Lecture 17

Question 1

Continental divide

Answer 1

Separates drainage basins that flow into different oceans

Question 2

Factors that influence stream behaviour

Answer 2

Average width and depth of channel

Channel gradient

Average velocity

Discharge

These properties change along the channel

Question 3

Width and depth

Answer 3

Both increase downstream as tributaries join

Width x average depth
=cross sectional area

Flux=cross sectional area x velocity

Question 4

Gradient

Answer 4

• vertical fall divided by horizontal distance (slope)
• typically decreases downstream
• express as m/km (e.g. 60 m/km means it drops 60 km for every m it travels)

Question 5

Velocity

Answer 5

• velocity of flow varies within channel
• slowest at base and sides because of viscous drag
• in a straight channel, fastest flow is at centre
• in a sinuous channel, fastest flow is near outside of bend

Question 6

Discharge

Answer 6

• flux
• volume of water passing a given point in unit time
• equal to cross sectional area x velocity
• increases downstream as tributaries merge
• units: m3/s

Question 7

Summary of downstream variation

Answer 7

• width increases
• depth increases
• discharge increases
• average velocity increases
• gradient decreases

Question 8

Sediment load

Answer 8

Bed load (sand, pebbles, boulders)

Suspended load (silt and clay)

Dissolved load

Question 9

Hjulstorm diagram

Answer 9

Review in notes

Question 10

Bed load

Answer 10

Sediment makes frequent contact with bed and moves by

• rolling
• sliding
• saltation (hopping effect)

Question 11

Suspended load

Answer 11

• small particles settle slowly
• in turbulent water, upward flow is faster than settling speed
• particles are maintained in suspension
• water appears cloudy and opaque

Question 12

Dissolved load

Answer 12

Ions in solution

Cations:
-calcium, sodium, potassium, magnesium

Anions:
-bicarbonate, sulfate, chloride

Ultimately precipitated as chemical sediments