Mississippi Watershed and River Dynamics

Mississippi Watershed

  • Area of land where all incoming precipitation drains to a common outflow point.
  • Feeds small streams.

Discharge

  • Volume of water passing through a channel section over a given time.
  • Calculated as the Area of wetted channel multiplied by the velocity of water.

Velocity Calculation

  • V = (R^{2/3} * S^{1/2}) / n
    • Where:
      • V = Velocity.
      • R = Hydraulic radius.
      • S = Slope of the water surface.
      • n = Manning's roughness coefficient.
  • Deeper and steeper channels result in faster flow.
  • Rougher channels result in slower flow.

Rivers of Sediment

  • Channels carry water and sediment from upstream.
  • Form and function depend on hydrology and sediment load.

Sediment Transport

  • Suspended: Moving along within the flow medium.
  • Saltating: Bouncing along the bed.
  • Rolling: Rolling along the bed.
  • Sliding: Sliding along the bed.

Channel Dynamics

  • Cutbank: Flow erodes sediment on the outside of bends.
  • Point Bars: Flow deposits sediment on the inside of bends.

Floodplains

  • Provide flood storage, attenuation, and sediment storage.
  • Support productive and diverse ecosystems.
  • Constantly replenished by floods and channel migration.

Mississippi River Sediment Load

  • Carries >200 million tons of sediment per year.

Avulsions

  • Abrupt change in the course of a river.
  • The Mississippi River avulses approximately once per 1,000 years.

Gradient

  • Defined as rise over run.

Levees

  • Earthen embankments built parallel to river channels to protect areas from flooding.
  • Larger levees are more destructive when they fail.
  • Reduce the width of the flood-prone area.
  • Lead to deeper peak flows that are steeper and faster.

Upper Mississippi River Modifications

  • Transformed into a series of flatwater steps to maintain minimum depth for barges.

Dams

  • Have cut half of the sediment supply from downstream channel and Gulf of Mexico.

River Regulation

  • The Mississippi River is controlled and regulated by dams and river engineering structures.
  • Benefits include flood control, recreation, and navigation.
  • However, there are also negative impacts:
    • Transformed hydrologic, sediment transport, geomorphic, water-quality, and ecological characteristics of the river.
    • Reduced overall channel capacity.
    • Increased flood stage.
    • Unsustainable conditions.