Running Water & Groundwater Chapter 6 (sec. 1 & 2 only) 200.
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Transcript of Running Water & Groundwater Chapter 6 (sec. 1 & 2 only) 200.
The Role of Streams & Rivers• What role do streams
& rivers play on Earth?– ~½ of the water that
falls to Earth’s surface eventually ends up in a stream or river• & most drain to oceans
– essential part of the water cycle
– Most important agent of surface erosion
River Systems
• What is a river system?– a river and all of
its tributaries• What is a
tributary?– a stream that
runs into another stream or river
• What is a drainage basin (or watershed)?– all the land that drains into the river either directly or
through its tributaries• Lgst drainage system in U. S. = Mississippi River system
Drainage Basins/Watersheds
Continental Divide
Appalachian Mountains
Divides• What is a divide?
– high land that separates one drainage basin from another• Continental Divide
(Rocky Mountains) is the major divide in the U. S.
– rain falling to east flows to (Gulf of Mexico &) Atlantic
» by way of Mississippi River system
– rain falling to west flows to the Pacific Continental Divide
Streamflow: Velocity
Measuring Stream Velocity Video
• What is “velocity”?– distance water travels in
a given amount of time
• How is velocity measured?– float method– flow meter
• What affects a stream’s velocity?– amount of energy a
stream has• What affects a stream’s
energy?– 1. gradient (slope or
steepness)– 2. discharge (amount of
water)– 3. channel (stream path)
» size (width & depth)» shape of the path
(straight/curved)
Streamflow: Velocity
• How is stream velocity related to the size of the particles it can transport?– faster = larger particles
Streamflow: Velocity
• What is “gradient”– steepness of the slope
• gradient = change in elevation
change in distance
Streamflow: Gradient
• What affects a stream’s gradient?– closer to head (source)
• steeper
– closer to mouth• gentler
Streamflow: Gradient
• What is “discharge”?– volume of water that
passes a certain point in a given amount of time (related to velocity)
Streamflow: Discharge
• What affects a stream’s discharge?– age of stream
• older– more discharge
• younger– less discharge
Streamflow: Discharge
Youthful
Mature
Old Age
• What affects a stream’s discharge?– time of year
• late winter to mid-summer– greatest discharge
• late summer to mid-winter– lowest discharge
Streamflow: Discharge
Mean of monthly stream flow at Peachtree Creek for each month (for the years 1958 to 2002) The January value of 174 cubic feet per second (ft3/s) was computed by averaging the 44 mean January stream flows (1959-2002).
• How is discharge related to velocity?– greater
discharge• greater
velocity
– lower discharge
• lower velocity
Streamflow: Discharge
• What affects a stream’s channel?– age of stream
• older– wider & deeper
• younger– narrower & shallower
Youthful
Mature
Old Age
Streamflow: Channel
• How is a stream’s channel related to its velocity?– straight, wide, &
deep• small surface area
in contact with the water
– less friction» faster (especially
at center near surface)
Streamflow: Channel
• How is a stream’s channel related to its velocity?– shallow & winding
• large surface area in contact with water
– greater friction» slower
• fastest on outside of curve
Streamflow: Channel
How Streams Weather & Erode Material• mostly mechanical
weathering• split rocks due to
pounding• abrasion
– by “cutting tools” sand, pebbles, boulders, etc.
» rounded & smoothed
• What is a “pothole”?– Deep, rounded basin
• formed when sand, pebbles, & small boulders swirl around in whirlpools & grind into the rocks
• What is a “plunge pool”?– basin worn away at the
base of a waterfall• formed by the action of
falling water and abrasion of churning particles
How Streams Weather & Erode Material
• Streams are effective agents of erosion.– gravity pulls water downhill
• soil & rock carried with it
Erosion/Transport
• What is a stream’s “load”?– eroded rock & soil materials that are
transported downstream• transported 3 ways:
– 1. solution (dissolved)– 2. suspension (floating)– 3. bed load
How Streams Transport Materials
Erosion/Transport
• What are two measures used to describe the ability of a stream to erode materials?– competence
• maximum size of particles stream can carry
– capacity• total amount of
sediment stream can carry
Erosion/Transport
The size and amount of sediment carried by a stream depends on:
• velocity (speed of stream)• discharge (volume of water)
Which stream carries larger/more sediment?
• When a stream flows to base level (lake/ocean), what happens to its energy?
– It decreases.• What happens…?
– graded (sorted) deposition occurs» largest closest to mouth & smallest farthest out» largest on bottom & smallest on top
Depositiondeposition patterns
vertical sorting animationsettling rate by size, shape, density animation
horizontal sorting in a stream animation
Depositional Features• What is a delta?
– fan-shaped deposit of silt & clay at mouth of river• forms when river flows into quiet or large body of water & energy
decreases
• How do the rates of erosion & deposition affect the size of the delta?– If deposition > erosion delta grows.– If erosion > deposition delta shrinks.
• A river flowing into a delta splits into distributaries.– smaller channels that bring sediment to front of delta
Stream Valleys: Youthful Streams• What is the gradient of youthful stream like?
– steep • How does the river flow?
– in a straight line
• In which direction does the river erode?
– downward
• What is the shape of the resulting valley?– V-shaped (a.k.a. canyon, gorge, chasm)
• steep, almost vertical sides and narrow bottom
• Is the discharge large or small?– small
• What happens to the gradient as a stream matures?– gradient decreases
• How does the river flow?
– slight curves b/c velocity decreases
• In which direction does the river erode?
– outward b/c less erosion of bed & more erosion of sides
Stream Valleys: Mature Streams
• What is the shape of the resulting valley?– wider, broad floor,
gently sloping walls
• Is the discharge large or small?– medium
Stream Valleys: Mature Streams
• What happens to the gradient in an old age stream?– gradient decreases to almost
horizontal (stream approaches base level) • How does the river flow?
– wide meanders across floodplain» can form oxbow lakes
• In which direction does the river erode?
– outward b/c less erosion of bed (bottom) & more erosion of sides
Stream Valleys: Old Age Streams
• What is the shape of the resulting valley?– valley floor widens into a floodplain
• Is the discharge large or small?– large
Stream Valleys: Old Age Streams
• Why does water move faster in center?– less friction
• What does the profile of a straight section of a stream look like?
Slow
Slow SlowFast
------------Slow SlowFast
Stream Flow, Erosion, & Deposition
Stream Flow, Erosion, & Deposition• What happens when a stream meanders?
– erosion in some areas• cut banks
– deposition in other areas• point bars
--------------
Cross-section on next slide
•point bar•deposition of small sediments•shallow
•cut bank•erosion of
small sediments,
large sediments left behind
•deep
• What does sediment size tell us?– large water moving fastest (only lg sediments left behind)– small water slowed down (& dropped small sediments)
Stream Flow, Erosion, & Deposition