Chapter 16: Streams and Floods

The Hydrologic Cycle

  • all water that cycles through various reservoirs (groundwater to atmosphere)

  • the sun drives this cycle from ice-water-water vapor

  • carries products of weathering

  • reservoirs

    • 94% ground water / saltwater

    • 76 % ground water / freshwater

    • 1.76% glacial water (cycle changes based on tilt of earth)

Precipitation: water vapor condenses to form rain or snow

Infiltration: precipitation that falls and interacts with the ground, moving through soil and cracks in rocks

Runoff: precipitation that does not infiltrate, moving along the surface (usually by streams)

Evaporation/Transpiration: loss of water through heat from the surface/plants


The Form of Streams

Stream: ANY flowing body of water CONFINED to a channel moving DOWNHILL

  • river, brook, creek

Channel: main location of flowing water

Valley: area that a stream channel can form. this area will flood

Floodplain: a flat area on either side of the channel; area where normal flooding occurs

Tributary: lesser streams that feed into the trunk of the main stream (MS River has many)

Natural Levees: sediment build-up at the margins of channels caused when a stream overflows its banks

  • out of the channel is lower velocity so largest sediments drop out

  • as a stream floods, it builds up its own system to prevent future flooding

Delta: a depositional platform build of sediments deposited in an ocean or lake at the mouth of a stream (MS River Delta)

  • How are they formed?

    • a dramatic decrease in velocity - sediments drop out of suspension


Stream Patterns

  1. Meandering streams: characterized by low sediment loads and low flow velocities

    • flows on very flat floodplains and forms meanders (curves) and oxbow lakes

    • ex. MS River

point bar: inside of a meander; lowest velocity, so deposition

cut bank: the outside of a meander; highest velocity, so erosion

  • makes the river’s meanders bigger

meanders get so large that it pinches off PB/CB and oxbow becomes a lake while the stream just cuts through to be more efficient.

PB/CB

Oxbow lake: a meander cut-off that is separated from the stream, but still holds water.

  • stream enlarges cut bank

  • meanders get closer together

  • stream cuts new channel (usually during a storm)

  • this isolates the old channel as a lake

oxbow lakes are not permanent… it eventually fills in.

  1. Braided streams: high sediment loads and high flow velocities. stream has many channels, divided into an interlacing network

  • often form in rivers with large volumes of sediment (ex. at the edge of melting glaciers, or mountain valleys) or arid regions (too much sed, not enough water)


Streams as Geosystems

Flood: increased discharge due to imbalance between inflow and outflow

Flood frequency curve: probability that a flood with a certain discharge will occur in any given year (m³/s)

Causes of Flooding:

  • excessive rainfall

  • heavy snow melt

  • human development on flood plain

  • storms (hurricanes)

Controlling flooding:

  • upstream dams or tanks that trap water and release it slowly

  • artificial levees keep water in channel to certain level

  • walls of stone/concrete on channel banks to reduce erosion (cutbanks)

  • bypasses that divert water to the side channel out of main channel