WHY STUDY STREAMS?
•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?
•71% of Earth’s surface is covered by water
•Of the 1.36 billion km3 of water on Earth
RUNNING WATER
•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
•Any surface water whose flow is confined to a narrow topographic depression is a stream
•Collecting Area
–WATERSHED OR DRAINAGE BASIN
–BOUNDED BY DRAINAGE DIVIDES
•Separates
•Most
direction water flows
famous is Continental Divide
– Separates Atlantic waters from Pacific waters
– Mississippi River drainage basin is 1.25 million square miles
STREAM SEGMENTS
•Tributaries
–OVERLAND FLOW MOVES WATER DOWNSLOPE
–THESE HEADWATERS ENCOUNTER DEPRESSIONS OR OTHER IRREGULARITIES AND
COALESCE INTO RILLS
–RILLS MERGE INTO LARGER BRANCHING CHANNELS CALLED TRIBUTARIES
•Main trunk stream
–COLLECTS WATER FROM TRIBUTARIES
–TRAVERSES THE LARGEST PART OF THE AREA
–CARRIES MOST OF THE WATER
STREAM SEGMENTS
•Distributaries
–NOT IN BOOK
–AT THE DOWNSTREAM END OF THE TRUNK STREAM
DELTAS
DELTAS
ALLUVIAL FANS—DELTAS ON LAND
A NEW ALLUVIAL FAN
GRADIENT
•Flow of a stream is driven by the gradient
–SLOPE
–DEPENDS ON TOPOGRAPHY
–GENERALLY DECREASES FROM HEAD TOWARD MOUTH
–EXAMPLES
•More than
•25
•.5
250 ft/mile is very steep
ft/mile in lowland plains
ft/mile in lower Mississippi River
STREAM CHANNEL
VELOCITY
•Distance/time (ft/sec or meters/second)
•Varies across the channel
STREAM VELOCITY VARIATIONS
VELOCITY
•Varies with depth in channel
–SLOWEST AT THE BOTTOM
–INCREASES UPWARD
–REACHES MAXIMUM NEAR SURFACE
–SURFACE VELOCITY CAN BE ALMOST ANYTHING DUE TO INTERACTION WITH THE
AIR
VELOCITY
•Texture of streambed and gradient act together to influence velocity
•Varies along the length of the channel
–FAST IN HEAD WATERS
–SLIGHTLY SLOWER A LITTLE FARTHER DOWNSTREAM
–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
•In a section across the stream
–IF STREAM IS STRAIGHT, DEPTH IS GREATEST NEAR CENTER
–IF STREAM BENDS, DEPTH IS GREATEST ON THE OUTSIDE OF THE BEND AND LEAST
ON THE INSIDE OF THE BEND
•In general, depth increases down stream
•Width tends to increase downstream
DISCHARGE
•The volume of water past a given point in a specified amount of time
•Cross sectional area times velocity
–USUALLY W X D X V
–EXPRESSED IN CUBIC FEET PER SECOND OR CUBIC METERS PER SECOND
–INCREASES DOWNSTREAM AS MORE WATER IS ADDED FROM TRIBUTARIES
DOWNSTREAM CHANGES
BASE LEVEL & GRADED STREAMS
•Lowest level to which a stream can erode
–ULTIMATE BASE LEVEL IS SEA LEVEL
–LOCAL BASE LEVEL IS ANY OBSTRUCTION SUCH AS A LAKE, RESISTANT LAYER,
FAULT, ETC.
•Graded Stream
–CONCAVE UPWARD
PROFILE
–STABLE
–STREAM NEITHER ERODES NOR DEPOSITS
CHANGING STREAMS
•Change of base level
–ALTERS PROFILE
–BUILDING A DAM RAISES BASE LEVEL
•Causes
sedimentation in reservoir
•May cause downstream
erosion to lessen the gradient
CHANGING STREAMS
•Adding sediment
–PERHAPS DURING CONSTRUCTION
–RAISES GRADIENT AND CAUSES INCREASED EROSION TO REDUCE GRADIENT AGAIN
TRANSPORTATION
•Streams carry lots of sediment
–45 TRILLION CUBIC METERS OF WATER AND 9-10 BILLION TONS OF SEDIMENT
EVERY YEAR
–MISSISSIPPI RIVER CARES 1 MILLION TONS OF SEDIMENT DAILY TO THE GULF OF
MEXICO
•Capacity
–THE MAXIMUM AMOUNT OF MATERIAL THE STREAM CAN CARRY
–IT IS PROPORTIONAL TO DISCHARGE
TRANSPORTATION
•Competence
–THE DIAMETER OF THE LARGEST GRAIN THE STREAM CAN CARRY
–IT IS PROPORTIONAL TO THE SQUARE OF THE VELOCITY
•Load—What the stream actually carries
–SUSPENDED
•Particles
•Most
carried along without ever touching the bottom
of the stream-borne sediment
•Fine solid
particles
–DISSOLVED—THE MATERIAL IN SOLUTION
TRANSPORTATION
•Load
–BED
•Carried
along the bottom of the stream
•Bounces
on and off the bottom
•Moves
by saltation
BED LOAD AND VELOCITY
•Coarsest material is associated with highest velocities
•Coarsest material occurs on the outside of meanders
VELOCITY AND TRANSPORTATION
STREAM DEPOSITION
•In-channel deposition
–HIGH SEDIMENT LOAD DEPOSITS MID-CHANNEL BARS
•Usually made of
the coarsest material
–EVEN HIGHER LOADS PRODUCE BRAIDED PATTERN
•Many intertwined
•Indicates
channels like braided hair
a stream with significantly more sediment than it can carry
–POINT BARS
•On
the insides of meanders
STREAM DEPOSITION
STREAM DEPOSITION
•Floodplain deposition
–NATURAL LEVEES
•Coarsest
•Highest
•Trees
material
point on the floodplain
often grow there
–TERRACES
•Stream establishes
•Later
a broad flood plain
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
•Headward erosion
–DOWNCUTTING AT HEAD OF STREAM CAUSES STREAM TO INCREASE ITS LENGTH
HEADWARD
–ERODES EDGES OF PLATEAUS AND OTHER UPLANDS
•Stream Piracy
–ONE STREAM HEADWARD ERODES UNTIL IT CAPTURES THE DRAINAGE OF A
NEARBY STREAM
STREAM PIRACY
STREAM PIRACY
STREAM ORDER
•Stream order is used to compare streams in different drainage basins
•Look at number of channels of each order and lengths of channels of each order
•Two streams of the same order join to create stream of higher order
•Highest order stream is the order of the basin
FLOOD PREVENTION
•Zoning
–IDENTIFY FLOOD-PRONE AREAS
–DETERMINE FREQUENCY OF FLOODING
–PREVENT CONSTRUCTION IN FLOODPLAIN
–USE FLOODPLAIN ONLY FOR USES THAT CAN STAND FLOODING, SUCH AS PARKS,
ETC.
•Artificial Levees
–KEEP MORE WATER IN CHANNEL
–THERE IS EVIDENCE THAT THEY DID MORE HARM THAN GOOD IN 1993
FLOOD PREVENTION
•Dams
–STORE WATER IN RAINY SEASONS TO EVEN OUT FLOW
–RETAIN WATER FOR IRRIGATION
–PROVIDE RECREATION
–DEPRIVE DOWNSTREAM AREAS OF SEDIMENT
–MAY LOWER WATER TEMPERATURE
–MAY CAUSE INCREASED EROSION DOWNSTREAM
–CAN CAUSE SALT BUILD-UP IN ARID AREAS
•Channelization
–SMOOTHES AND STRAIGHTENS CHANNEL — LARGER DISCHARGE
–INCREASED VELOCITY CAN INCREASE EROSION
EFFECTS OF DEVELOPMENT
•Changes runoff
–DECREASES LAG TIME OF PEAK RUNOFF AFTER A STORM
–CONCENTRATES RUNOFF INTO A SHORTER PERIOD
–INCREASES AMOUNT OF RUNOFF BY DECREASING INFILTRATION
–REDUCES AMOUNT OF WATER COLLECTED AND RETURNED TO THE ATMOSPHERE
BY PLANTS
•Changes temperature
–RUNOFF FROM PAVED AREAS IS WARMER IN SUMMER
–RUNOFF FROM PAVED AREAS IS COLDER IN WINTER
–INCREASES TEMPERATURE RANGE OF STREAM
STREAM HYDROGRAPHS
EFFECTS OF DEVELOPMENT
•Narrowing of floodplain
–BUILDINGS TAKE UP SPACE THAT COULD HAVE BEEN OCCUPIED BY WATER
DURING A FLOOD
–MAKES FLOOD LEVEL HIGHER
•Pollution
–RUNOFF CONTAMINATED BY DROPPINGS FROM CARS AND PEOPLE
–OVERLOADED SEWAGE SYSTEMS
–LEACHING FROM LANDFILLS
–THERMAL POLLUTION FROM POWER PLANTS
FLOOD DEBRIS
NEW CHANNELS
WATER LEVEL INDICATORS
PICNIC TABLE
PICNIC TABLE AND SIGN
GUARD RAIL
BULL ISLAND STATE PARK