Water on the land

Water on the land

1. Formations

   • Upper course
     • V-shaped valley
       • In the upper course a river has a high GPE thus a low KE and discharge
         • There is therefore downward vertical erosion as particles in saltation erode the bed
           • This leaves a steep sided valley
             • As erosion continues, the sides become exposed to weathering
               • The rock weakens and falls into the river channel to be carried away
                 • Leaves a valley shaped like a 'V'.
     • Interlocking spurs
       • In the upper course river has high GPE and so low KE and discharge
         • This means that it has insufficient energy to laterally erode through hills using hydraulic action
           • Instead it flows around the hills
     • Waterfall
       • Waterfalls are formed where a layer of hard rock overlays a layer of soft rock.
         • The soft rock is more easily thus quickly eroded, forming a step in the river bed.
           • Hydraulic action expands the step to form a notch and plunge pool.
             • Hydraulic action erodes and expands the plunge pool, meanwhile splashback erodes and expands the notch, until it becomes so big that the rock overhang above can no longer be supported.
               • The rock fragments help to abrade and further expand the plunge pool, whilst splashback expands the notch, until the rock above collapses again.
                 • As this continues, the waterfall retreats upstream and leaves a steep-sided groge
   • Middle course
     • Meanders
       • River cliff
         • On the outside of the bend the river is fastest so the river has the most kinetic energy and the highest discharge
           • This means that it has the greatest erosional potential here, so lots of hydraulic action takes place on the outer bank, eroding it away.
       • River beach
         • On the inside of the bend the river is the slowest. So, it has low kinetic energy and discharge.
           • There is a lower erosional potential here, so no hydraulic action occurs. In fact, the river doesn't have enough energy to carry its load so deposition occurs, forming a river beach.
     • Oxbow Lake
       • As a river flows around a meander, most erosion (hydraulic action/abrasion) occurs on the outside of the bend, as here the water is fastest so there is the greatest erosional potential.
         • As this erosion continues, the meander becomes S-shaped and a neck forms.
           • As this erosion continues the neck becomes so small that the river breaks through it (maybe during a flood), leaving a straight channel and a channel the shape of the old meander.
             • For a while water flows across the neck and around the backwater.
               • Over time earth and mud are deposited by the stright channel, by the backwater, eventually blocking it off to leave a kidney-shaped oxbow lake next to the straight channel of the river.
   • Lower course
     • Delta
       • At its mouth, a river is carrying too much load for its slow velocity.
         • So, this material is deposited.
           • Fine load such as clay is deposited last in deeper water and silt forms the steep delta edge.
             • Coarser material forms the flat top of the delta. as it's first to be deposited
               • The finest particles, clay, are carried out into deeper water.
     • Floodplains
       • When a river floods, it slows down.
         • The finest material carried by the river (i.e. the alluvium) is deposited last on the area surrounding the river. This is because least energy is required to carry the alluvium.
           • Repeated flooding builds up a rich, fertile floodplain (?)
     • Levees
       • When a river floods, it slows down.
         • As it slows down, the biggest load particles (large rocks and boulders) are deposited first (as they need the most energy to be carried). This is often on the river's immediate banks.
           • With repeated flooding, levees are built up.

2. River processes

   • Erosion
     • Transportation
       • Deposition
         • Deposition occurs when:
           • River velocity drops
           • The river becomes shallower
           • Water volume decreases
         • boulders
           • cobbles
             • pebbles
               • sand
                 • silt
                   • clay: stays in suspension until it reaches deeper sea-water
         • Deposition occurs near the mouth
       • Traction: largest load (boulders and large rocks) roll along the bed.
       • Saltation: Stones and pebbles bounce along the river bed.
       • Solution: rivers flow over rainwater-soluble rock (e.g. chalk, limestone) and the rock dissolves in the water to become a part of the water flow.
       • Suspension: fine particles such as sand, silt and clay are suspended in the river and move as the water does.
       • It is affected by:
         • Rock type (e.g. shales vs. granite)
         • River velocity
         • Water volume
     • Hydarulic action: the sheer force of the water hitting the bed and banks.
     • Abrasion: force of load repeatedly hitting (and scraping?) bed and banks.
     • Attrition: load particles repeatedly hitting thus weakening each other and breaking down..
     • Solution: only occurs when rivers flow over rainwater-soluble rock (e.g. limestone, chalk). Rock dissolves in river.
     • It is affected by:
       • Gradient
       • River velocity
       • Rock type (e.g. sharper, more angular load increases erosional potential)

3. Case studies

4. River features

   • Discharge
     • Measured in cumecs (m3/s)
     • It is affected by:
       • RRPPLT
       • Relief (gradient)
       • Rock type ((im)permeable)
       • Precipitation
       • Previous weather conditions (ground saturation)
       • Land use (rural/urban)
       • Temperature

5. River definitions

   • Source: the place where a river begins
   • Tributary: a smaller river that joins onto the main river
   • Mouth: the place where the river ends as it flows into the sea
   • Confluence: where and 2 rivers/ a river and a tributary meet
   • Drainage basin: the area around the river and its tributaries that is drained by that river.
   • (Line of) watershed: the dividing line between drainage basins)