Topic 4 - River process and Pressures Flashcards
What is a drainage basin?
An area of land drained by a river an it’s tributaries.
What are the Features of Drainage basins?
Watershed: Highland or hill that separates one drainage basin from another
Confluence: the point where two rivers/streams meet/join
Tributary: a smaller stream or river that joins a bigger stream or river
Source: the starting point of a river or stream
Mouth: the point where a river leaves the drainage basin and enters the sea
Weathering
Weathering is the breakdown of rock by natural processes. There are three key
weathering processes that affect river valley’s
Physical
(Freeze thaw)
water enters cracks in rocks and freezes when temperatures drop
below zero, the water expands, putting pressure on the rock. This
process of expanding and contracting causes the rick to break
into smaller pieces.
Chemical (acid rain)
slightly acidic rainfall, polluted by factories and vehicles, reacts with weak minerals causing them to dissolve and decay.
Biological weathering
the roots of plants grow in cracks and split the rock apart.
Mass movement
is the transfer of material down the valley/slope due to gravity.
Soil creep
Individual particles soil move slowly down a slope due to gravity
Slumping
At the bottom of a valley slope the river erodes the valley side.
Material above slides downwards rotating as it does often after
times of heavy rain saturating the rock and soil making it heavy.
River erosion
The action of water wearing away rocks and soil at times of flood and on steep
gradients. There are four key processes of erosion.
Abrasion
Load is dragged by water wearing away the banks and bed of the river and causes most erosion.
Attrition
Load collides with load and wears down/breaks up
Solution
Weak acid dissolves rocks such as Limestone
Hydraulic Action
The shear force of the water trapping air in cracks fracturing the rock on the banks and bed of the river
Traction
large, heavy pebbles are rolled along the river bed. This is most common near the source of a river, as here the load is larger.
Saltation
pebbles are bounced along the river bed, most commonly near the source.
Suspension
lighter sediment is suspended (carried) within the water, most commonly near the
mouth of the river.
Solution
the transport of dissolved chemicals. This varies along the river depending on the presence of soluble rocks.
Deposition
When a river loses it’s energy deposition occurs. Heaviest material is deposited first.
Interlocking spurs
At the source rivers have less power and flow around valley slopes (spurs) instead of eroding them. The spurs then inter
lock from one side to the other.
Waterfalls
Occur where water flows over bands of rock with differing resistance. Weaker less resistant rock erodes quicker due to
increased velocity and creates a step in the river bed gradually undercutting the more resistant rock. Continued abrasion and
hydraulic action creates an overhang and a plunge pool is created at the base from abrasion and the force of falling water. Eventually the overhang will collapse and make the waterfall steeper. Repetition causes the waterfall to retreat upstream forming a steep-sided gorge.
Meander
Large bends that swing from side to side (sinuosity) on the floodplain. Faster flowing water erodes the outside of the bend
through lateral erosion creating a steep bank (river cliff) whilst the inside of the bend due to slower shallower water
deposition takes places creating a gently sloping bank (slip-off slope).
Oxbow Lake
When a meander grows its neck narrows then at times of flood the river simply cuts straight through it leaving an old meander
cut off (horseshoe-shaped lake). Deposition blocks up the old bend.
Levees
Levees are natural embankments formed by the deposition of
sediment at times of flood. Large sediment is dropped first as the
river floods onto the floodplain and loses velocity. Smaller
sediment is deposited afterwards and when this process is
repeated the banks get higher forming Levees.
