Rivers and their sediments part 1 Flashcards
What produces fluvial?
What produces lacustrine?
Rivers.
Lakes.
character of sediment is a function of what?
flow velocity, sediment type and size.
when do major velocity changes occur? name 4.
During a release from confinement and when going overbank. Both of these have a rapid deceleration and deposition.
Flowing into standing water causes rapid deceleration
Tributary inflows increase in water and flow velocity increased erosive power also.
Where do the largest bedforms form?
Highest velocity areas so deeper parts of the river in the main channel
What do lower velocity areas represent in terms of bedforms?
They show smaller structures representing the larger bedform, so they will have ripples instead of dunes.
Large cross bedding is seen in bedforms or the river channel?
Bedforms
Release from confinement can leave what shape of feature? what kind of gradient do these have?
A fan shaped body (lobe shape) which has a low gradient (about 4)
In an alluvial fan: where it has steep slopes and a small catchment what kind of flow is dominant?
Debris flow
In an alluvial fan: where it has lower gradient and a large catchment what kind of flow is dominant?
Stream flow (river)
Debris flow is common in what size and shape of alluvial fan?
Stream flow is common in what size and shape of alluvial fan?
Debris flow: Steep slopes and small catchment
Stream Flow: Low angle and large catchment
Lobe shaped alluvial fans may have a changing channel pattern, why?
due to river avulsion.
Name debris flow characteristics
Matrix supported with fine grained sand within it, often reverse grading.
Do gravity driven mass flow deposits need water to help them move? how can these become reverse graded?
No they dont need water to move. They become reverse graded because the finer sediment can fall through the rocks and end up at the bottom
when is most work done in a river?
During flooding
What causes water to flow over the banks of a river?
When the amount of water flowing down the river has increased; by lots of rainfall etc. If the banks are resistant to erosion and cannot widen to support more water then it forces the water to go over the banks.
what 2 factors change over climatic setting?
Frequency and magnitude
Frequency and magnitude change due to what?
Climatic setting
Describe the shape of a river if it has a) low sinuosity and b) high sinuosity.
Low sinuosity rivers go straight or wind less whereas high sinuosity rivers meander and wind alot.
Discuss what kind of channel and sinuosity braided and meandering rivers have.
Braided rivers: Low sinuosity, multi channel
Meandering rivers: high sinuosity, single channel.
what changes a river from meandering to braided?
increased water and stream power
sinuosity changes with what?
Slope, the higher the slope the more it meanders q
If a river has resistant banks is it more likely to be meandering or braided?
If a river has erosive banks is it more likely to be meandering or braided?
Discuss the discharge and slope on both of these.
Braided if banks are easily eroded, discharge and slope are high.
Meandering if banks are resistant, discharge and slope are low.
braided rivers leave what kind of signature?
sheet-like deposits with roughly
symmetrical x-section, internally complex
Discuss high and low flow stage.
High stage: where the work is done, the river is eroded and bars form.
Low stage: where reworking occurs, channel is exposed. Cross beds can be apparent here because of the reworking.
How would you identify a bar on a sedimentary log?
They have thick cosets which fine upwards. Sometimes have cross bedding or trough cross bedding (from dunes) or evidence of rooting
Discuss how a bar forms
Bars are low relief mounds of sediment which build up in a river as they move downstream, the downstream end of the bar sticks out the most. They can be bank attached or not bank attached.
why do we get pebble lags?
They can be left behind because of the collapse of a river and if the bedload is not moving very fast.
conglomerates with mud clasts in them are evidence of what?
The mud clasts are there because the banks collapsed into the river, this may be all you see of the floodplain because it has been eroded.
if a river has switched position it has …..
avulsed
when does evulsion tend to occur?
In low sinuosity rivers, it will eat into its own floodplain
can bars form on gravels?
Yes if the sediment is very coarse
Where do point bars form?
They form where the sediment is getting deposited on the inner bank of a river that is meandering. Lower velocity areas.
What is the ultimate signature of a meandering river?
Point bars
what are you looking for in a sedimentary log to identify a point bar?
Fining upwards sequence
What is lateral accretion?
When sediment is accumulating on the low angle sides of a point bar, the bar is accreting (building) sideways across the river. Lateral accretion is a process.
What is IHS (Inclined Heteolithic Stratification)
Strata that is inclined where you get rapid alterations between sands and muds as you go higher up the accretion surface.
