Transcript Streams

Streams
•To geologists, a stream is any flowing body of water in a
channel—brook, creek, wash, wadi, slough, stream, river…
•Streams flow in valleys they have carved. Small streams
have small valleys, big streams have big valleys.
•Streams form hierarchical networks of connected streams,
where one stream feeds into another that eventually flows
into the ocean or other closed depression.
•Streams are fed by 1) other streams; 2) groundwater
discharge into the channel; and 3) runoff from slopes.
• Most landscapes (spatially and temporally) are dominated
by stream flow.
–Present Upper Midwest is unusual because glaciation has
produced a non-stream landscape.
–Areas like Missouri and Texas are more typical.
•When geologists speak of stream processes, they include
not only the channel processes, but also the mass wasting
and sheetflow and erosion associated with channels.
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Streams
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Hydrologic Cycle
Anatomy of a stream
Vocabulary
Streamflow>Overview
7 Streamflow>Variation
8 Streamflow>Meanders
9 Erosion and Transport of Particles by Water
10 Stream Processes>Overview
11 Stream Processes>Erosion
12 Stream Processes>Deposition
13 Stream Processes>Deposition>Terminal
14 Stream Processes>Deposition>Channel
15 Stream Processes>Deposition>Terraces
16 Stream Processes>Erosion Hills, Slopes, Valleys and Stages of Landform Development
17 Stream Processes>Erosion>Landform Development>Youth
18 Stream Processes>Erosion>Landform Development>Mature
19 Stream Processes>Erosion>Landform Development>Old Age
20 Drainage patterns
21 Channel Types
Hydrologic Cycle
• Major components
–Main location of evaporation is oceans. Main
location of precipitation is oceans.
–Because land is dry, there is not as much
opportunity for evaporation, so land experiences
net precipitation
• What happens to rain that falls on land?
–Evaporation directly to atmosphere
–Transpiration by plants
–Infiltration into soil then groundwater then into
a stream
• Infiltration ultimately becomes transpiration
or runoff (or [rare] groundwater discharge
into an ocean or [rare] evaporation)
–Runoff to channels
• Rainfall onto land either evapotranspires or runs
into stream channels
Anatomy of a stream
• The region that drains into a particular stream is the drainage basin.
• The region that usually or most often has water in it is the channel.
• Often, there is a broad flat plain adjacent to the channel that gets inundated during floods
and is called a floodplain.
• Outward from the floodplain are the valley wall and uplands
• Frequently, there are channel-parallel ridges next to the channel called levees, that result
from deposition of suspended load during floods.
• The sediment, if present, underlying the channel and floodplain is called valley fill.
Valley
Wall
Vocabulary
• Gradient is the slope of the channel, measured along the
channel. Decreases downstream.
• Width of channel is measured perpendicular to direction
of flow, and varies with time. Increases downstream.
• Thalweg is the line down a channel connecting the
deepest point at each cross section. This is where fastest
flow occurs.
• Depth of channel varies tremendously, with time, along
channel, and across channel. Depth usually increases
downstream. Usually, people mean the deepest depth at a
given location at a given time. Stage refers to depth of
water in channel measured at a gage station.
• Discharge is the amount of water flowing past a point,
usually measured in cubic feet per second or cubic meters
per second.
• Longitudinal profile a cross-section along the direction of
flow. They have a characteristic shape that looks like an
exponential decay. They don’t change rapidly.
• Transverse or latitudinal profile goes across the channel.
They have a variety of shapes and change rapidly.
• Graded streams A graded stream, in net and on
average, neither erodes nor deposits Most streams are
graded for most of their length, but streams almost always
erode into their headlands and deposit at their mouth.
Streamflow>Overview
• Streamflow is expressed as volume per unit time, usually
cubic meters per second, cubic feet per second, sometimes
cubic kilometers per second, or acre-feet per second or day.
• Streamflow varies tremendously with time. Short term
controls include rainfall, snowmelt, and evaporation
conditions. Long term controls include landuse, soil,
groundwater state, and rock type.
• Streamflow is measured at gauging stations, where the
stream is routed into an artificial channel of known cross
section, often a V, or the dimensions of the channel have been
measured (and need to be re-measured as channels shift and
sediments accumulate...) The height in the channel is
recorded continuously, and converted into stream flow.
• A graph of flow vs. time is called a hydrograph. The shape
of a hydrograph depends on rainfall, landuse, geology, and
soils.
• Example Hydrograph: Sangamon River at Riverton IL
http://waterdata.usgs.gov/il/nwis/uv?cb_all_00060_00065=o
n&format=gif_stats&period=31&site_no=05576500
Streamflow>Variation
• At a given location, as discharge increases, the channel deepens, widens, and flows faster.
• As you go downstream in a humid region, discharge increases, so
– the channel deepens and widens, and
– velocity usually slows down, but this varies from stream to stream, depending on how much the
channel widens
• In an arid region, water lost to evaporation might exceed water gained from tributaries and groundwater,
resulting in a decrease in discharge as you go downstream.
• Velocity in a stream depends on discharge (largely controlled by recent weather), channel shape, and
channel roughness. The velocity of flow in a stream, especially a small one, varies tremendously with time,
because the short-term discharge varies with the weather. For small streams, variations in discharge of a
factor of 1,000 are very common. Larger streams tend to be more stable.
Streamflow>Meanders
• Water tends to move downstream in slugs
that extend all the way across a channel.
• When the channel bends, water on the
outside of the bend (the cut-bank) flows
faster and water on the inside of the bend
(the point) flows slower. This distribution
of velocity results in erosion occurring on
the outside of the bend (cut) and deposition
occurring on the inside of the bend.
• Because of this positive feedback loop,
meanders get bigger (meander more), until
finally the meander gets cut off, usually in a
flood.
Erosion and Transport of Particles by Water
• Velocity and Competence and Capacity Discuss Figure 10.7 (velocity versus competence curve). In
short, higher velocity is required to move larger particles.
• Modes of transport
–Bedload
• Sliding
• Rolling
• Traction
• Saltation
–Suspended
1cm/sec=3600cm/hr=36m/hr
–Dissolved
100cm/sec=3600m/hr=3.6km/hr=~2 miles per hour
Stream Processes>Overview
Streams do a lot of things. These processes are interwoven, and often occur simultaneously, sometimes in
seemingly contradictory ways (For example, at a meander, a stream is eroding one side of its channel and
depositing on the other.) Any, all, or none of these processes may be happening at a location, and the result
of stream processes is largely the combination of them.
• Move water is the essence of a stream.
Because streams move water, they:
• Transport sediments
• Deposit sediments when they slow down
• Erode Headward: extending the drainage into un-drained uplands, if there are any. Gully formation.
• Downcut: eroding down into stream bed
• Erode Laterally their banks, valley fill, and valley walls. This also leads to oversteepened slopes that are
susceptible to mass wasting. Combined, lateral erosion and mass wasting produce slope retreat.
Because stream flow varies dramatically, at a single location, deposition and erosion may alternate on a daily
or yearly or ?centurily? … basis.
Stream Processes>Erosion
Fig. 10.37a
Fig. 10.37b
Streams erode by a combination of direct stream
processes, like downcutting and lateral erosion,
and indirect processes, like mass-wasting
accompanied by transportation.
Fig. 10.37c
Fig. 10.37d
Stream Processes>Deposition
• Any place or time the stream slows down, it loses capacity and competence, and deposits
sediments. Stream sediments are often re-eroded by the stream itself, but may get preserved or
are visible in their initial state.
• Channel sediments may be subject to continuous re-working and result in very well-sorted
sediments.
• In cut-off meanders, deltas, and alluvial fans, sediments are not re-worked and they are less
well-sorted.
• “Deposits” can also form by removing other material. For example, there are channel lag
"deposits" consisting of material too coarse for the stream to move and often contain fossils like
dinosaur bones and petrified wood.
Stream Processes>Deposition>Terminal
Where a stream loses gradient suddenly, such as where it enters a large body of standing water or flows out
onto a valley floor, it slows down and rapidly drops a lot of sediments. These pile up and further reduce
gradient, until an easier (steeper/shorter) path is found and the stream takes that path. The result on land is
alluvial fans, which are surprisingly circular cones. In water, there are more processes (waves and tides
especially) and more complicated shapes arise for deltas, such as bird’s foot deltas or delta-shaped deltas.
• Deltas (deposits left when a stream stops where it hits a standing body of water, like an ocean or a lake)
• Alluvial fans (deposits left when a stream stops where it hits a great decrease in slope, going from
mountains to valley)
Stream Processes>Deposition>Channel
• Meanders
–Cutbank On the outside of a meander,
the current is fast and erosion occurs.
–Pointbars On the inside, the current is
slow and deposition occurs. These start
as coarse deposits and, as the location
gets shallower, the sediments get finer,
so you get a fining upward sequence.
• Levees When a river overflows its banks,
the water loses velocity and coarse,
suspended load (mainly silt, some sand)
gets deposited in linear ridges parallel to
the channel.
• Floodplains When a river overflows its
channel, there is usually an extensive, flat
lowland area (the floodplain) that gets
flooded. The water here flows very
slowly, so only clay and some silt can get
transported here. Very fertile soils due to
the sporadic influx of fresh soil materials.
Fig. 10.18
Stream Processes>Deposition>Terraces
• Terraces These are floodplain or channel deposits that are now above the channel level. They are often
very well drained, because the water table is about level with the river and there are permeable materials
(usually old channel-bed deposits) that conduct water very well.
Stream Processes>Erosion
Hills, Slopes, Valleys and Stages of Landform Development
The channel is not the only stream-sculpted part of the landscape. The slopes that feed
into it (slopes, and hills) are also shaped by streams and surface runoff. Most
landscapes are dominated by streams.
Stages of stream development Landscape development as a series of steps ("cycle")
Diagrams at right from http://www.uwgb.edu/dutchs/EarthSC202Notes/erosion.htm
NEXT THREE SLIDES HAVE DETAILS AND PHOTOS
• Youth In youth (initial state) an area has been uplifted and you have a flat upland.
There are few streams, they usually have straight, narrow V-shaped valleys with
steep gradients.
– Upland swamps are common. Part of the upland area does not drain into streams.
Headward erosion, down cutting and slope retreat are all active. No floodplains.
Streams are, in net, eroding. Deposits are few.
– Good for mosquitoes. Soils may be very well-drained if the underlying materials
are permeable.
– Common in Midwest.
• Mature Erosional processes are slowing down, headward erosion has more or less
stopped. An integrated drainage system has formed, and the entire area drains into
streams. (The whole area is in slope.)
– Streams are broad V-shaped or \_/,.all major streams and some minor ones
meander, the meanders occupy the entire floodplain, and gradients are medium.
Floodplains are narrow. Both depositional and erosional features are found, with
erosion slightly outpacing deposition.
– Bad for mosquitoes. Soils are well-drained.
– Common in “shallow” South (Missouri, Kentucky, Tennessee… )
• Old Age Erosion has largely ceased and deposition dominates.
– Valleys are very broad, but meanders occupy a small part of the floodplain,
gradients are low to very low. Levees are common, and the stream may be higher
than the floodplain, resulting in backswamps and yazoo streams. Lowland
swamps are common, as are oxbow lakes and cutoff meanders.
– Good for mosquitoes. Lots of poorly drained soils, flooding common.
– Common in Deep South (Louisiana
, Mississippi…)
Stream Processes>Erosion>Landform Development>Youth
Diagrams at right from http://www.uwgb.edu/dutchs/EarthSC202Notes/erosion.htm
• Youth In youth (initial state) an area has been uplifted and you have a flat upland.
–There are few streams.
–Streams usually have straight, narrow V-shaped valleys with steep gradients.
–Upland swamps are common. Part of the upland area does not drain into streams.
–Headward erosion, down cutting and slope retreat are all active.
–No floodplains have formed.
–Streams are, in net, eroding. Deposits are few.
–Waterfalls are present.
–Rocky streams are common.
–Good for mosquitoes (upland swamps).
–Soils may be very well-drained if the underlying materials are permeable.
–Common in Midwest.
0094
Fig. 10.37d
Stream Processes>Erosion>Landform Development>Mature
• Mature
–Erosional processes are slowing down.
–Headward erosion has more or less stopped (no undrained uplands left).
–An integrated drainage system has formed, and the entire area drains
into streams. (The whole area is in slope.)
–Streams are broad V-shaped or \_/.
–All major streams and some minor ones meander, the meanders occupy
the entire floodplain, and gradients are medium.
–Floodplains are narrow.
–Both depositional and erosional features are found, with erosion
slightly outpacing deposition.
–Bad for mosquitoes. Soils are well-drained.
–Common in “shallow” South (Missouri, Kentucky, Tennessee… )
Stream Processes>Erosion>Landform Development>Old Age
• Old Age Erosion has largely ceased and deposition dominates.
–Valleys are very broad, but meanders occupy a small part of the
floodplain.
–Gradients are low to very low.
–Levees are common, and the stream may be higher than the floodplain,
resulting in backswamps and yazoo streams.
–Lowland swamps are common, as are oxbow lakes and cutoff
meanders.
–Good for mosquitoes. Lots of poorly drained soils, flooding common.
–Common in Deep South (Louisiana, Mississippi…)
Drainage patterns
• Drainage patterns (shape of connected hierarchy of
streams) reflect the underlying geology.
–Dendritic (looks like a veins in a maple leaf, or branches
in a tree) results from uniform material. If there is a
regional tilt, the drainage network looks more like palm
leaves.
–Radial indicates flow out of a central, circular upland, like
a volcano.
–Rectangular indicates a jointed or faulted pattern, with
the joints or faults intersecting at right angles, or a
rectangular distribution of rocks type as found in an area
of downdropped fault blocks. A variant is angular
drainage, where the intersections are oblique. In addition
to meeting at right angles, streams have right angle bends.
–Trellis system develops in areas of dipping (tilted)
sediments where alternating beds are resistant or
susceptible to erosion. The main valleys are over
susceptible-to-erosion rocks, with short/side valleys
draining the ridges formed over resistant-to-erosion rocks.
–Centripetal (streams flow inward, rare) indicates a
central, circular lowland like a sinkhole.
Channel Types
• Rills
• Gully
• Straight Usually V-shaped, typically
steep gradient, often symmetric.
Common in uplands and early stages of
stream development.
• Meandering Usually broadly U-shaped,
asymmetric. Common in lowlands and
delta regions, late stages of stream
development. Mature (meanders occupy
whole floodplain) and old age (channel
occupies small part of floodplain)
• Braided multiple interconnecting
(anastomosing) channels. Common in
arid climates, sands, and channels
carrying much of its load in coarse
materials.
• Wadi, Arroyo
Streams
• Most landscapes (spatially and temporally)
are dominated by stream flow. Present Upper
Midwest is unusual because glaciation has
produced a non-stream landscape. Areas like
Missouri and Texas are more typical.
• Stream-dominated landscapes vary greatly.
• Stream processes often occur over/with other
processes. (e.g., McLean County, IL was
glaciated and is showing youthful to mature
stream landscapes)
• Streams and mass wasting work together to
create stream eroded landscapes with
integrated slopes leading into streams.
• The basic stream processes occur in all stream
landscapes, but the particular processes
(downcutting, headward erosion, meandering
through valley fill…) depend on the materials
available and how far the processes have gone
(age)
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