2.2 Rivers Flashcards
The hydrological cycle:
- Drainage basin: the area of land drained by a river
- Watershed: the edge of highland surrounding a
drainage basin, marking the boundary between two
drainage basins - Source: the beginning or start of a river
- Confluence: the point at which two rivers or streams
join - Tributary: a stream/smaller river which joins a larger
stream or river - Mouth: point where river comes to end, usually when
entering sea
River processes:
Drainage basins act as a system with:
- Inputs: precipitation
- Transfers: infiltration, percolation, surface runoff,
throughflow & groundwater flow
- Stores: interception, surface storage, soil moisture
storage & groundwater storage
- Outputs: evaporation & transpiration or
evapotranspiration
Characteristics of the River:
- Bed
- Bank
- Width
- Depth
- Speed of flow
- Wetted perimeter
- Channel
- Thalweg
Purpose of river characteristics:
- Bed: The bottom of the river channel
- Bank: The sides of the river channel
- A river has two banks - Width: The distance between the two banks of a river
- Depth: The distance from the water surface to the bed
of a river - Speed of flow: how fast the water in a river is moving;
different speeds arise at different parts
of the river - Wetted perimeter: length of bed and banks in contact
with river - Channel: The route course (between bed and banks)
that a river flows. The flow of the river is
often described as channel flow. - Thalweg: The fastest part of the river, always near the
middle of the river channel, where there is
least friction
Erosion stages:
- Attrition
- Hydraullic Action
- Abrasion
- Solution
Erosion:
- Attrition: large particles such as boulders collide and
break into smaller pieces; occurs at higher part of river - Hydraulic action: the sheer force of the river dislodges
particles from its banks and bed - Abrasion: smaller particles rub against the river banks
and bed like sand-paper; occurs at low part of river - Solution: acid in rivers dissolve rocks; occurs at any
part of river
Transportation stages:
- Traction
- Saltation
- Suspension
- Solution
Transportation:
- Traction: rolling stones along the bed
- Saltation: small particles bounce along bed in a
leapfrog motion - Suspension: silt and clay-sized are carried within the
water flow - Solution: minerals dissolve in the water
Deposition:
- When a river lacks the energy to carry its load; it begins
depositing the heaviest particles - Happens when there is less water or where the current
slows down - Large boulders are deposited at the top, and very small
particles are deposited at the end, resulting in sorting
River profiles:
Upper
Middle
Lower
Upper (of river profile):
- long profile
- cross profile
Long profile:
Steeply sloping towards the lower sections of the river
Cross profile:
Steep sided v-shaped valley, thin river channel, deep in places
Middle (of river profile):
- long profile
- cross profile
Long profile:
Shallow slopes towards the mouth of the river
Cross profile:
V-shaped valley remains with a wider valley floor, river begins to meander, channel is wider and deeper
Lower (of river profile)
- long profile
- cross profile
Long profile:
Almost at sea level, gently sloping to its mouth
Cross profile:
Wide, shallow valley, with large flood plains and meanders; channel is wide deep and smooth sided.
Landforms:
- V-shaped valley
- Potholes
- Meanders
- Waterfalls
- Ox-bow lakes
- Levees
- Deltas
- Flood plain
V-shaped valley:
(landforms)
- Is narrow with a narrow, shallow river channel
- Has steep sides
- Channel has a steep gradient
- Water is mainly slow flowing
- Load is mainly large, angular and rough
Potholes:
(landforms)
Can be found in the upper & middle valley
where a river flows over solid rock
Meanders:
(landforms)
- Wide sweeping bends found in the lower part of the
river - They are formed by a combination of lateral erosion &
deposition
Waterfalls:
(landforms)
- They occur because the river flows over hard rock
which erodes slowly - Beneath is softer rock which is eroded faster to form
a “step” - The force of the water erodes the bottom of the
waterfall to form a plunge pool - The hard rock gets undercut as the soft rock erodes
so that it eventually collapses
Levees:
(landforms)
when a river floods, the coarsest material is
deposited first, on the edges of the river, forming a
natural embankment called a levee
Deltas:
(landforms)
- Deltas occur where a river that carries a large amount
of sediment meets a lake or the sea - This meeting causes the river to lose energy and drop
the sediment it is carrying - Deltas form where river mouths become choked with
sediment, causing the main river channel to split into
hundreds of smaller channels or distributaries
Flood plains:
(landforms)
- Area of alluvial deposits found beside the river in its
lower course - As meanders move slowly down the course of the
river they erode the valley to create a wide valley floor - Deposits layers of alluvial material on the slip off
slopes building up into a large flood plain
Causes of river flooding:
- Steep-sided channel
- Lack of vegetation or woodland
- Drainage basin, consisting of mainly impermeable rock
- Drainage basin in an urban area
- Deforstation, overgrazing and overcultivation, and
population pressures
Steep-sided channel:
a river channel surrounded by steep slopes causes fast surface run-off.
Lack of vegetation or woodland:
surface run-off will be high as trees and plants won’t intercept precipitation.
Drainage basin, consiting of mainly impermeable rock
water cannot percolate through rock layer, and will
runoff surface
Drainage basin, in urban area:
these consist largely of impermeable concrete, which encourages overland flow.
Deforestation, Overgrazing and Overcultivation, and population pressures:
cause soil erosion causes sediment to go into rivers decreasing the cross-sectional area
Floods:
Pros and Cons
Pros:
- Recharges groundwater stores
- Alluvium (mat deposited by rivers) is deposited on
floodplain, good for farming
- Deposition from increased river discharge can make
new land
- Pollutants washed off of land
Cons:
- People can be killed
- Homes can be destroyed
- Spread of water borne diseases
- Shortage of clean water and food
- Infrastructure damaged
- Businesses can be destroyed
- Fires can occur due to electricity & water
Flood management techniques:
- Dams
- Afforestation
- River engineering
- Managed flooding
- Planning
Dams:
- Built across a river to control the amount of discharge
- Water is held back by the dam in a reservoir
- Released in a controlled way to control flooding
- Is expensive to build, can affect farmers & cause.
erosion downstream
Afforestation:
- Trees planted near to the river
- Greater interception of rainwater
- Lower river discharge
- Relatively low cost option, enhances environmental
quality of the drainage basin
River engineering:
- Channel widened/deepened to carry more water
- Channel straightened so water travels faster
- Course altered to divert floodwater away from homes
- Alterations may lead to a greater risk of flooding
downstream, as the water is carried there faster
Managed flooding:
Allow river to flood naturally in places, to prevent flooding in other areas
Planning:
- Authorities & government introduce policies to
control urban development near/on floodplain
reducing risk of flooding & damage to property - Enforcing regulations may be difficult in LEDCs
