Continental divide
Separates drainage basins that flow into different oceans
Factors that influence stream behaviour
Average width and depth of channel
Channel gradient
Average velocity
Discharge
These properties change along the channel
Width and depth
Both increase downstream as tributaries join
Width x average depth
=cross sectional area
Flux=cross sectional area x velocity
Gradient
- 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)
Velocity
- 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
Discharge
- 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
Summary of downstream variation
- width increases
- depth increases
- discharge increases
- average velocity increases
- gradient decreases
Sediment load
Bed load (sand, pebbles, boulders)
Suspended load (silt and clay)
Dissolved load
Hjulstorm diagram
Review in notes
Bed load
Sediment makes frequent contact with bed and moves by
- rolling
- sliding
- saltation (hopping effect)
Suspended load
- small particles settle slowly
- in turbulent water, upward flow is faster than settling speed
- particles are maintained in suspension
- water appears cloudy and opaque
Dissolved load
Ions in solution
Cations:
-calcium, sodium, potassium, magnesium
Anions:
-bicarbonate, sulfate, chloride
Ultimately precipitated as chemical sediments
