Waterfalls and Rapids
Occur when there is a sudden change in the gradient of a river.
- Rivers flow over bands of soft and hard rock.
- The softer rock erodes more quickly, leaving a plateau of the more resistant rock.
- This creates an undercut, leaving an overhang of the hard rock. Where the water falls down over this hard rock, it creates a plunge pool below. The force of the water landing in the plunge pool and the weaker rock in its load deepens the plunge pool.
- Eventually, the resistant rock collapses, falling into the plunge pool.
- The waterfall is moved upstream, leaving a gorge.
Cylindrical holes drilled into the bed of a river by water at high velocity with pebbles in its load. They occur in the upper/middle course.
- Vertical eddies in the water are strong enough that sediment grinds a hole in the rock.
- Can be from a few centimetres to a metre wide.
Braided channels are when the river is forced by islands of deposited sediment to split into several channels. They occur due to large levels of sand/gravel and a variable discharge. Braiding also occurs in semi arid areas that recieve rivers from mountainous areas and glacial streams with variable annual discharge.
- The channel becomes very wide in relation to its depth.
- The river becomes choked with sandbars.
Meanders are sinuous bends in a river.
- In low flow conditions, straight channels have alternating bars of sediment on their beds, forcing the water to move around them.
- This creates alternating shallow sections (riffles) and deeper areas (pools).
- The maximum velocity is therefore directed towards one of the banks, making an outer concave bank. It erodes, and the river's sinuosity increases.
- The cross section of a river is asymmetrical.
- The outer bank forms a river cliff/bluff with a deep pool close to the bank, which is undercut by erosion. The inner bank becomes a point bar.
- Meanders flow across to the concave outer band with a compensatory subsurface return - helicoidal flow.
- Eroded material on the outer bank is transported along and deposited on the inner.
A horseshoe lake separated from the river, created where there are meanders.
- The river is eroding laterally.
- The meander becomes narrower, due to high discharge -The river erodes the outside of the bends and deposits on the inside so its course is changing.
- This erosion narrows the neck of the meander
- To take the path of least energy usage, during a flood, the river cuts through the neck of the meander, creating the new channel.
- A horseshoe shape is left behind. New deposition seals off the ends and the cut-off becomes an ox-bow lake
- On the middle and lower courses, a river is at risk from flooding during times of high discharge.
- If it floods, the velocity falls, resulting in deposition and forming small raised banks along the sides of the channel.
- Subsequent floods increase the size of these banks.
Flood plains are created as a result of erosion and deposition. A floodplain is the area around a river that is covered in times of flood.
- They are relatively flat areas of land on either side of the river which form the valley floor in the middle and lower courses.
Deltas are a feature of deposition at the mouth of a river.
- They occur as fresh water mixes with sea water and clay particles coagulate. Clay then settles on the riverbed.
- Deltas only form when the rate of deposition exceeds that of sediment removal.
They are usually composed of:
- larger particles (the first to be deposited), which make the topset beds,
- medium grade particles (deposited as steep angled wedges), which make the foreset beds,
- the finest particles (which travel furthest), which form the bottomset beds.
Types of delta
- Arcuate - the land around the river mouth arches out into the sea and the river splits many times on the way to the sea, creating a fan effect. curving shoreline, dentritic (branched form resembling a tree) pattern of drainage
- Bird's foot - the river splits on the way to the sea, each part of the river juts out into the sea, rather like a bird's foot
- Cuspate - the land around the mouth of the river juts out arrow-like into the sea. Shaped by gentle opposing currents/longshore drift