity water.
Meanders are usually found in the middle course of a river.

Over time, meanders become larger, until eventually an ox-bow lake forms.
Erosion causes the bends to move closer together to create a swan’s neck meander.
During a flood, high energy water will breach the meander neck.
This makes the river flow in a straighter and faster course.
Over time, deposits will fill in the meander bends.
The meander becomes cut off from the river, forming an ox-bow lake.