Rivers Revision

4 main processes:

• Interception
• Evapotranspiration
• Infiltration
• Precipitation

Potential Evapotranspiration: Amount of water that could be lost by evapotranspiration e.g. this is potentially high in deserts but amount that takes place is limited due to minimal moisture available

Water Flow:

• Throughflow - water moves downward through soil layers
• Channel Flow - downhill movement of water in rivers
• Groundwater Flow - Lateral movement of water from water table

Definitions/Equations

Discharge - Volume of water passing a measuring point in a given time

Drainage Basin Discharge - Precipitation - evapotranspiration +/- changes in storage

Lag Time - Difference between peak rainfall and peak discharge

Factors affecting hydrograph

Drainage basin size: larger catches more precipitation, higher peak discharge

Gradient: steeper sided, water flows quickly, shorter lag time

Shape: Circular, all points on watershed same distance from discharge measurement, increased p. discharge

Streams: more, drain quickly, shorter lag times

The Water Table - Level of saturated ground in the soil (rises and falls depending on amount of rain)

Watershed - Area of high land which seperates to drainage basin

Drainage Basin - Catchment area from which a river system obtains it's water

Water Balance - Balance between outputs and inputs

• Precipitaion (P) = Streamflow (Q) + Evapotranspiration (E) +/- Changes in Storage

Factors Affecting Discharge:

Rock Types - Impermeable don't store or allow flow, reudces infiltration, increased surface runoff and decreased lag time

Soil Type - Sandy soils allow a lot of infiltration, clay soils allow little, this increases run off, reducing lag time and increasing peak discharge

Vegetation - More vegetation, more precipitation, reduced peak discharge

Precipitation - More precipitation from intense storms, greater peak discharge

Temperature - Hot, dry/ freezing, cold create hard ground, reduced infiltration, increases run off, decreases lag time and peak discharge

Human Activity:

• Urbanisation: reduced lag time and increased discharge
• Man made drainage systems: reduced lag time and increased discharge

Hydraulic Action - Pressure of water breaks rock particles away from bed and banks

Abrasion - Eroded pieces of rock in the water scrape and rub against the bed and banks

Attrition - Eroded rocks smash into each other and break into smaller fragments. Doesb't erode bed and banks just makes rock particles smaller and rounder

Corrosion - Dissolving of rock through chemical processes. C02 dissolves in water to form a weak acid which reacts with rock like limestone and chalk breaking them down

Deposition occurs when a river loses energy

Energy and speed can be reduced by:

• Reduced rainfall: lower discharge, river slows and loses energy
• Increased evaporation: lowere discarge
• Friction: reduces speed and energy
• River meeting sea: reduced energy

Solution: Substances that dissolve are carried along in the water 

Science, chemical, acidic, dissolve

Suspension: Very fine material, clay and silt particles, whipped up by turbulence and carried along the river

Car, carried along

Traction: Boulders and pebbles rolled along river bed at times of high discharge

Tractor, large, rolled like wheels

Saltation: Sand sized particles are bounced along river bed by flow of water

Salsa, bouncy, flow

Upper:

Erosion -

• Mainly vertical
• Rough channel causes turbulence and large angular bed load is dragged along bed causing erosion

Transportation -

• Many large particles e.g. boulders are carried by traction of saltation in high energy conditions

Deposition -

• Little deposition, mainly larges particles as energy levels drop

Middle:

Erosion -

• Mainly lateral
• Attrition of larger particles means sediment size decreases from source to mouth

Transportation -

• More material carried in suspension as particle size decreases
• Some by saltation

Deposition - 

• Sand and gravel deposited across floodplain as river floods and friction reduces energy

Lower:

Erosion - 

• Low erosion due to lower turbulence and reduced particle size (reducing abrasion)
• Some lateral during meader formation

Transportation - 

• Mainly smaller particles carried by suspension
• Some substances in solution

Deposition - 

• Smaller particles are deposited on flood plain where river floods and in the mouth as the sea absorbs energy