Rivers and their sediments part 1

Question 1

What produces fluvial?

What produces lacustrine?

Answer 1

Rivers.

Lakes.

Question 2

character of sediment is a function of what?

Answer 2

flow velocity, sediment type and size.

Question 3

when do major velocity changes occur? name 4.

Answer 3

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.

Question 4

Where do the largest bedforms form?

Answer 4

Highest velocity areas so deeper parts of the river in the main channel

Question 5

What do lower velocity areas represent in terms of bedforms?

Answer 5

They show smaller structures representing the larger bedform, so they will have ripples instead of dunes.

Question 6

Large cross bedding is seen in bedforms or the river channel?

Answer 6

Bedforms

Question 7

Release from confinement can leave what shape of feature? what kind of gradient do these have?

Answer 7

A fan shaped body (lobe shape) which has a low gradient (about 4)

Question 8

In an alluvial fan: where it has steep slopes and a small catchment what kind of flow is dominant?

Answer 8

Debris flow

Question 9

In an alluvial fan: where it has lower gradient and a large catchment what kind of flow is dominant?

Answer 9

Stream flow (river)

Question 10

Debris flow is common in what size and shape of alluvial fan?
Stream flow is common in what size and shape of alluvial fan?

Answer 10

Debris flow: Steep slopes and small catchment

Stream Flow: Low angle and large catchment

Question 11

Lobe shaped alluvial fans may have a changing channel pattern, why?

Answer 11

due to river avulsion.

Question 12

Name debris flow characteristics

Answer 12

Matrix supported with fine grained sand within it, often reverse grading.

Question 13

Do gravity driven mass flow deposits need water to help them move? how can these become reverse graded?

Answer 13

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

Question 14

when is most work done in a river?

Answer 14

During flooding

Question 15

What causes water to flow over the banks of a river?

Answer 15

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.

Question 16

what 2 factors change over climatic setting?

Answer 16

Frequency and magnitude

Question 17

Frequency and magnitude change due to what?

Answer 17

Climatic setting

Question 18

Describe the shape of a river if it has a) low sinuosity and b) high sinuosity.

Answer 18

Low sinuosity rivers go straight or wind less whereas high sinuosity rivers meander and wind alot.

Question 19

Discuss what kind of channel and sinuosity braided and meandering rivers have.

Answer 19

Braided rivers: Low sinuosity, multi channel

Meandering rivers: high sinuosity, single channel.

Question 20

what changes a river from meandering to braided?

Answer 20

increased water and stream power

Question 21

sinuosity changes with what?

Answer 21

Slope, the higher the slope the more it meanders q

Question 22

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.

Answer 22

Braided if banks are easily eroded, discharge and slope are high.
Meandering if banks are resistant, discharge and slope are low.

Question 23

braided rivers leave what kind of signature?

Answer 23

sheet-like deposits with roughly

symmetrical x-section, internally complex

Question 24

Discuss high and low flow stage.

Answer 24

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.

Question 25

How would you identify a bar on a sedimentary log?

Answer 25

They have thick cosets which fine upwards. Sometimes have cross bedding or trough cross bedding (from dunes) or evidence of rooting

Question 26

Discuss how a bar forms

Answer 26

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.

Question 27

why do we get pebble lags?

Answer 27

They can be left behind because of the collapse of a river and if the bedload is not moving very fast.

Question 28

conglomerates with mud clasts in them are evidence of what?

Answer 28

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.

Question 29

if a river has switched position it has …..

Answer 29

avulsed

Question 30

when does evulsion tend to occur?

Answer 30

In low sinuosity rivers, it will eat into its own floodplain

Question 31

can bars form on gravels?

Answer 31

Yes if the sediment is very coarse

Question 32

Where do point bars form?

Answer 32

They form where the sediment is getting deposited on the inner bank of a river that is meandering. Lower velocity areas.

Question 33

What is the ultimate signature of a meandering river?

Answer 33

Point bars

Question 34

what are you looking for in a sedimentary log to identify a point bar?

Answer 34

Fining upwards sequence

Question 35

What is lateral accretion?

Answer 35

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.

Question 36

What is IHS (Inclined Heteolithic Stratification)

Answer 36

Strata that is inclined where you get rapid alterations between sands and muds as you go higher up the accretion surface.