Drainage Basin Networks and Sources to Sink Flashcards
How does topography determine drainage direction? Give an example
Mountainous regions act as barriers and slopes that guides the flow of water within them. In the USA three rivers are guided by the Rocky mountains, Appalachian Mountains and post-glacier landscape to flow south in to the Mississippi river
What is discharge?
The measure of flow rate within rivers. The amount of water that passes by time unit thorugh a section of river.
What unit is used to measure discharge and how is it calculated?
Cumecs (m^3S^-1) = Area of River (Height x Width) x velocity (ms^-1).
What is bankful discharge?
the maximum volume that a channel can hols without flooding
What are the three determinants of river velocity?
Slope, Depth, Roughness
How do you calculate river velocity using its determinants?
Roughness x square root of (depth x slope)
What is river capture?
When the headward erosion of a river leads in to another channel. It captures that river within it.
What is an example of river capture?
Indus River used to flow through the Ganges but is now flowing through the Indus Fan.
What is river splitting?
This is when a flat plane develops in to two drainage basins following changes of the previously uniform rainfall pattern
Explain how river splitting happens
Initially there is an area which experiences uniform rainfall and tectonic uplift. Changes to the uniform rainfall over an area cause part of a plane to receive more than the other. This means that the same part will experience greater denudation. This will cause the area to rise at different rates creating a ridge of which either side there will be different sloped basins.
What is the most dominant and everyday determinant of changes within drainage basins?
Hydrological Cycle
What does the Hjulstrom curve represent?
The different flow velocities required to erode, transport and deposit different sized sediments along a logarithmic graph.
Explain clay within the Hjulstrom curve?
It initially requires a high velocity to be eroded due to it being so compact and coagulated. However, once release it is very fine and so forms part of the channel and has a very very low deposition velocity so much so that it cannot be represented on the curve
What is transport capacity?
Total amount of sediment that a body of flowing water can hold.
What is the transport rate?
The velocity at which the flow of water can carry the maximum transport capacity
What is the sediment delivery ratio?
the fraction of sediment eroded from the slope that reaches the drainage basin outlet
What tends to happen to sediment delivery ratio as you move downstream and why?
Sediment delivery ratio decreases downstream because there are more places within the channel that can trap and store sediment.
What are some of the parts of the channel that can trap and store sediment?
Floodplains, Braided Channels, Deltas, Meander Bars
How do you calculate sediment delivery ratio?
Sediment Output from Basin / Sediment eroded from slope
What is the sediment to sink concept?
The notion that as water and sediment travels downstream it goes through a ‘jerky conveyor belt’ system. This means that it gets stopped at different point within the system for different durations until it finally finds its way to the mouth. For example, in braided rivers it is only stored for a short period whereas in floodplains it can be stored for ever possibly.
What is Sediment Accommodation Space?
The space available to store sediment.
How does sediment accommodation space vary across the long profile and why?
Upstream there is very little because it is so steep and volatile. Downstream there is a greater accumulation of sediment and water that can carve space.
How can you increase sediment accommodation space upstream artificially?
Dams
What is the main site of study for sediment accommodation space?
Downstream
What are the two determinants of sediment accommodation space downstream?
Sea Level Rise
Tectonic Uplift
How does sea level rise affect sediment accommodation space?
If sea level rises then there is more space available for the sediment to be stored underneath the surface on a continental shelf
How does tectonic uplift affect sediment accommodation space?
In the event of uplift this causes the elevation of the system to rise which increases potential energy increasing erosion which forces downstream to create more space so it can still function
What is the Lane Diagram?
A representation of how changes in the slope of the stream and sediment size and the balance between them can cause either degradation and aggradation
What is the proportional relationship between Qs (sediment discharge) and D (Sediment Diameter) with Q (water discharge) and S (slope) relating to the Lane Diagram?
Qs x D proportional to Q x S
What are the controls on river long profile?
Discharge: Increases downstream
Sediment Load: Increases downstream but at reduced rate due to increased sediment storage
Sediment Size: Decreases in Size
How do rarely occurring straight rivers create meanders?
They deposit material on alternating side of the channel in bars creating an oscillating pattern. this diverts flow to channel sides and increases erosion there leading to a meander
What impact do tropical forests have on the shape of meanders?
Vegetation stabilises flows and causes the winding to bunch up and become more dense in a given area.
How do braided rivers form?
When a channel receives large amount of sediment that it covers up the channel vegetation reducing its depth. Small changes in this huge constant supply cause the channel to produce multiple mini channels.
What sort of river systems are produced in post-glacial landscapes?
Similar to braided rivers due to the lack of vegetation
What are the controls on channel patterns?
Slope - This affects the rate of vertical erosion braider rivers rely upon it to erode and transport the massive amounts of material it receives
Bank Strength - If the banks are strong then this limits lateral erosion and therefore lateral sediment supply.
What are determinants on the bank strength of channels?
Composition - It may be composed of clay which is compact and difficult to erode
Vegetation - This can act as a binder increasing strength
How does long profile aggradation happen?
If aggradation occurs within the channel then this also occurs within the floodplain as it overflows during flooding events where it is then deposited.
How can long profile aggradation downstream lead to channel migration?
The aggradation causes overflowing of the channel and its sediment which creates an alluvial ridge (levee if you will). When the channel floods, some of the water cannot overcome the alluvial ridge and so is forced to form a new channel somewhere else through incision in the floodplain of the original river.
What is avulsion?
Overflowing of a channel’s sediment on to its floodplain that can create an alluvial ridge
Give an example of where a river has migrated due to avulsion?
Kosi Fan since 1736
