Rivers

• Explain the profile of a river in the lower course

  The lower course of a river flows through low lying land, and has a wide and deep channel. Therefore, there is less friction, resulting in a high velocity. The speed is boosted by the additional discharge from all the tributaries. Deposition from floods builds up the flood plains and meanders migrate. This builds up and widens the valley as it reaches the mouth of the river.
• Interlocking spurs and their formation

  What: Pieces of land weaving through eachother

  Formation:

  1. River wants to take the easiest route of least resistance using fluvial erosion.
  2. When they meet bands of hard rock, they weave around them, eroding only the soft rock.
  3. This creates a zig zag shape and the hills become interlocked.
  4. Eroded scree is deposited at the edge of the river.
• Waterfall formation

  A river flows over bands on hard rock, overlying bands of soft rock. The soft rock erodes as it is less resistant, through processes of hydraulic action.An example of the soft rock is sandstone. This undercuts the hard rock, which doesn't eroded due to it being more resistant, leaving behind a ledge, which overtime develops into an overhang. An example of the hard rock is limestone. The overhang collapses due to gravity, as there's no rock underneath to support it. Every time the rock collapses, the waterfall retreats a few metres upstream.
• Plunge pool formation

  There is high energy water at the drop of the waterfall, as the gravitational potential energy transfers into kinetic energy, which erodes the base of the drop through hydraulic action, forming a plunge pool which receives the collapsed rock from the overhang. This increases rate of erosion as attrition and abrasion of the rock takes place within the plunge pool, making it deeper and wider.
• Gorge formation

  The waterfall formation process repeats, causing the waterfall to retreat backwards, forming a steep sided valley gorge as it does so. This can be seen at high force in the lake district.

  
• Ox bow lake formation

  1. Meanders migrate through lateral erosion, leading to the narrowing of the necks of the meander.
  2. When the river floods, it can cut through the neck, forming a new, straightened channel. Deposition cuts off the old loop
  3. Continuous deposition cuts off the loop, creating an ox bow lake.
  4. Overtime, they may become mashes (meander scars) due to a lack of water supply from the river.
• Meander formation

  In a river, pools and riffles form around obstacles like boulders as water twists and turns. Pools are deeper with more erosion due to less friction. Riffles are formed by sediment deposition and cause the river to flow in a winding course. Helicoidal Flow moves water, depositing material, while Thalweg's energy erodes outside bends, forming a river cliff. Water slowly deposits load on the inside bend to create a river beach. Erosion on the outer bank and deposition on the inner bank form a changing meander.

  
• Levee formation

  1. When a river floods, it carries sediment.
  2. As the water breaks its banks, it drops the heavier sediment immediately, as it hasn't got the energy to move it further, and the finer sediment later on, as it takes less energy to carry.
  3. This process repeats as the river floods over many years, and levees are composed of gravel, stones, and alluvium, which makes for very fertile land, which is good for farming.

  
• Characteristics of floodplains

  Wide and flat

  Layers of alluvium (silt)
• Characteristics of levees

  • Natural embankments
  • Higher than the adjacent flood plains
  • Steep sides on the channel side
• Characteristics of estuary

  • Mud flats or slat marshes
  • Point where fresh water meets salt water
• Factors increasing flood risk: Deforestation

  Human/Physical: Human

  What happens: Trees are removed, meaning much more water is suddenly available and transferred rapidly to the river channels, increasing flood risk.

  Why: Much of the water that falls on trees is evaporated or stored temporarily on leaves and branches. Trees also use up water as they grow in Photosynthesis. The removal of trees means this water now must flow into the river channel.
• Factors increasing flood risk: Heavy rainfall/Precipitation

  Human/Physical: Physical

  What happens: Torrential rainstorms or prolonged rainfall lead to flooding (winter) or sudden rainfall leading to flash floods (summer).

  Why: Sudden: Less infiltration time due to increased rate of rainfall,so the water is forced to flow as surface run off. Prolonged: River reaches capacity and discharge is to large leading to flooding.
• Factors increasing flood risk: Relief/Steep slopes

  Human/Physical: Physical

  What happens: Steep land leads to quick flow, and less steep leads to slower flow.

  Why: Steep slopes encourage rapid water transfer as surface run off. The ground has less time to infiltrate waters as it reaches rivers to quickly.
• Factors increasing flood risk: Rock type/Geology

  Human/Physical: Physical

  What happens: 2 types of rocks - Impermeable (e.g. granite), no water can soak in. - Permeable (e.g. sandstone), allows water to soak in.

  Why: Impermeable rocks don't allow perculation, so it forces water to flow as surface run off, so the river exceeds capacity to cope.
• Rivers case study: River Tees


  Where: North-East England, source in the Pennine Hills near cross fell, height of 893 metres, flows 128 km to the North Sea, Middlesbrough

  Hig Force: Located in Teesdale in the upper course of the Tees, drops 20 metres, hard rock example = dolerite, soft rock example = limestone

  Meanders, Levees, and Flood plains: Sock burn meander, flows near Darlington from west to east, low lying in the middle course, contour lines are far apart.

  Middle course: 95% of land used for farming

  Lower course: Marsh land used for industries e.g. oil refineries and nuclear energy
• Bedload at different stages of River Tees


  Upper: Angular, large, rough edged due to low levels of erosion

  Middle: Rounded, smaller, all corners knocked off as erosion is at its most powerful

  Lower: Estuary mud, dried out to become marshes
• River Tees energy


  95% of energy flowing through the river channel is lost through friction.
• Factors increasing flood risk: Farming/Agriculture

  Human/Physical: Human

  What happens: In farming, soil is left unused and exposed to the elements for long periods of time. This leads to more surface run off.

  Why: Animals walking across the soil compact it down, so there are no pores for water to flow through, making the soil impermeable. Also, since there are no crops, the ground has less time for infiltration. These lead to soil erosion, making the soil infertile, so no crops can grow and no air spaces in the soil,so water is forced to flow as surface run off, increasing river discharge
• Factors increasing flood risk: Urbanisation

  Human/Physical: Human

  What happens: Building on floodplains creates impermeable surfaces such as tarmac roads, concrete driveways, and slate (link to geology).

  Why: This increases surface run off, so water is transferred quickly to drains to head into the river channel, meaning there's more risk of flood, as the ground can't infiltrate the water.
• Hydrographs

  Shows how river discharge changes overtime in relation to precipitation or rainfall.
• Peak rainfall

  The highest amount of rainfall during the flood event
• Peak discharge

  The highest volume of water flowing in a river channel per seccond (m3/s or cumecs) within the given time frame.
• Lag time

  The time between peak rainfall and peak discharge (hours), occurs as not all rainfall falls directly into the river, so it needs to travel.
• What does lag time tell us

  Longer lag time = Les steep rising limb = water reachs channel slower, so has more time to infiltrate into the ground.

  Shorter lag time = steeper rising limb = water falls quickly into channel, so has no time to infiltrate = more flood risk
• Rising limb

  Increase in discharge as it enters the channel. Steep = fast moving water due to physical and human factors of flooding
• Falling limb

  Decrease in river discharge as the river returns back to normal capacity (base flow)
• Drainage basin characteristics: Basin size

  Flashy (high peak) hydrograph (summer): Small basins often lead to a rapid water transfer.

  Low peak hydrograph (winter): Large basins result in a relatively slow water transfer.
• Drainage basin characteristics: Drainage density

  Flashy (high peak) hydrograph (summer): High density speeds up water transfer.

  Low peak hydrograph (winter): Low density leads to a slower transfer.
• Drainage basin characteristics: Rock type

  Flashy (high peak) hydrograph (summer): Impermeable rocks encourage rapid overland flow

  Low peak hydrograph (winter): Permeable rocks encourage a slow transfer as ground water flow.
• Drainage basin characteristics: Land use

  Flashy (high peak) hydrograph (summer): Urbanisation encourages rapid water transfer.

  Low peak hydrograph (winter): Forests slow down water transfer due to interception and Photosynthesis.
• Drainage basin characteristics: Relief

  Flashy (high peak) hydrograph (summer): Steep slopes lead to rapid water transfer

  Low peak hydrograph (winter): Gentle slopes slow down water transfer
• Drainage basin characteristics: Soil moisture

  Flashy (high peak) hydrograph (summer): Saturated soil means rapid overland flow as it exceeds capacity.

  Low peak hydrograph (winter): Dry soil socks up water and slows down transfer.
• Drainage basin characteristics: Rainfall intensity

  Flashy (high peak) hydrograph (summer): Heavy sudden rainfall may exceed the infiltration capacity of vegetation, and lead to rapid overland flow.

  Low peak hydrograph (winter): Light rain overtime will transfer slowly and most will soak into the soil.
• Explain the profile of a river in the upper course

  The average velocity and discharge of a river increases along its course. Although the upper course has a steep gradient and is V shaped, the velocity depends on how much water comes into contact with the Channel banks and bed. This is because in the upper curse the rock is harder, and so the valley sides are not widened out much by weathering and erosion. Therefore, velocity is low.
• Explain the profile of a river in the middle course

  In the middle course, the river is flowing through lower country. The gradient is less steep than in the upper course,so the river begins to meander and erode laterally into the valley sides. The rate of erosion increases as the rocks that make up the valley sides are softer. As the river uses more energy in lateral erosion it is not able to remove al the eroded material, so this builds up the valley floor to give it a more gentle profile.
• Floodplain formation

  Erosion of the outer bends of a meander will gradually wear down and flatten the land on either side of the channel. When the river overflows its banks during a flood, the river will leave the channel and flow across the neighbouring land. Due to increased friction between the water and the flood plain, the river water will lose energy and deposit its sediment load. Flood plains are used for farming, as the soil is very fertile. As the alluvium builds up, it forms levees, which raise the river beds.