Streams/Rivers
– A stream is any body of water that flows under the force
of gravity in a relatively narrow channel.
– Streams affect more of the Earth’s surface than any
other agent of erosion.
– Streams merge together to form larger streams or rivers.
– Over 2 million streams in the continental U.S.
– The single most important agent of land erosion: cut
down into soil and bedrock
- Increase their length by headward erosion
- Increase their width by lateral erosion
- Transport and deposit sediments
The drainage basin of the Mississippi River, covers about 3 million
square km (1.2 million square miles) and consists of many smaller
drainage basins. The drainage basin of the Yellowstone River is
one of many that contribute to the Missouri River which is one of
many that make up the drainage basin of the Mississippi.
What are the largest rivers (based on their drainage
basin)?
• Amazon River –2,231,000 sq. mi.
• Congo River –1,550,000 sq. mi.
• Mississippi River – 1,150,000 sq. mi.
• Yenisei River –1,000,000 sq. mi.
• Parana River – 890,000 sq. mi.
• Yangtze River –750,000 sq. mi.
• Orinoco River –340,000 sq. mi.
• Ganges River –409,000 sq. mi.
• Brahmaputra River –361,000 sq. mi.
• Mississippi's watershed drains all or parts of 31 US
states and 2 Canadian provinces between the Rocky
and Appalachian Mountains
Tributaries of the Lower Mississippi River
• Ohio River is the primary branch
• White River flowing in at the White River National
Wildlife Refuge in east central Arkansas
• Arkansas River
• Big Black River in Mississippi
• Yazoo River meeting the Mississippi at Vicksburg
• Red River in Louisiana
• The widest point exceeds 1 mile which occurs in several
places.
Streams Transport and Deposit Sediment
Channel material influences erosion
If a channel is sandy, the particles are easily dislodged
from the bed and banks. Sand particles are lifted into
the water. Undercutting and dumping will cause lots of
material to be carried downstream.
Banks consisting of coarse gravel and cohesive clay and
silt are more resistant to erosion. Channels with silty
banks tend to be narrower compared to ones with
sandy banks.
Dependent on:
1. Discharge
2. Gradient
3. Load
Discharge - volume (amount) of water flowing past a
certain point over a given unit of time
A. Dictated by the size of a stream’s drainage basin
 Discharge of the Mississippi River is roughly 18,000
cubic meters per second.
 Amazon discharge is about 200,000 cubic meters per
second
B. Influenced by climatic conditions
 Timing of precipitation
 Quantity of snowmelt
 Local soil’s ability to absorb water
Stream loads – divided into three components
1. Suspended load – material is carried in suspension
- cloudy water, turbulent flow
- Mainly fine sand, silt and clay
2. Bed load – material transported along the channel
bottom, principally sand and gravel
- Grains may slide, roll or bump along the bottom = traction
- Grains may bounce along the bottom and take suspended
leaps – saltation
3. Dissolved load – material is carried and transported in
solution as ions
- Detect by taste
Gradient
• The slope of a stream channel
• Gradient = vertical difference in elevation /
horizontal distance.
• The lower Mississippi River has a very low gradient,
about 10 cm or less per kilometer.
• Some mountain streams have a gradient more than
40 m/km
• When the gradient is steeper more gravitation
energy is available to drive channel flow (see
handout).
A. Calculate the average gradient along the stream from the red dot at A to the red dot at B.
Answer: DIFFERENCE IN ELEVATION: Point A is on the 580' contour; point B is on the 520'
contour. The difference in elevation is 60 feet.
DISTANCE ALONG THE STREAM - each inch on the map = 2400 inches on the ground, or 200
feet. If it is 4.75 inches from A to B then 4.75 x 200 feet = 950 feet
Gradient is 60 ft/ 950 ft.
• Often the gradient is expressed in 'vertical feet' per
'horizontal mile'.
• To do this, convert the 950 horizontal feet into
miles.
• There are 5,280 feet in a mile, so 950 feet =
950/5,280 miles = 0.180 miles. The gradient now is
60 vertical feet/0.180 horizontal miles.
• X/60 = 1 /0.180 or X = (60*1)/0.180
• Thus, X = 333 and the gradient is 333 feet/mile
• Lower part of the Mississippi River is characterized
as a low gradient/ low relief (0.1 m/km)
– Gentle overall slope
– Erosion, transport, and deposition of fine-grained
sediment
– Features include meanders, floodplains, and low river
terraces
The oldest preserved river sediments indicate that the
lower Mississippi began draining the continent during
Mesozoic time.
Adjustments to the river date back during the last Ice Age
(2 million to 12,000 years ago).
The Upper Mississippi River is young
• It formed since the retreat of the last ice sheets
10,000 years ago.
• Ice sheets and glaciers covered the northern half of
North America so northern rivers like the upper
Mississippi did not exist.
• The weight of the ice sheets depressed the crust;
causing large regions to slope northward (opposite of
today).
• Melting of the ice released huge discharges of water
and sediment that carved completely new river
channels, including the upper part of the Mississippi.
• Today the Mississippi River begins in a lake in
Minnesota and winds its way southward, across the
center of the continent. Its length is not constant
because of its shifting meanders but it is 2,300 mi.
(3,700 km) long.
– At the mouth of the Mississippi River is a large
delta southeast of New Orleans, LA.
• Capacity – the maximum load of solid particles that
a stream can transport
- Depends on velocity of water and on the discharge
• Competence – a measure of the maximum size of
solid particles that a stream can transport.
Deposition of Sediment by Streams
• Deposition occurs whenever a stream slows causing
a reduction in competence. The flow velocity has
decreased so sediments begin to settle.
• Largest particles settle first
• Process called sorting – particles of similar size are
deposited together.
• Alluvium – general term for sediment deposited by
streams. Many different depositional features are
composed of alluvium: levees, point bars, deltas,
etc.
Stream Channel
• Stream flow can be thought of as an open conduit
consisting of the streambed and banks that act to
confine flow except during floods.
• Two basic types of channels:
1. Bedrock channels – form in headwaters
• Steep slopes, waterfalls, rapids, energetic flow, potholes
2. Alluvial channels – form in sediment that was previously
deposited in the valley
• Material deposited by the stream can form floodplains
• Shape of the channel is based on the channel’s gradient,
discharge, and sediment being transported.
Meandering stream channels – follow a winding
course of looping bends and turns
• has one main channel
cutbank – outer bank of a meander, often collapses
and slumps into a stream
point bar – flow along the inside of the meander is
slower
- Sand deposits form
- Minimum velocity
- Most visible deposit within the stream channel for many
a hiker
Floodplain – flat land immediately surrounding a stream
channel
– would be submerged if the stream were to overflow its banks
Oxbow lakes/ cutoff lake/horseshoe lake- crescent
/arcuate shaped water bodies
– Beings as a curve or meander
– A lake forms when the river finds a different and shorter
course.
– Form in low-lying plains.
– Erosion and deposition cause a new channel to be cut
through the piece of land.
– These are still water lakes because water does not flow into
them.
– They lack an outlet and can become swamps or bogs
Mississippi River Oxbow lakes
• Lake Chicot – largest oxbow lake in North America
near Lake Village, Arkansas
• Lake Bolivar, North of Greenville
• Lake Washington, south of Greenville
• Eagle Lake, Warren County, northwest of Vicksburg
• De Soto Lake, Coahoma County (west of Clarksdale)
• Beulah Lake – south of Great River Road State Park,
north of Greenville.
• Lake Whittington – north of Greenville
• The floodplain of the Mississippi River has
countless crescent-shaped scars of ancient
meanders that have been abandoned .
• Many scars have dried up as their water evaporates
• During floods, a stream may cut through meander
loops
• Subsequent flood deposits may fill oxbow lakes
-form meander scars
– Rich wildlife habitats
Levee
• Form along the edges of many channels and is a raised
embankment; a depositional feature
• Natural levees are created by the river and act as a
barrier to water flow from the channel to the
floodplain.
• During flooding, sediment-carrying floodwater rises
about the channel and begins to spreads out. As it
does, the current slows and so deposits sediment in
long mounds next to and paralleling the channel.
• When the flood recedes, sediment that piled up next to
the channel remains as an elevated rise or levee.
• Coarser material is deposited closest to the river
channel and finer material is deposited further away
from the river channel.
• Humans construct artificial levees to try to keep
floodwaters from spilling onto the floodplain. This
strategy commonly works for awhile.
• Without artificial levees much of the local towns
and cities near the river would be flooded to often
– Hinder agriculture and settlements
– Nearly impossible to engineer an affordable levee
system that can handle the largest flood events
(Hurricane Katrina in 2005)
– Failure of a single levee can put lives in jeopardy, cause
hundreds of millions of dollars in damages, and cause
towns and neighborhoods to be abandoned as unsafe.
Mississippi Delta
• A delta forms when a stream enters an ocean or a lake.
• Depositional landform
• Grow outward from the shoreline which causes the
stream gradient to decrease.
• Distributaries form (a branch of a river that flows away
from the main channel)– numerous shifts in the main flow
• Triangular form occurs resembling the Greek letter
delta
• Not all rivers have deltas. For example, powerful
ocean waves associated with the Columbia River in
the Pacific Northwest redistributes the material
soon after it is deposited.
• Rivers may not carry sufficient quantities of
sediment to build a delta.
• The delta of the Mississippi River resulted from the
accumulation of huge quantities of sediment
derived from the vast region drained by the river
and its tributaries.
• It extends over thousands of square kilometers and
began about 6,000 years ago.
• New Orleans rests where there was ocean 5,000
years ago.
• The delta consists of a series of seven coalescing
sub-deltas. Each delta was formed when the river
left its existing channel to find a shorter, more
direct path to the Gulf of Mexico.
-Each subdelta interfingers and partially covers one another to produce
a complex structure.
-- After each portion was abandoned, coastal erosion modified the
features.
- The present subdelta has been built by the Mississippi in the last 500
years, and is called a bird-foot delta.
• At present, the active bird-foot delta has extended
about as far as natural forces will allow.
• The river is trying to cut through a narrow neck of
land and shift its course to that of the Atchafalaya
River. It this occurs, then the Mississippi would
abandon its lowermost 500-km path in favor of the
Atchafalaya ’s much shorter 225-km route.
• To prevent this from occurring, a damlike structure
was erected at the site where the channel was
trying to break through. A flood in 1973 weakened
the control structure; another massive auxiliary
dam was completed in the mid-1980’s.
• http://www.k5geosource.org/2activities/1invest/ro
cks/pg5.html