Fluvial Systems Flashcards
how much of earth’s weather is in stream channels?
0.001%
what is the annual volume of stream flow?
up to 36,000 km3
what is the most important agent of erosion?
rivers
why are rivers important?
Major link in hydrologic cycle Dominant agent of landscape alteration Human history ? Interpretation of ancient rocks Potential hazard e.g. floods
why are fluvial systems open?
inputs and outputs of water and sediment
what are the components of a fluvial (river) system?
1) Collecting basin: precipitation into tributaries in headwater region (catchment, watershed)
2) Transporting system: via slope runoff & groundwater into channel network
3) Dispersing system: output to deltas, lakes, ocean … deposition is dominant
what are some examples of North American drainage basins?
Mississippi
rio grande
which rivers have the largest fluvial sediment output?
colorado, Mississippi, ganges
what is the fluvial sediment output of the Fraser river?
17x10^6 tons/year
What controls river form and function at a regional scale?
climate
geology
topography
vegetation
What controls river form and function at the drainage basin scale?
flow discharge (Q, measured as m3/s) [volume of water flowing past a certain point in a given time period] sediment load slope (stream gradient) base level (elevation at the outlet) land use activities
All rivers ‘grade’ to achieve …?
dynamic equilibrium
how does the form of a river change from head to mouth
steep -> shallow gradient
what is base level?
lowest elevation that a river ‘grades’ to
how does a river get a new equilibrium profile?
(1) Profile disrupted
(2) Headward erosion; downstream deposition
(3) New profile - equilibrium
dams affect the….
equilibrium profile
result -> silting
what is the Niagara Falls escarpment an example of?
a river working to achieve equilibrium
how much has headward erosion at Niagara Falls increased the valley length by?
11.5 km in 12,300 years
what can headward erosion lead to?
stream piracy and beheading
how do rivers change overtime?
Continual base level change, deposition and downcutting
what does the amount of sediment moved depend on?
discharge, velocity
gradient
sediment available
what is stream competence?
ability to transport a certain size of sediment particle
what is stream capacity?
ability to transport a certain volume of sediment
what are the types of fluvial sediment transport?
(1) bedload: rolling, sliding, saltation (bounce) of coarse particles
(2) suspended load: fine particles in suspension
(3) dissolved load: dissolved material in solution
what is erosion?
net removal of material
How do rivers erode the landscape?
Removal and transport of regolith (weathered rock & soil)
Down cutting of valleys due to abrasion by load
Headward erosion of channel network
what are the main channel pattern types?
Straight
Meandering
Braided
in a straight channel where is flow fastest?
in the centre of the river
in a meandering channel where is flow fastest?
on the outside of the bend
what is velocity pattern controlled by?
curvature
what will a meandering river create?
cut banks, point bars, oxbow lakes
what are examples of meandering rivers?
Colorado River
Nowitna River, Alaska
what are the characteristics of a braided channel river?
Multiple channels
High sediment load
Unstable bars/islands
Strong seasonal changes in flow
what are some Canadian examples of braided channel rivers?
Mackenzie, Yukon, St. Lawrence, Ottawa and Bow Rivers
where are braided streams common?
in systems of high sediment supply from glaciers
what are alluvial fans?
Fan-shaped deposits where rivers emerge from uplands into a broad valley or plain
what is a delta?
Deposition at mouth where rivers flow into sea, lake, reservoir or estuary (i.e. into a larger water body)
e.g. Mississippi delta, fraser delta
what is the most frequent natural disaster?
flooding
why does natural flooding occur?
Prolonged rainfall or high spring snowmelt
- > flow discharge (Q) exceeds channel banks
- > Levees and deposits in floodplain
what is ‘Recurrence interval’ (R, yrs)?
average time between floods of a given size
100-yr flood = 1% chance in any given year…(not every 100 yrs!)
50-yr flood = 2% chance…
10-yr flood = 10% chance…
2- yr flood = 50% chance…
what was the Red River flood (Manitoba, 1997) caused by?
spring melt of high snowpacks, saturated soil from heavy rainfall & a major storm
How do human activities influence river systems?
Alter most factors that control river form and function:
Amount & timing of discharge
Amount of sediment load
Slope & base level
what is flooding encouraged by?
Paved surfaces - increase amount and rate of surface runoff
Bridges, concrete banks - constrict natural flow of floodwaters
What percentage of rainfall is surface run-off in a forested area?
0-20%
What percentage of rainfall is surface run-off in a urban area?
90-100%
