Running Water
Behavior of Streams
Man’s Interaction with Streams
Why Study Streams?
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Water is the most important erosional agent in modifying the Earth’s surface
Source of water for industrial, domestic, and agricultural use
8% of electricity used in North America is generated at hydroelectric plants
Streams are major transportation and commercial routes
One of the prime areas where mankind interacts with natural processes
Where’s the Water?
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71% of Earth’s surface is covered by water
Of the 1.36 billion km3 of water on Earth
Running Water
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Laminar flow—Flow lines are parallel
Turbulent flow—Flow lines are not parallel
Channel flow—Surface runoff confined to long, trough like channels
Sheet flow—A more or less continuous film of water flowing over the surface
Introduction
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Any surface water whose flow is confined to a narrow topographic depression is a stream
Collecting Area
o Watershed or drainage basin
o Bounded by drainage divides
o Separates direction water flows
o Most famous is Continental Divide
 Separates Atlantic waters from Pacific waters
 Mississippi River drainage basin is 1.25 million square miles
Stream Segments
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Tributaries
o Overland flow moves water downslope
o These headwaters encounter depressions or other irregularities and coalesce into
rills
o Rills merge into larger branching channels called tributaries
Main trunk stream
o Collects water from tributaries
o Traverses the largest part of the area
o Carries most of the water
Stream Segments
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Distributaries
o Not in book
o At the downstream end of the trunk stream
Deltas
Deltas
Alluvial Fans—Deltas on Land
New Alluvial Fan
Gradient
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Flow of a stream is driven by the gradient
o Slope
o Depends on topography
o Generally decreases from head toward mouth
o Examples
 More than 250 ft/mile is very steep
 25 ft/mile in lowland plains
 .5 ft/mile in lower Mississippi River
Velocity
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Distance/time (ft/sec or meters/second)
Varies across the channel
Velocity Variations
Velocity
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Varies with depth in channel
o Slowest at the bottom
o Increases upward
o Reaches maximum near surface
o Surface velocity can be almost anything due to interaction with the air
Velocity
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Texture of streambed and gradient act together to influence velocity
Varies along the length of the channel
o Fast in head waters
o Slightly slower a little farther downstream
o Increases toward mouth
 Contrary to what you might think
 Happens even though gradient decreases
 Happens partly due to smoother bed than up stream
 Happens due to increased amount of water to handle
Depth and Width
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In a section across the stream
o If stream is straight, depth is greatest near center
o If stream bends, depth is greatest on the outside of the bend and least on the inside
of the bend
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In general, depth increases down stream
Width tends to increase downstream
Stream Channel
Discharge
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The volume of water past a given point in a specified amount of time
Cross sectional area times velocity
o Usually W x D x V
o Expressed in cubic feet per second or cubic meters per second
o Increases downstream as more water is added from tributaries
Downstream Changes
Base Level & Graded Streams
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Lowest level to which a stream can erode
o Ultimate base level is sea level
o Local base level is any obstruction such as a lake, resistant layer, fault, etc.
Graded Stream
o Concave upward
profile
o Stable
o Stream neither erodes nor deposits
Changing Streams
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Change of base level
o Alters profile
o Building a dam raises base level
 Causes sedimentation in reservoir
 May cause downstream erosion to lessen the gradient
Changing Streams
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Adding sediment
o Perhaps during construction
o Raises gradient and causes increased erosion to reduce gradient again
Transportation
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Streams carry lots of sediment
o 45 trillion cubic meters of water and 9-10 billion tons of sediment every year
o Mississippi River cares 1 million tons of sediment daily to the Gulf of Mexico
Capacity
o The maximum amount of material the stream can carry
o It is proportional to discharge
Transportation
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Competence
o The diameter of the largest grain the stream can carry
o It is proportional to the square of the velocity
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Load—What the stream actually carries
o Suspended
 Particles carried along without ever touching the bottom
 Most of the stream-borne sediment
 Fine solid particles
o Dissolved—the material in solution
Transportation
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Load
o Bed
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Carried along the bottom of the stream
Bounces on and off the bottom
Moves by saltation
Bed Load and Velocity
Velocity and Transportation
Stream Deposition
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In-channel deposition
o High sediment load deposits mid-channel bars
 Usually made of the coarsest material
o Even higher loads produce braided pattern
 Many intertwined channels like braided hair
 Indicates a stream with significantly more sediment than it can carry
o Point bars
 On the insides of meanders
Stream Deposition
Stream Deposition
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Floodplain deposition
o Natural levees
 Coarsest material
 Highest point on the floodplain
 Trees often grow there
o Terraces
 Stream establishes a broad flood plain
 Later uplift causes stream to downcut producing terraces
 Additional uplift can produce additional terraces
Terrace Evolution 1
Terrace Evolution 2
Terrace Evolution 3
Stream Terraces
Meander Cutoff
Drainage Pattern—Dendritic
Drainage Patterns—Radial
Drainage Patterns—Rectangular
Drainage Patterns—Trellis
Pattern Changes
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Headward erosion
o Downcutting at head of stream causes stream to increase its length headward
o Erodes edges of plateaus and other uplands
Stream Piracy
o One stream headward erodes until it captures the drainage of a nearby stream
Stream Piracy
Stream Piracy
Stream Order
Flood Prevention
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Zoning
o Identify flood-prone areas
o Determine frequency of flooding
o Prevent construction in floodplain
o Use floodplain only for uses that can stand flooding, such as parks, etc.
Artificial Levees
o Keep more water in channel
o There is evidence that they did more harm than good in 1993
Flood Prevention
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Dams
o Store water in rainy seasons to even out flow
o Retain water for irrigation
o Provide recreation
o Deprive downstream areas of sediment
o May lower water temperature
o May cause increased erosion downstream
o Can cause salt build-up in arid areas
Channelization
o Smoothes and straightens channel — larger discharge
o Increased velocity can increase erosion
Effects of Development
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Changes runoff
o Decreases lag time of peak runoff after a storm
o Concentrates runoff into a shorter period
o Increases amount of runoff by decreasing infiltration
o Reduces amount of water collected and returned to the atmosphere by plants
Changes temperature
o Runoff from paved areas is warmer in summer
o Runoff from paved areas is colder in winter
o Increases temperature range of stream
Stream Hydrographs
Effects of Development
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Narrowing of floodplain
o Buildings take up space that could have been occupied by water during a flood
o Makes flood level higher
Pollution
o Runoff contaminated by droppings from cars and people
o Overloaded sewage systems
o Leaching from landfills
o Thermal pollution from power plants
Flood Debris
New Channels
Water Level Indicators
Picnic Table
Picnic Table and Sign
Guard Rail
Bull Island State Park