The Water Cycle - Rivers key processes Flashcards
What is erosion?
The breaking down of rocks which are removed by water
What is hydraulic action?
When water forces its way into cracks and air trapped inside increases in pressure, cracking and breaking the surrounding rock
What is abrasion?
When water carries suspended rocks and scrapes them along the river bed
What is attrition?
When large rocks collide together, breaking into smaller, smoother fragments
What is solution?
When water dissolves soluble minerals/chemicals in rocks which weakens them for other processes
What is transportation?
How sediment moves
What is traction?
When large boulders are rolled along the river bed as there is not enough energy to pick them up - either in high energy rivers or floods
What is saltation?
When the water has enough energy to pick up pebbles but not enough to carry them so they “bounce” along the river bed
What is suspension?
When fine sediment like sands, silts and clays are carried by the water which can make rivers look muddy
What is solution (transportation)?
When minerals dissolved from rocks remain dissolved and carried in the water
What is deposition?
Conditions under which sediment is dropped by water
How are V shaped valleys formed?
Under the force of gravity due to the steep gradient, the narrow river vertically erodes the bed downwards. This leaves steep, unstable valley sides, which with additional weathering such as freeze-thaw, collapse under mass movement to form sloping sides and thus a V shaped valley
How are interlocking spurs formed?
Towards the source the river is narrow and shallow and does not have much energy for lateral erosion through the hills. Therefore the river weaves its way around them instead
How are waterfalls and gorges formed?
When water flows over different rock types, the soft rock erodes faster than the hard rock by hydraulic action and abrasion which creates a small drop. As the water flows over the drop, the softer rock continues to erode faster and over time this drop becomes larger to become a waterfall. A plunge pool is then eroded at the bottom of the drop by abrasion. This causes undercutting and the hard rock on top is left overhanging which eventually collapses, causing the waterfall to retreat back up the valley. This process repeats many times and leaves behind a steep sided valley called a gorge
How are meanders and oxbow lakes formed?
Water flows through pools and riffles which causes the thalweg to be thrown to one side and begin to erode the bank by hydraulic action and abrasion. Over time, this erosion of the bank causes a bend in the river which is a meander. A river cliff forms on the outside of the meander due to the erosion whereas on the inside the water flows slower so there is less energy meaning deposition occurs, forming a slip off slope. Over time this process continues and the meander gets bigger, causing the neck to become narrow. During a high energy event like a flood, the river will break through the neck to create a straight river channel. The meander bend no longer has water flowing through it thus deposition occurs due to low energy, forming an oxbow lake. As meanders migrate, floodplains are formed
How are levees and floodplains formed?
During floods, the river discharge exceeds its capacity which causes water to overflow onto its banks. When the water floods onto the land, friction causes the speed and energy to drop, causing the coarser sediment to be deposited first due to the reduction in energy. The finer sediment (alluvium) is carried further and deposited across the floodplains. This process repeats over many years and the deposition of coarse sediment next to the river channel forms levees while the deposition of finer sediment further from the channel forms floodplains.
What are estuaries and deltas?
An estuary is a water body where rivers meet the sea and water flows into the sea on one side of the river and on the other side the tide pushes water up the river meaning no deposition can occur
A delta is a landform formed at the mouth of a river which has a fan shape
