Rivers

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The Water Balance

The water balance shows a state of equilibrium in the drainage basin between the inputs and outputs and changes in the groundwater storage.

The water balance can change with seasons.

In the spring there is water surplus causing considerable run off. This is because precipitation. 

When the water balance is posotive the increases in infiltration allowing ground water stores to be recharged. 

When vegitation is growing and temperatures rise there may be a water defict. 

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Factors affecting river discharge

The storm hydrograph shows variations in a rivers discharge over a short period of time. 
River discharge is the volume of water in the river and is measured in cumecs.

Start and finish level = base flow.

As storm water enters the drainage basin the discharge rises causing a rising limb

peak = highest flow in the chanel

receeding limb shows fall in the discharge back to base level 

the delay between maxium rain fall and peak discharge is lag time.

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Factors affecting river discharge

  • If intensity and duration of the storm are high = rising limb      
     
  • heavy rainfall on saturated soil = steep rising limb
     
  • porus soil types and permiable rock = less steep as slower regulated
     
  • a small drainage basin tends to respond more rapidly to a storm = shorter lag
     
  • vegetation reduces peak discharge 
     
  • human activities have an influence e.g roads mean that water is able to get to the river channel quicker reducing lag time                      
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Factors affecting the long profile

The long profile of a river shows the changes in altitude from source to mouth and it is steeper in upper course. 

Gradient -  Steeper in the upper part of a basin. 

Varying Rock types   Resistant rocks produce kinks e.g waterfalls and rapids

Resoviours  Flatten out the long profile

Rejuvination  a fall in sea level relative to the level of the land or the rise of land relative to the seal level which revives the erosional activity of the river. The steepening of the long profile results in a knickpoint


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Types of erosion

potential energy - a result of the weight of the water and kinetic energy - produced by gravity. 

 Headward erosion - at the source as the river erodes back to the watershed as it undercuts soil and rock.

Upper course, cuts down vertically to the base level 

Base level is sea level

lower courses - erosion latterly, widening the valleys. 

  • Abrasion is the erosion of the bed and banks by load transported by fast flowing current
  • corrosion is chemcial action disolving
  • hydraulic action is sheer force and power of moving water 
  • Attrition is erosion of the bed load by contact with other load, smoothing. 
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Transportation

The capacity of the river is the largest amount of material that can be transported and the competence is the size of the largest particle. They both increase as discharge increases. 

  • Traction is the movement of large particles in the bed load. rolling along the river bed
     
  • saltation is the movement by bouncing along the river bed
     
  • suspension is smaller particles floating in the river
     
  • solution is chemical load that has been disolved into the river 

     

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Deposition

Deposition occurs when the river has reduced energy. This happens when velocity and river discharge increase. 
 

Reduced precipitation leading to reduction in discharge

the river entering a lake or sea

a sudden increase in river load

shallow water within a channel

as a river flows onto the flood plain 

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