A thorough insight into erosional and transport processes and river landforms including: waterfalls, potholes, meanders, oxbow lakes, deltas, levees.
Source: the starting point of a river, the place from which a river flows
Mouth: the ending point of a river, usually where it enters a sea or lake
Tributary: a small river that flows into the main river
Confluence: the location where a tributary meets the main river
Distributary: a small river that flows away from the main river
Estuary: the section of the river near the mouth that is influenced by tides
Drainage basin: the area of land from which water will flow into a river system
Watershed: the imaginary line that separates two drainage basins
The water cycle
Sections of a river
A river can be divided into the upper, middle and lower course. The long profile of rivers can be used to portray the gradients in each of the river courses. Overall, the long profile of rivers tends to be concave, with a more gentle slope towards the lower course (river mouth).
The cross profile of rivers is used to display the channel changes (width, depth, etc.)
The upper course is marked by a V-shaped valley, with steep sides and a shallow channel.
The middle course is marked by gently sloping valley sides and a wider and deeper channel.
The lower course is a very wide, neary flat valley and usually has a wide, deep channel.
- Hydraulic action – Air trapped in the cracks of the river bed by the water exerts pressure on the rock wall, causing it to break.
- Abrasion – Eroded rocks wear off the river channel.
- Attrition – Eroded rocks collide and rub together.
- Solution – Soluble components of the rocks are dissolved in the water (eg. limestone, chalk)
- Traction – Heavy boulders are rolled along the river bed
- Saltation – Small rocks are bounced along the river bed
- Suspension – Light particles (eg. sand grains) are transported in the water
- Solution – Dissolved particles are carried along by the river.
- Flotation – Light objects, eg. wood, float on the river surface.
Deposition occurs when the river does not have enough speed to carry the load (eg. delta, inner bends of meander or during dry spells). The velocity of a stream may slow down due to a reduction in rainfall, increased evaporation and more friction with the river bed.
Waterfalls, gorges and plunge pools
- form in the upper course where hard rock is layered horizontally above soft rock
- soft rock is less resistant to erosion and is worn away
- hard rock forms an overhang, creating a Knick point
- plunge pool develops at base of Knick point as force of water erodes soft rock
- hard rock eventually unsupported by soft rock below, causing the overhang to collapse
- plunge pool enlarges as hard rock erodes it
- waterfall retreats upstream and gorge may form
Image from: https://commons.wikimedia.org/wiki/File:Waterfall_formation23.png, by Jerry Crimson Mann
Potholes are smooth, rouded hollows in the bedrock of the river
- in the upper course
- when flowing water encounters bedload (stones are trapped in hollows on the river bed)
- forced over bedload and downcuts behind it (eddie currents)
- currents cause differential erosion (hydraulic action and abrasion)
- small holes are formed and widened by currents
- deep vertical erosion lowers river bed
- form in the upper course
- hard rock and soft rock layered diagonally (no obvious breaks of slope)
- soft rock is less resistant, so it is eroded faster than hard rock
- hard rock remains, resulting in bumps along the river bed
Meanders and oxbow lakes
Meanders are large bends common in the middle and lower courses of a river.
Image from: Flickr by U.S. Fish and Wildlife Service Headquarters, License: Attribution 2.0 Generic
- Meanders form in the middle course due to lateral erosion
- Helical flow of the river results in deposition in inner bends (creating a slip-off slope) and erosion in the outer bends (creating river cliffs)
- Combined action of erosion and deposition narrows meander neck
- River cuts through the meander neck (usually during a flood) and uses the shorter course
- Deposion occurs on the banks of the river as the Thalweg (fastest route) is now in the centre of the river
- Old meander is blocked by deposited material, leaving an oxbow-lake
Deltas, levees and flood plains
Deltas form in the lower course, where the velocity decreases, resulting in the deposition of sediment. The absence of major tidal currents prevents the deposited materials from being carried away and the impact of salt water causes suspended particles to flocculate (stick together), resulting in further deposition. Eventually, the accumulation of sediment is raised above sea level by the growth of vegetation. This forces the river to divide into many branches (distributaries).
Nile delta from : NASA ,Credit: Jacques Descloitres, MODIS Rapid Response Team, NASA/GSFC
How floodplains form:
- river overflows at times of high discharge and spreads over surrounding flat land
- river velocity decreases, resulting in deposition
- flood plains build up with success floods over the years
Levees may form in course material is deposited at the edge of the river channel and forms and natural embankment.