Freshwater Flashcards
Factors that impact drainage basins
Geology Land use Precipitation Relief Time
Relief
Steep slopes tend to reduce the amount of infiltration of water into the ground, this water can then flow quickly down to rivers as overland flow.In addition, steep slopes also cause more through flow within the soil.Both can raise river levels.Gentle slopes or flat land allow water to penetrate into the soil and increase lag times.
Geology
Soil and rock typecan also influence what happens to precipitation when it reaches the ground.Impermeablesoils and rocks such as clay or shale do not allow water to infiltrate, this forces water to run off reducing river lag times and increasing flood risk.Permeable rocksallow water to infiltrate into them.If permeable rocks allow water in through cracks, fissures and bedding planes but not through their pores they are said to be pervious (such as limestone).Porous rocks allow water to penetrate into their pores such as sandstone.
Precipitation
If its rain then it causes more flooding.
If its snow then when it melts it causes surface runoff.
Land use
Vegetation type and coverageplay a big role, with forests intercepting more rainfall than grasses.This interception increases lag time and reduce the risk of a flood.Indeed, deforestation (the removal of trees) can increase soil erosion, reduce interception and increase flood risk.Afforestation, where trees are planted, can have the opposite effect.
The velocity of a river
Chanel shape
The roughness of the channel beds and banks
Gradient
Erosion types
Hydraulic action
Attrition
Abrasion
Solution
Types of transportation
Traction - rolling
Saltation - bouncing
Suspension - floating
Solution - dissolved
Hydraulics - the study of water flow in a channel
The effect of friction creates an uneven distribution of velocity in a stream. Water closest to the bed and bank travels slowest, while water nearest the centre travels fastest.
The highest velocity is in the middle about a third of the way down The shape of the channel also affects velocity.
Velocity varies with a number of factors: volume of water; the roughness of the river bed; gradient of the stream; width, depth and shape of the channel.
The efficiency of a stream’s shape is measured by its hydraulic radius. The higher the ratio, the faster the water flows
There is a close relationship between velocity, discharge and the characteristics of the channel.
These include depth, width and channel roughness.
The width: depth ratio is a good measure of comparison.
Channel roughness causes friction, which slows down the velocity of the water. Friction is caused by irregularities in a river such as boulders, trees and vegetation, and contact between the water and the bed and bank.
The smallest particles (silts and clays) are carried in suspension as the suspended load.
Larger particles (sands, gravels, very small stones) are transported in a series of “hops” as the saltated load.
Formation of V-shaped valley
They are formed by strong streams, which over time have cut down into the rock through a process called downcutting. These valleys form in mountainous and/or highland areas with streams in their “youthful” stage. At this stage, streams flow rapidly down steep slopes.
Formation of waterfalls
A waterfall is a sudden drop along the river course. It forms when there are horizontal bands of resistant rock (hard rock) positioned overexposed, less resistant rock (soft rock).
A waterfall forms when water falls down hard rock in a steep-sided gorge. A plunge pool forms. The overhang erodes and waterfall retreats.
The soft rock is eroded quicker than the hard rock and this creates a step.
As erosion continues, the hard rock is undercut forming an overhang.
Abrasion and hydraulic action erode to create a plunge pool.
Over time this gets bigger, increasing the size of the overhang until the hard rock is no longer supported and it collapses.
This process continues and the waterfall retreats upstream.
A steep-sided valley is left where the waterfall once was. This is called a gorge.
Formation of floodplains
Floodplains form due to both erosion and deposition. Erosion removes any interlocking spurs, creating a wide, flat area on either side of the river. During a flood, material being carried by the river is deposited (as the river loses its speed and energy to transport material). Over time, the height of the floodplain increases as material is deposited on either side of the river.
Formation of deltas
Delta are found at the mouth of a river, where
the river meets the sea. At this point the river
is carrying too much load for its velocity and
so deposition occurs.
Formation of meanders
Meanders are created by the lateral erosion that the river undertakes in this part of its journey to the sea.
Water flows around the bend in the river and as it does so, it swings to the outside of the bend - this means the fastest flowing and highest volume of water is concentrated on the outside of the bend causing it to erode the bank at this point.
On the inside of the, the flow is extremely slow and because of this lack of energy, the river is depositing sediment.
Flood hydrograph
is a means of showing the discharge of a river at a given point over a short period of time - know how to draw it
