Geography- Rivers

Inputs

• Precipitation, eg snow, rain, hail, frost.

Storage

• Interception, when preciptation lands on vegetation or buildings before the soil, only temporary.
• Vegetation sotrage, water taken up by plants.
• Surface storage, egs puddles, ponds and lakes.
• Groundwater storage, stored in soil or porous rocks (aquifer) in the water table.
• Channel storage, water held in river.

Water balance

• Difference between inputs and outputs.
• Affects how much water is stored in the basin.

Flows & Processes

• Surface runoff (overland flow)- water flowing over the land.
• Throughfall- water dripping from plant to plant.
• Stemflow- water runnig down a plant stem/tree trunk.
• Throughflow- water moving downhill through the soil.
• Infiltration- water soaking into the soil, rates depend on soil trype, structure and what's in the soild.
• Percolation- water seeping through soil into water table.
• Groundwater flow- water flowing below water table through permeable rock.
• Baseflow- groundwater flow that goes into the river through river banks and beds.
• Interflow- water flowing downhill through permeable rock above water table.
• Chanel flow- water flowing in river.

Outputs

• Evaporation
• Transpiration
• Evapotranspiration- evaporation and transpiration happening together.
• River discharge/flow.

• Is volume of water that flows in a river per second, measured in cubic metres per second.
• Affected by precipitation, hot weather, remobal of water.

Hydrographs

• Graphs of river discharge over time, showing volume of water changing over time.

1. Peak discharge- highest point, river discharge greatest.

2. Lag time- delay between peak rainfall and peak discharge.

3. Rising limb- part of graph up to peak discharge, river discharge increases as rainwater flows into river.

4. Falling limb- part of graph after peak, river discharge decreases because less water flowing into river.

1. Larger drainage basin, will have higher peak discharge, smaller drainage basins and steep-sided will have shorter lag time. Basins with high drainage density have shorter lag time.

2. If ground is waterlogged surface runoff increases- decreasing lag time.

3. Rock type- impermeable rocks increase surface runoff reducing lag time, increasing peak discharge.

4. Soil type- sandy soild increase infiltration (compared to clay soils) decreasing surface runoff, increasing lag time and decreasing peak discharge.

5. Vegetation- intercepts precipitation, increasing lag time and decreasing peak discharge.

6. Precipitation- high precipitation leads to high peak discharge.

7. Temperature- hot, dry or cold, freezing conditions hardens the ground, increasing surface runoff, reducing lag time but increasing peak discharge.

Human activity- concrete is impermeable decreasing lag time, increasing peak discharge.

• Vertical erosion deepens river channel, in upper course of river.
• Lateral erosion widens river, in middle and lower course of river.

4 ways of erosion:  1. Hydraulic action- pressure of water braks rock particles from bed and banks.

2. Abrasion (corrasion)- rocks scrape and rub against bed and banks, remobing material.

3. Attrition- rocks smash into each other and break into smaller fragments, edges get rounded as they rub togerther.

4. Corrosion (solution)- rocks dissolve by slightly acidic water, reacting with limestone or chalk.

4 ways of transportation:    1. Solution- rocks dissolved in slightly acidic water.

2. Suspension- fine material (sand) carried by turbulant water.

3. Saltation- larger particles, eg pebbles/gravel, bounce along river bed.

4. Traction- very large particles, eg boulders, roll along river bed.

Deposition occurs when:

1. Reduced rainfall- lower discharge- river slows- has less energy.

2. Increased evaporation or abstraction- lower discharge.

3. Friction- reduces speed of river and its energy.

4. River forced to slow, before a narrow section- loses energy.

5. Energy lost when river meets sea.

• Capacity is total load a river can transport at on time.
• Competence is maximum particle size a river can transport at one time.

Long Profile

• The gradient of channel from sourcce to mouth- by showing height of river bed and base level.
• Base level is lowest point a river can erode to (sea level)
• Total erosion and deposition is balanced, so overtime the long profile will become a smooth curve- graded profile.

Efficiency

• The more energy- the more erosion and transportation- the more efficent.
• Efficient river has high velocity, high discharge and little friction.
• Larger hydraulic radius- more efficient.
• Hydraulic radius= cross section/length of wetted perimeter.

Upper stage:

• Low hydraulic radius- innefficient.
• Low discharge.
• Turbulant flow.
• Large cobbles and boulders- large range of sediment size.
• Steep channel gradient.
• V-shaped valleys, and narrow channels.
• Vertical erosion dominant.
• High friction.

Middle stage:

• Reduced range of sediment size- more sand sized particles.
• U-shaped valleys.
• Channel may meander- lateral erosion dominant.
• Channel more efficient.
• More tributaries- higher discharge.

Lower Stage

• Channel is widest.
• Hydraulic radius highest- most efficient.
• Deposition prominant process.
• Channel capacity highest.
• Smallest range of sediment size- most has been eroded.
• Discharge highest.
• Laminar flow dominates.
• Wider flood plain due to deposition.

Waterfalls:

• Form where band of hard rock meets soft rock.
• Soft rock eroded more quickly by abrasion and hydraulic action, causing a step in river bed.
• Water flowing over speeds up- greater erosive power- further eroding soft rock, undercutting hard rock. Plunge pool formed by abrasion as collapsed rock are swirled.
• Hard rock is undercut, can collapse. Continues leaving steep sided gorge.

Potholes:

• Small, circular hollows in river bed.
• Formed by abrasion as turbulant flow swirls bedload in circular motion, rubbing and scraping out holes.

Rapids:

• Steep sections of river with turbulant flow, with sections of hard rock.
• Mini-waterfalls.

Meanders:

• Large, sweeping curves in middle and lower stages, formed by erosion and deposition.
• Form where alternating pools and riffles develop.
• River is deeper in pools so has more energy and erosive power, river loses energy in riffles.
• River's flows is uneven and maximum flow concentrated to one side.
• Turbulant flow increases around pools as water speeds up.
• Corkscrew like current called helidoidal flow- causing more erosion deepening pools.
• Eroded material deposited on inside of next bend, where river loses energy. 
• Erosion and deposition exaggertaes bends into meanders.

Oxbow lakes:

• Formed when neck of loop of a meander is broken, during flooding.
• Deposition dams the loop.

For all:  The presence of a large amount of sediment is essential – brought down by large rivers

Braiding

• Rivers velocity drops so sediment deposited.Causes river to divide ino small, winding channels that rejoin into a single channel.

Levees

• Natural, raised embankments formed when river overflows, as loses velocity so sediment deposited. Heaviest material deposited first closest to channel. Builds up on river bank.

Deltas.

• River reaches sea, loses energy, deposits load. Load builds up, alluvium rises above sea level, blocking mouth. River braids to reach the sea.
• Flocculation- salt present in the water in the sea generates an electrical charge causes particles to stick together, increasing cohesion and weight, encouraging deposition.

Flood plains: Land either side of river, when river overflows, wetted perimeter increases, hydraulic radius decreases, so sediment deposited on flood plain.

• Is the change in energy of a river due to changes in base level, due to isostatic change (ground level change) or eustatic change (change in sea level).
• Drop in base level gives greater potention energy, increasing vertical erosion.
• Long profile extended and knickpoint forms, marking junction between original and new long profile meet. Sudden break in long profile of river due to change in land level causing increase in erosion.
• River terraces are former floodplains- left above present floodplains due to vertical erosion eroding the riverbed and becoming deeper. Creating a new floodplain, and old flood plains are river terraces (Oxford)
• Incised meanders are formed rejuvenation due to eustatic change (global change in sea level). Vertical erosion prominent river has more potential energy, which erodes meander making it much deeper, for example into an entrenched meander. Result is deep, winding valley with steep sides and the river is far below former flood plain.

Physical:

• Sparse vegetation, little interception, more rain reaches ground, increases discharge.
• Impermeable ground, clay soils and granite don't allow infiltration, increases surface runoff, increasing discharge.
• High drainage density, short lag times, water flows into main stream quickly, increasing discharge.
• Steep slopes, short lag time, increasing discharge,
• Heavy rainfall- ground saturated, increase surface runoff, increasing discharge. 

Human:

• Urbanisation- impermeable concrete, surface runoff rapid. Drains quickly take water to rivers, reduce lag time, increase discharge.
• Deforestation- reduces interceptio, increases volume of water and discharge.
• Climate change- could increase rainfall and storms, increasing flooding.
• Agriculture- overgrazing less vegetaion. Overgrazing and ploughing increase soil erosion.
• Flood management strategies can increase flooding risk.

Social Impacts (higher in LEDCs- flood defences poor):

• People and animals killed.
• Floodwater contaminated with sewage- increase risk of diseases eg diarrhoea.
• Possessions can be damaged, homes destroyed- homelessness.

Economic Impacts:

• Businesses shut down.
• Recue work and repairs costly.
• Unemployment increases.
• Public transport, roads and bridges destroyed.
• Crops destroyed- can increase food prices.

Environmental Impacts:

• Contaminated floodwater pollutes rivers.
• River banks eroded.

Hard Engineering disadvantages:

• Expensive to build and maintain.
• Floods happen less often, but more hazardous eg. if dam breaks.
• Natural processes disrupted.
• Ugly.

Soft Engineering advantages:

• Cheaper to maintain.
• Flooding more predictable- less unexpected disasters.
• Improve opportunites for recreation- fishing.
• More attractive.