TYPES OF RUNNING WATER
Running water is the most important agent in eroding, transporting, and
depositing sediment and developing landscapes.
Stream- Any body of running water confined to a channel (of any size), often
starting in mountains and ending at the sea.
Headwaters- Place where streams start (often in the mountains).
-Narrow, V-shaped stream valley
-Steep banks made of bedrock.
Mouth or delta- Place where the stream enters the sea.
-By this time, the stream valley has changed shape into a broad, flatbottomed stream valley.
Sheetwash- Running water that is not confined to a stream. During heavy rains,
water may move unconfined over land as sheets of water
DRAINAGE BASINS
Drainage basin (also called a watershed)- The
total area of land drained by a stream and all
of its branches or tributaries/feeders.
Drainage basins are separated by topographic
highs called divides. For example. the
“Continental Divide” separates streams that
flow to the Pacific from streams that flow to
the Atlantic.
DRAINAGE PATTERN
Drainage Pattern- Pattern formed by a stream and all of its branches and
tributaries. Drainage patterns are controlled by the rock or sediment
underlying the stream so they tell you about bedrock.
1) Dendritic - Treelike pattern that forms on rock or sediment that easily
erodes (very common).
2) Radial- Pattern that mimics the spokes of a wheel. They form around
conical features (common in volcanic terrains).
3) Rectangular- Pattern of angular (often 90°) bends of tributaries due to
fractured bedrock which serves as pathways for streams (common in
granitic/basaltic bedrock).
ARE STREAMS FEATURES OF EROSION OR
DEPOSITION? IT DEPENDS ON SEVERAL
FACTORS:
1. VELOCITY OF THE STREAM
Velocity is the key factor that determines a stream’s ability to
erode, transport, and deposit sediment.
Average speed of stream is ~ 5 km/hr. During flood stage, speed
may be up to ~25 km/hr.
The maximum velocity of a stream is at the center on straight
segments and at the outside curved segments.
Each size of sediment (clay, silt, sand, gravel) has certain speed
requirements for erosion, transportation, and deposition.
2. GRADIENT
Gradient is the slope of a stream (rise/run). It is usually
measured in feet/mile.
Gradient decreases as stream moves from the mountains
towards the sea. Mountain stream gradients are 50 to 200
ft/mile whereas lowland stream gradients are less than 5
feet/mile.
Streams with steeper gradients have higher velocities.
3. CHANNEL SHAPE AND ROUGHNESS
Flowing water drags against stream channel and is slowed down
by friction.
The more surface area of the confining channel, the slower the
stream moves. Therefore, narrow, deep channels produce fast
moving streams and wide, shallow channels produce slow
moving streams.
Streams will flow rapidly over a smooth channel bottom and
more slowly over a channel bottom filled with boulders.
STREAM DISCHARGE
Discharge is the volume of water that flows past a given point
per unit of time.
Discharge = average stream width (ft) x
(ft) x average velocity (ft/sec)
average depth
The units of discharge are ft3/sec (cfs) or m3/sec.
Example: A stream that is 100 ft wide, 15 ft deep, and flows
at 4 mi/hr (6 ft/sec) has a discharge of 9,000 cfs.
STREAM EROSION
There are 3 primary ways that streams erode
into their channel:
1) Hydraulic action – flowing water pries rocks
loose.
2) Solution – flowing water may gradually
dissolve some rock types or cements.
3) Abrasion - flowing water causes the stream
channel bedrock to erode from impact with
the sediment load (ex. potholes).
MECHANISMS FOR TRANSPORTING SEDIMENT
A stream’s sediment load can be classified as:
1) Bedload- Large or heavy sediment particles that travel on the streambed (sand and
gravel). Bedload moves by traction (rolling, sliding, dragging) or saltation (bouncing).
2) Suspended load- Fine-grained sediment light enough to stay suspended in turbulent
water (silt and clay). This load stays suspended due to the turbulent motion of water
making streams appear muddy.
3) Dissolved load- Soluble material which is transported in solution and may end up in the
ocean.
Vast amounts of sediment are dumped into the ocean each year. It is estimated that rivers
in the US carry 250 million tons of solid load (bedload + suspended) and 300 million tons of
dissolved load each year.
TYPES OF DEPOSITS MADE BY STREAMS
1) Bar deposit: A stream deposit consisting of sand or gravel deposited in the center of the channel.
Bars form when the stream’s velocity or discharge decreases and its bedload is dropped.
Braided stream: A stream that has many bar deposits.
Braided streams are wide, shallow, and heavily loaded with sediment.
Braided streams are characteristic of deserts and mountainous regions where sand
and gravel are abundant.
TYPES OF DEPOSITS MADE BY STREAMS
2) Meandering Stream: A stream that is sinuous.
They develop in areas with low gradients (lower energy than braided streams).
MEANDERING STREAMS
Red River on the Texas-Oklahoma border.
MEANDERING STREAMS
Meandering streams carry a lot of silt and clay and look muddy.
Water in meandering streams moves faster on the outside of the meander than on the inside of the
meander. Erosion occurs on the outside of the curve while deposition occurs on the inside. The
sediment in the inside of curves produces point bars.
As meanders become extremely sinuous, there is more and more chance that the river will choose a
shorter path and cut thru a meander (meander cutoff). The isolated meander then becomes a
crescent-shaped oxbow lake.
3. FLOODPLAIN DEPOSITS
Floodplain: The area on both sides of a stream channel.
Typically this area is dry, but in times of flood the stream will overflow its bank and the
floodplain will be covered with water.
As the water recedes, it leaves behind silt and clay. Thus, floodplain deposits build
successive, horizontal layers of silt and clay.
The coarsest sediment is deposited directly adjacent to the stream and may form a
natural levee.
4. DELTA DEPOSITS
Delta- A deposit where a stream flows into a
large body of water and is forced to drop its
load.
Deltas often have a triangular shape in map
view, like the Greek letter delta, D.
Deltas are sand-rich and can be braided, just
like braided streams.
5. ALLUVIAL FAN DEPOSITS
Alluvial fan- A large, fan-shaped pile of sediment that forms where a stream emerges
from a narrow mountain canyon.
The stream loses it energy as it emerges from canyon and drops its gravelly sediment load.
As the fan builds outward, the stream will move out over the fan, depositing more
sediment.
Braided stream channels cover the top of the fan
Deposition on the fan is discontinuous due to infrequent rainstorms.
Alluvial fans are best preserved in the deserts of the southwestern U.S.
FLOODING
Flooding is beneficial in the sense that it replenishes nutrients on the floodplain and
provides fertile planting ground.
Flooding can also be detrimental to towns built along the streams.
Urbanization contributes to flooding by paving land with concrete. The concrete
reduces infiltration and increases runoff.
DEVELOPMENT OF STREAM VALLEYS
All streams travel on a gradient toward the lowest elevation.
Streams attempt to flatten out the gradient by downcutting, or eroding into the stream
bed, creating a valley.
The lowest level to which a stream may erode is called base level.
Base level for streams that travel to the ocean is sea level. However, base level does not
have to be sea level in cases where streams flow to a lake or dry valley (for example,
Death Valley).
DEVELOPMENT OF STREAM VALLEYS
Streams cannot erode below base level or else they would have to flow upstream to
reach their final destination.
Streams with large gradients (mountain streams) downcut more than streams with gentle
gradients (meandering streams).
Streams will begin rapid downcutting and erosion when base level drops due to either a
drop in sea level or uplift of the land.
Meandering streams deposit huge amounts of sediments on the floodplain. However,
with a drop in base level, a meandering stream will begin downcutting again, producing
incised meanders.
DEVELOPMENT OF STREAM VALLEYS
Lateral erosion is a second way in which a stream valley develops.
The stream will undercut the banks, causing the overlying rocks to fall down (mass
wasting). Over time the V-shaped valley becomes wider and the floodplain grows. The
river can meander back and forth within this valley.