L14: Surface Water

Question 1

River morphology

Answer 1

Refers to the shape of a river channel and the evolution of the channel shape over time

Question 2

What are the 2 subtypes of river morphology?

Answer 2

• Bedrock channels

- Alluvial channels

Question 3

Bedrock channels

Answer 3

• Places where the river is directly incised in bedrock, and are typical of the headwaters (higher gradient) region of a river
  - Tends to be further up near headwaters; steeper
• Typically form a series of pools and falls, and rock is removed both by physical erosion to form alluvium/sediments and chemical dissolution of the underlying rock
  - Tend to be v shape
  - Erosion is intense to some extent bc high gradient
  - Constant chemical dissolution of rock

Question 4

Alluvial channels

Answer 4

• Made up of unconsolidated sediment – called alluvium. These channels are normally further downstream in lower gradient environments, and the channel morphology changes more rapidly than for bedrock channels. There are two primary types of alluvial channels: braided channels and meandering channels
  - Bed will be made over alluvial sediments (come from erosion upstream)
  - Can move back and forth; physical position of the river can move
  - Graded and meandering = the 2 types of alluvium channels

Question 5

Braided channels

Answer 5

• Comprised of multiple channels separated by bars typically comprised of relatively coarse material (sand and gravel)
• Channels and bars change position and size as the stream erodes and deposits sediment

Question 6

Where do braided channels typically form?

Answer 6

1. Discharge is variable overtime

2. Banks are easily eroded

Question 7

Where are braided channels common?

Answer 7

• Common in glacier-fed rivers
  - Head down gradient; kess steep
  - Sand and gravel = close to mountains

Question 8

Meandering channels

Answer 8

• In meandering rivers, the channel forms large sinuous loops – each bend is a “meander”
• Most common in fine-grained alluvium (clays and silts, primarily), and in low gradient channels
• Coarser bed material is located on the outside of meanders, where flow is faster
• Meanders migrate downstream over time due to active erosion and deposition processes in the river channel. If the migration of a meander slows due to a more resistant bank material and resulting slower erosion, the meander behind it may catch up, leading to the formation of an oxbow lake.

Question 9

Erosion on outer bank causes . . .

Answer 9

• causes cutbank and downstream migration of meander
• Erosion on the outside must move faster
• Since water is faster on the outside, rocks are able to be picked up and eroded

Question 10

Depostion on inner bank forms . . .

Answer 10

Pointbar

Question 11

Rivers

Answer 11

sit in river valleys, which include the river channel and the region nearby that feeds water to the river. Bedrock channels usually sit in deep, narrow, V-shaped valleys, while alluvial channels usually sit in wide, shallow valleys.

Question 12

Lateral (side-to-side) erosion

Answer 12

• caused by the migration of meanders causes the widening of shallow valleys. This feature is called the floodplain.
• Rapids - steep slows down

Question 13

Flood

Answer 13

occurs when a river’s discharge exceeds the capacity of its channel

Question 14

Natural levees

Answer 14

embankments of sediment along the sides of the river – form due to repeated flooding.

Question 15

During floods, river discharge is ________ and the river is carrying ________ amounts of sediments.

Answer 15

• High

- Elevated

Question 16

As water exceeds the banks of a river and moves laterally, it __________, depositing the sediments in the _________. Most of the sediments are deposited near the river, forming _________.

Answer 16

• Slows down
• Floodplain
• Levees

Question 17

Flood prediction and magnitude

Answer 17

Notebook

Question 18

How may downcutting occur?

Answer 18

1. sea level falls
2. tectonism removes an obstacle 3. river discharge increases
3. River discharge increases

Question 19

Terraces

Answer 19

• Many rivers alternate between lateral erosion Within the current floodplain, and downcutting
• Downcutting may leave old floodplains as terraces.

Question 20

Alluvial fan

Answer 20

• fan-shaped body of alluvium at base of upland area

Question 21

Delta

Answer 21

a triangular shaped deposit formed when a stream enters standing water of a sea or lake

Question 22

Lakes

Answer 22

• a standing bodies of water filling depressions on land.

- In general, an obstacle to flow is necessary to produce a lake

Question 23

What can cause lakes?

Answer 23

• Glacial erosion and deposition of sediments
• Volcanic activity (for example, crater lakes)
• Tectonism
• Deposition and erosion of sediment by water (e.g. oxbow lakes in meandering river systems)

Question 24

Most lakes are geologically ___________.

Answer 24

• short-lived.

- Lakes can be open or closed

Question 25

Open lakes

Answer 25

have inlet and outlet streams, which causes the water level in the lake to stay relatively constant in the short term. The sediments in open lakes are typically muds, with sands near shore.

Question 26

Closed lakes

Answer 26

have no outlet stream. This causes water levels to fluctuate due to evaporation. The sediments formed in closed lakes typically include salt deposits formed by evaporation

Question 27

Oxbow Lakes

Answer 27

After the cutoff, silt and sand are deposited across the ends of the abandoned channel, producing an oxbow lake. The oxbows fill with fine sediment and organix matter produced by aquatic plants and eventually turn into swamps

Question 28

Natural Levees

Answer 28

Created during overbank flooding, when sand and silt are deposited enxt to the channel creating belts of higher land on either side of the channel creating belts of higher land on either side of the channel. Deposition is heavier closest to the channel, so the levee surface slopes away from the channel

Question 29

Floodplains

Answer 29

The meandering river channel dominates the flood plain, along with stretches of abandoned channels. These were abandoned after cutoff events, when the river cuts across a meander loop to create a shorter, more direct path