Understanding River Systems: Gradient, Discharge, and Base Level

Chapter 5 - River Systems

How rivers and streams work – Gradient (know this!)

• Streamflow
  • Factors that determine velocity
  • Gradient, or slope
  • Channel characteristics
    • Shape
    • Size
    • Roughness
    • Discharge – volume of water flowing in the stream (generally expressed as cubic feet per second)
• Gradient highest nearest river source, where is it lowest?
• Upstream-downstream changes
  • Profile
  • Cross-sectional view of a stream
  • From head (source) to mouth
  • Profile is a smooth curve
  • Gradient decreases from the head to the mouth
  • Factors that increase downstream
    • Velocity
    • Discharge

Base level

Rivers flow and/or erode to the lowest level, ultimately to sea level

• Base level
  • Lowest point a stream can erode to
  • Two general types
    • Ultimate – sea level
    • Temporary, or local
  • Changing causes readjustment of the stream – deposition or erosion
  • Sealevel drop = streams downcut to reach new base level

What can change base level? How do gradients change when base level changes?

What is discharge? Types of sediment load (three types)?

Discharge – volume of water flowing in the stream (generally expressed as cubic feet per second)

The work (energy) of streams results in:

• Erosion (of riverbed)
• Transportation
• Transported material is called the stream’s load
  • Dissolved load
  • Suspended load
  • Bed load

Where along a river channel does a stream move fastest? Slowest? What is a point bar and where does it form?

Meandering vs. Braided streams: what sediment conditions characterize each?

Floodplains: widen by erosion during flood times, sediment deposition occurs on floodplain, and natural levees

What are meanders? Oxbox Lakes? Deltas? (identified in the lab)

• Stream valleys
  • Characteristics of wide valleys
  • Floodplain
  • Features often include
    • Meanders
    • Cutoffs
    • Oxbow lakes

The work of streams

• Transportation
• Features produced by deposition
  • Deltas – exist in ocean or lakes
  • Natural levees – form parallel to the stream channel
  • Area behind the levees may contain back swamps or yazoo tributaries

Depositional Landforms

• Floodplain: part of the stream channel that is occupied during high discharge flows (floods)
• Floodplains widen by river meandering
• Deposition of fine-sized sediment occurs when river overflows channel (natural levees and on floodplain)

What are stream terraces, and how do they evolve?

Flash floods: occur where high rainfall occurs over a small period of time in areas with steep terrain

Regional floods: high rainfall over a long period of time, i.e. Mississippi River drainage basin.

Regional vs. Flash Floods

• Regional Floods (i.e. Mississippi River)
  • Build-up slowly over large areas as a result of snow melt and/or prolonged rainfall
  • Time for warning, evacuations
• Flash Floods
  • High rainfall event in areas of steep topography
  • A single thunderstorm can cause a flash flood

What roles do manmade levees play in flooding?

Floods and flood control

• Engineering efforts
• Artificial levees
• Flood-control dams
• Channelization
• Nonstructural approach through sound floodplain management

Urbanization and floods: Why (and how) does developing land (i.e. paving, roofing) affect flood frequency and the magnitude of flooding? (lab question also)

Urbanization and Floods

• Hydrograph = Graph of discharge vs. time
• Flood magnitude is measured in peak discharge and/or flood height
• Flood frequency and magnitude increases with urbanization
• More pavement and impermeable surfaces leads to increased runoff
• Higher stream levels in shorter periods of time