STREAMS

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STREAMS RIVERS

• Chapter 6

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REVIEW. The Water Cycle
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6.1 Streams and Rivers

• River Systems
• (structure)
• Tributary- a stream that
• runs into another stream
• or river
• River system-
• a river and all of its
• tributaries

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• Drainage basin, or watershed- all the land
  that drains into the river either directly or
  through its tributaries
• ex) Mississippi River system
• Divide- high land that
• separates one drainage
• basin from another
• Ex) Continental Divide in
• the Rocky Mountains

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Characteristics

• Velocity- distance that water travels in a given
  amount of time
• Related to the amount of energy that the water
  has
• Erosion occurs more quickly
• Other characteristics affect the velocity
• Gradient, Discharge, and Channel Characteristics
• Gradient- steepness of the slope of a stream or
  river
• Steep at its source gradual at sea level

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Characteristics continued

• Discharge- the amount, or volume, of water that
  passes a certain point in a given amount of time
• Not constant over length of a river
• Increases downstream as tributaries add more
  water
• Not constant year-round- increased
  precipitation/melting snow
• Channel- the path through which the water flows
  in a stream or river
• Shallow, winding stream with many boulders has
  contact with surface area that causes it to slow
  down by friction
• Straight channel that is wide and deep has less
  surface area in contact with water, so velocity
  is greater
• Velocity is greatest at the top center of the
  stream
• When it curves, zone of max. speed shifts to the
  outer bank
• SEE FIGURE 3 (p.160)

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Different types of Stream Channels
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Base Level

• Base Level lowest point to which a stream can
  erode its channel.
• Base level is usually the level at which the
  mouth of a stream enters the ocean or the next
  body of water.
• Temporary Base Level vs. Ultimate Base Level
• Temporary lake Ultimate sea level
• A stream in a broad,
• flat-bottomed valley
• that is near its base level
• often develops many meanders.

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6.2 Stream Erosion and Deposition

• Erosion
• Running water wears down Earths surface by
  breaking up bedrock and by removing eroded rock
  and soil materials
• Breaks up materials by mechanical means
• Abrasion mainly in time, creates rounded
  boulders, pebbles, and sand grains from the
  cutting tools

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• Deposition-
• the process by which materials are deposited
• Occurs because the river or stream no longer has
  enough energy to transport them

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Stream Flow Erosion

• There is more stream velocity in the middle/top
  of the stream
• Erosion will be greater in the middle of the
  straight stream (increase depth)
• Deposition will occur on the stream/river banks
  where the channel turns and the velocity of the
  waterway drops
• SEE FIGURE 3 (p.160)

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Erosion Deposition Features in an Old
Stream/River
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Erosion cont

• Potholes-
• deep oval or circular basins formed when water
  in a river develops small whirlpools, and
  materials grind potholes in the bedrock

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• Plunge pool-
• a basin that has been
• worn away at the base
• of a waterfall by the
• action of falling water
• Dunn River Falls
• Chemical weathering
• consists of dissolving soluble minerals
• Calcite is dissolved, which may form pits and
  holes in the riverbed
• Widens existing cracks and holes

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Depositional Features

• Delta- a fan-shaped deposit that forms when a
  river flows into a quiet or large body of water
• River water comes to almost a standstill at a
  delta
• Distributaries- branches formed by rivers flowing
  over its delta
• Responsible for
• deltas shape
• Formation is
• delicate balance
• between deposition
• and erosion

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• Alluvial fan- fan-shaped deposit
• May form when a steep mountain stream meets dry,
  level land at the base of a mountain
• Differs from a delta
• Deposit is formed on land, not in water
• Sediments are coarse sands and gravels rather
  than fine silt and clay
• Surface is sloping,
• not flat like that of a
• delta

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Transported Material

• Load- the eroded rock and soil materials that are
  transported downstream by a river
• Suspension- a state in which rock materials
  carried by a river are stirred up and kept from
  sinking by the turbulence of stream flow
• Materials are heavier than water, but the
  turbulence keeps them stirred up
• The faster a stream flows, the more turbulent and
  muddy it becomes
• Bedload- sand, pebbles, and boulders that are
  moved along the bed of a stream and that are too
  heavy to be carried in suspension

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Transporting Materials

• Two measures are used to describe the ability of
  a stream to transport materials
• Competence- a measure that describes the maximum
  size of the particles a stream can carry
• Capacity- a measure of the total amount of
  sediment a stream can carry
• Depend on the velocity and discharge
• Vary along a stream and change throughout the
  year
• High velocity High discharge large amount of
  sediment and larger sizes of sediment particles

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Stream Deposition

• River will deposit part of its load when either
  velocity or discharge decreases
• Velocity may decrease if channel widens or river
  meets an obstruction (curving bank/rock outcrop)
• Speed decreases at the inside of the curve and
  when the river empties into the sea- deposits the
  most material
• Discharge and velocity increase during a flood,
  then decrease as flood waters subside

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Meanders write your own notes!

• Erosion occurs along the OUTSIDE of a meander
• because the water is FASTER there (more
  velocity, more energy to erode)
• Deposition occurs along INSIDE curves, (its
  slower water cannot transport sediment any
  longer)

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Stream Valleys

• Narrow valleys
• narrow V-shaped valley shows
• that the streams primary work
• has been down cutting toward
• base level
• Rapids and waterfalls
• Wide valleys
• Downward erosion is less dominant
• More energy is directed side to side
• Floodplain- the flat, low-lying portion
• of a stream valley subject to periodic flooding

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Linville GorgeCarved out of the bedrock by the
river forming a river valley.
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Floodplains

• Streams that flow on floodplains move in meanders
• Cutoffs- shorter channel segments
• Oxbow lakes- abandoned bend of a river/stream

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Oxbow Lake Formation

• Meander forms (because the stream is close to
  base level)
• Far side of curves erode faster, inching closer
  closer together.
• Curves join, and water moves past the bypass loop
• Deposition occurs at the ends of the loop
  (because velocity drops), cutting off the oxbow
  lake.

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Floods

• Most caused by rapid spring snow melt or storms
  that bring heavy rains over a large region
• Ex Mississippi River in 1993
• Measures to control flooding
• Artificial Levees- earthen mounds built on the
  banks of the river
• Flood-control dam- store floodwater and then let
  it out slowly
• Limits on floodplain development- minimizing the
  development on floodplains

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Flood plains Levees

• Developing in flood plains causes great economic
  loss.
• What can we do to help this??

• Build smaller levees farther apart
• Less expensive to build
• Improves habitat in the flood plain.

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