Geography - Rivers

•        Attrition – material (load) carried by the river bump into each other resulting into smoother, smaller particles.

•        Abrasion – This is the process which the bed and banks of the river are worn down by the river load.

•        Corrosion – This is the chemical action of river water. The acids in water slowly dissolve the bed and banks (limestone).

•        Hydraulic action – This process involves the force of the water action against the river bed and the banks.

•        Saltation – Sand sized particles bounce along the river bed.

•        Solution – Some minerals dissolve in the water of the river.

•        Suspension – Silt and clay particles are carried within the water.

•        Traction – Rolling stones along the river bed.

•        Meander – The current is fastest on the outside of the bend because the river channel is deeper so there’s less friction meaning the water isn’t as slow as it is on the inside of the bend. Therefore more erosion takes place on the outside of the bend forming river cliffs .  Eroded material is deposited on the inside of the bend forming slip off slopes.

•        Oxbow Lakes – erosion causes the outsides of the bends to get closer until there’s only a small bit of land left between the bends, this is called the neck.  The river breaks through the land more than likely during a flood. The river flows along the shortest course, deposition eventually cuts off the meander forming an oxbow lake.

•        Levees – Levees are natural embankments along the edges of a river channel, during a flood eroded material is deposited, the heaviest material is deposited closest to the river channel. Over time the deposited material builds up, creating levees along the edges of the channel.

•        Deltas – Rivers are forced to slow down when they meet the sea, this causes them to deposit the material, the material builds up and the channel gets blocked, this forces the channel to split up into lots of smaller rivers called distributaries. 

•        Canalisation - The straightening of a river, this stops erosion, increases the flow of water and it allows big ships to transport goods up and down the river without having to navigate tricky meanders.

•        Afforestation - Planting trees along the river, these trees intercept rainfall and stop throughflow in order to prevent floods. Afforestation lengthens the lag time as it prevents surface runoff, drip flow and stem flow also take place increasing the lag time further.

•        Surface runoff - The water that runs over the surface of the land.

•        Groundwater flow - When water in rocks moves downhill

•        Drip flow- When water drips off vegetation, after interception the water is either absorbed by the plant and used for photosynthesis, or it drips off the leaves and onto the ground.

 •        Stem flow - Stemflow is the flow of intercepted water down the trunk or stem of a plant.

•        Interception - Interception is when the leaves of a plant or trees stop the rain from hitting the ground (it intercepts the precipitation).

•        V-shaped valley - A v-shaped valley is eroded vertically, rather than latarally as the river does not have enough power, so gravity takes control, A v-shaped valley has steep sides, but a narrow, and shallow channel.

•        Percolation - Percolation is when water moves vertically down through soil and rock.

•        Through flow - Throughflow is when water in soil flows downhill.

•        Infiltration - Infiltration is when water soaks into the soil.

•        Watershed - The watershed is the edge of the highland surrounding a drainage basin, it marks the boundary between two drainage basins.

•        Confluence - The confluence is the point at which two rivers or streams meet.

•        Tributary - A tributary is a smaller stream or river which joins to a bigger stream or river.

•        Source - The start of the river.

•        Mouth - The point where the river meets the sea or the end of the river.

•        Long profile - The long profile of a river shows you how the gradient changes over different courses.

•        The cross profile - The cross profile shows you what a cross section of the river looks like.

•        Chemical - Chemical weathering is the breakdown of rock by changing its chemical composition.

•        Biological - Biological weathering is the breakdown of rocks by living things such as plant or tree roots growing into cracks and pushing them apart

•        Mechanical -Mechanical weathering is the breakdown of rock without changing its chemical composition,  for example freeze thaw weathering where water freezes in cracks, expands, then when the water thaws it contracts releasing the pressure on the rock, this process repeated causes the rock to break down.

•        A hydrograph shows the following;

-Peak rainfall: Peak rainfall is the highest amount of rain/precipitation that takes place.

- Peak discharge: The highest amount of discharge in the time period you are observing.

- Lag time: The lag time is the delay between peak rainfall and peak discharge.

- Rising limb: The rising limb is increase in river discharge as rainwater flows into the river.

- Falling/Recession limb: The decrease in river discharge as the river returns to its normal level.

•        The lag time is affected by the following factors;

- The area of the storage basin: Large basins recieve more precipitation than a smaller basin, it also takes longer for the water to get to a river or watersource therefore resulting in a longer lag time.

- The shape of the drainage basin: An elongated basin with flat slopes will cause the lag time to increase as there is less surface runoff meaning water reaches the river in a slower time.

- Rock type: Permeable rock means that water can infiltrate into the rock which means there is less surface runoff and a longer lag time.

-Rainfall: Low rainfall causes less surface runoff and a longer lag time.

- Previous conditions: Previously dry conditions mean that water can infiltrate into dry soil so there is less surface runoff.

Amount of vegitation: Areas of high vegetation are more likely to have a longer lag time as more water is intercepted by the leaves, drip flow and stem flow slow down the time at which it takes for the water to reach the ground. Throughflow is also slowed by the roots meaning a longer lag time.

- Rainfall: High, intense rainfall causes more surface run off and a shorter lag time.

- The area of storage basin: If the area of the storage basin is smaller it will take less time for the water to flow into the river meaning a shorter lag time.

       - Rock type: Impermeable rock means that water can't infiltrate into the rock, this                   means there's more surface runoff and less of a lag time

- Previous conditions: Previously wet conditions means the soil is already saturated with water, the water is unable to infiltrate so there is more runoff and again the lag time is reduced.

- Soil: Clay is impermeable because it is already saturated in water meaning less infiltration and more surface runnoff.

- Land use: Urban areas are covered with impermeable materials incrasing surface runoff decreasing the lag time.

•        Prolonged/Heavy rainfall - After a long period of rain the soil becomes saturated, meaning any access water can't be absorbed increasing surface runoff which could eventually lead to a flood.

•        Snow melt - When a lot of snow or ice melts it means that a lot of water goes into a river in a short space of time increasing the amount of discharge which could result in a flood.

•        Deforestation - If trees are cut down there is less interception taking place meaning more of the rainwater reaches the ground in a quicker time, there are no roots which means there is more throughflow resulting in more discharge incrasing the risk of flooding. If there are no roots there is nothing binding the soil together therefore it is washed away, the soil then sinks to the bottom of the riverbed, this in turn raises the rivrbed reducing the volume of water the river channel can hold in turn increasing the risk of flooding.

•        Urbanisation - Many impermeable materials are in an urban area, buildings, roads and paths are made of materials such as concrete and tarmac, these impermeable surfaces increase runoff. The drains also take water to the river incrasing the likelihood  of a flood.

•        Dams and reservoirs - Dams are huge walls which are built across rivers mainly in the upper course leaving a reservoir behind. This strategy allows you to control the amount of water that is flowing down the rivers, if there is heavy rainfall forecast water can be released slowly reducing the volume of water and decreasing the chances of a flood. The water in the reservoir can be used as drinking water and to generate hydroelectric power. The downside to dams are that they are very expensive to build and creating a reservoir can flood existing settlements. Eroded material is also deposited in the reservoir and not along the rivers natural course making farmland down stream infertile or at least less fertile.

•        Channel straightening/Cannalisation - The clue is in the name, Cannalisation or channel straightening is when the rivers course is straightened, meanders are cut out by building artificial straight channels, the benifits are that the water moves more quickly and there is less erosion taking plae meaning less soil raises the river bed, reducing the risk of flooding. The disadvantages are that flooding may occur downstream as the floodwater is carried faster, it is also extremely expensive.

•        Man-made levees - These are man made embankments on either side of the river, the embankments mean the river channel is able to hold more water, reducing the risk of flooding, these are also a cheap alternative to damming or cannalisation. The only problem is that if the levee breaks it could cause massive floods.

•        Flood plain zoning - Flood plain zoning is a soft engineering method which involves restricing urbanization on flood plains, this means that there aren't any houses or infrastructure damaged by flooding, the downside is that there arent many rural areas by the river as settlements have been located there for the source of water.