Running Water & GroundwaterChapter 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

• How is gradient related to velocity?– steeper

• faster

– gentler• slower

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?– # (& volume) of tributaries

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

Streamflow: Discharge

• What is a stream’s “channel”?– path through which the water flows

Streamflow: Channel

• 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

Life of a

Stream

Youthful

Mature

Old Age

• 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

• In a meander where does water move fastest?– outside of bend

• In a meander where does water move slowest?– inside of bend

• What does the profile of a meandering stream look like?

cut bankerosiondeep

point bardepositionshallow

fastslow

Stream Flow, Erosion, & Deposition