2.2 - Rivers Flashcards
What is a drainage basin? Describe 7 features of one:
-drainage basin (area that is drained by a river)
-watershed
-source
-tributary (smaller, adjoining river)
-trunk stream (main channel)
-confluence (joining of 2 rivers)
-flood plain (land next to river which floods, made of alluvium)
-mouth
What is a watershed?
a ridge that surrounds a drainage basin
Describe drainage density and its formula:
DD = total length of all rivers ÷ area of DB
-high DD, high chance of flash floods (water can get into river faster)
Drainage basin’s flowchart:
What is discharge?
volume of water carried by a river
Describe the processes of transportation in a river:
-traction (rolling)
-saltation (bouncing)
-suspension (floating)
-solution (dissolved)
Describe the processes of erosion in a river:
-abrasion (wearing away, sandpapering effect)
-attrition (loose rocks impacting each other)
-hydraulic action (force of water impacting bed/banks)
-solution (rocks dissolve in water over time)
Describe the courses involved in a river’s journey from source to mouth:
-upper (V valley, waterfalls, rapids, interlocking spurs)
-middle (meanders, floodplains)
-lower (oxbow lakes, floodplains, levees, deltas)
Long profiles are used to portray this information
How does a river change as it flows downstream?
(through the upper/middle/lower course)
-wider, deeper, less steep, more sloping sides
-higher discharge, higher sediment load
Why does a river’s depth and width usually increase downstream?
-increased discharge (from tributaries joining trunk stream)
-beds and banks are eroded more by increased HA and abrasion
How are waterfalls formed?
-less resistant rock (clay) underneath more resistant rock (limestone)
-undercut by erosional process (HA and abrasion)
-overhang develops
-overhang gets too big, collapses into plunge pool (area under waterfall)
-process repeats, waterfall retreats (forms gorge)
What are river potholes and how are they formed?
round holes found in a river bed
-sediment scours bed and forms depressions in river bed
-turbulent flow causes pebbles (grinders) inside it to spin and erode the depression deeper by abrasion
-hole largens, larger material can get trapped and support further erosion
Explain how a meander/oxbow lake is formed:
-obstruction in river so it flows faster around it
-outside of bend eroded faster, forms river cliff
-inside eroded slower, more deposition, forms slip-off slope
-meander erodes laterally (outside of bend undercuts river cliff), causes collapse and retreat, meander gets bigger
-neck narrows and breaks through, cutting off an oxbow lake
-former meander sealed off by deposition
Describe a meander cross section:
-asymmetrical channel
-slip off slope on inside, river cliff on outside
-inside is shallow (more deposition), outside is deepest (more erosion)
-thalweg (fastest river flow)
What is a delta?
a depositional landform that is made when the deposition rates (of alluvium) exceeds erosion rates at the mouth of a river where it flows into an ocean/estuary/lake
Describe the structural features of a delta:
Not the layers
-a braided channel
-distributaries (river splits up)
-deltaic plain (made of alluvium, clay/silt/gravel/sand)
-eyot (sediment island)
What kinds of deltas are there? Give an example for each:
-arcuate (Nile)
-bird-foot (Mississippi)
-cuspate (Ebro, Spain)
Explain how a delta/braided channel is formed:
Name the layers
-deposition rates exceed erosion rates
bottomset - fine clay/silt flung out by turbidity current
foreset - coarse gravel/sand, built by avalanching
topset - coarse build by fluvial deposition
-eyots build up and form distributaries and braided channels
Mention deposition rates in any delta formation question
Requirements for a delta to form:
-deposition rates must exceed erosion rates
-high sediment load
-river + sea water densities must be different
-no strong sea currents
How are levees + floodplains formed?
-when river floods, velocity drops (increased friction)
-coarse material deposited closer to banks, creates a taller levee
-fine material deposited further out across floodplain, creates sloped levees
-after many floods, layers of the rich + fertile alluvium builds up the floodplain + levees
Pros and cons of larger levees:
-increases channel capacity, holds more water in future so it needs more to flood, so people can build on the flood plain
-larger flood risk as there is now more water
What are some benefits of living near a river?
-fishing
-transportation of people/materials
-flat, fertile land (good for farming)
-water supply
Why might living near a river be a problem?
-flood risk
-difficulty of crossing
-erosion of land
Suggest 4 hazards caused by flooding:
-damage to buildings/roads/possessions (eg cars)
-loss of water/electricity/gas supply
-risk of drowning
-spread of disease (cholera)
Draw a storm hydrograph:
bars are rainfall, line is discharge
What is the danger of having a shorter lag time and why?
-higher chance of having a flash flood
-rainfall gets into river too fast and floods
Give some reasons as to why a flash flood might happen:
-short-lived but intense rainfall
-short lag time (water gets into river faster)
-shortage of permeable surfaces (eg in an urban area with lots of concrete)
-higher drainage density
Why do pebbles generally get smaller downstream?
-added discharge at confluences from tributaries
-increased force of HA on the rocks, making loose rocks impact each other more frequently (more attrition)
How does a culvert work?
-culvert restricts width of river upstream so less water gets to the town
-backed-up water is kept in field storage away from the town
Name 2 human features of a river:
-bridges/roads over it
-marinas
Suggest why most of the major flooding occured at Cape Girardeau:
-further downstream and several tributaries have joined the Mississippi, higher discharge
-likely to have a large flat flood plain around the river
