Streams
Streams 85% sediment transport,
glaciers 7%, waves 1.5%, wind 1%
Lab Hose, sand in beaker
Fluvial (Latin fluvius: river)
Evapotranspiration and Drought
•
Globally, something like 64 percent of precipitation occurs over land as a
result of evapo-transpiration from lakes and wetlands and dense vegetation,
in particular forests pumping groundwater held in the soils into the air.
Sources of Earth’s water
(saltwater)
freshwater
Infiltration
• Infiltration is controlled by
– Intensity and duration of rainfall
– Soil texture
– Slope of the land
– Nature of the vegetative cover
– Runoff occurs in excess when soil is
saturated or when slope is too great.
Streamflow
• Factors that determine velocity
–Gradient, or slope
–Channel characteristics including
shape, size, and roughness
–Discharge – the volume of water
moving past a given point in a
certain amount of time
Stream Changes
• Changes from upstream to downstream
• Factors that decrease downstream
– A. Longitudinal Profile is the side-view of a stream
– From head (headwaters) to mouth of stream
profile flattens out
– B. So gradient (slope) decreases downstream
– C. channel roughness (grain size) also
decreases downstream
• Factors that increase downstream
– A. Velocity
– B. Discharge
– C. Channel size all get larger
Longitudinal profile of a
stream Tributaries
Drainage Divide
Stream capture,
decapitation
Trunk Stream
Distributaries
And Delta
Base Level
• Elevation below which a stream cannot
erode
• Local and ultimate
A waterfall results from change
in local base level
Local change in base level affects river profiles
Graded Streams
Demo: bank collapse, stream cross
section reduced
Pinched hose demo
Graded Streams
• Erosion and Deposition Equal
• When local conditions in a graded stream
are disrupted, velocity changes.
• Original conditions are restored.
• DEMOS
• Pinched hose demo, change in velocity at
restrictions
• Hose and sand demo: faster water is
better at erosion
More important definitions
Stream - Sediment Terminology
• Transported material is called sediment load
• Types of load
– Dissolved load - ions in solution
– Suspended load - fine clays, etc
– Bed load Moves during flood - high velocity
Sandy Portion – Saltation (bouncing)
Boulders – Traction (rolling)
• Capacity –maximum load stream transports
• Competence – Largest particles it can move
• Proportional to velocity squared
Suspended load - confluence Green & Colorado Rivers in Canyonlands, Utah
•
Competence
• Maximum particle size a stream transports
• Proportional to velocity squared –K.E.
Deposition of sediment by a stream
Caused by a decrease in velocity
Competence is reduced
Sediment begins to drop out
• 1. Narrow valleys uplift or drop in base level
• V-shaped
• Downcutting toward base level
• Can include rapids and waterfalls
•
•
Straighter streams
Typical of steep slopes in
uplifted areas
An Alluvial Fan in Death Valley
Sudden drop in competence
Alluvial fans
Develop where a
high-gradient
stream leaves a
narrow valley and
spreads out onto
a broad plain
Slopes outward
in a broad arc
2. Wide valleys
–Meanders "Meandering Stream"
• Cut bank and point bar
• Cutoffs and oxbow lakes
–
•
•
Floodplains, either:
Erosional floodplains
Depositional floodplains
Erosion and deposition along
a meandering stream
Discussion: Crack the whip
Floodplain
Levee
Meanders get more extreme with time. Deposition at point bar keeps up with erosion at cutbank
• Deposition of sediment by a meandering
stream
• Channel deposits
– Point bars cover bed load as meandering
stream’s channel loops move
–
Floodplain deposits cover point bar
Natural Levees
–form parallel to the stream
channel by successive floods
over many years
Channel moves in direction of cutbank
Pointbar advances as crossbed drapes
Meandering
Stream
Cut off loops silt up; called oxbow lakes
Old cut-off loop fills with
mud, an oxbow lake
OxBow
Floodplain
http://hays.outcrop.org/gallery/rivers/arid_meander?full=1
Point Bar Sequence from Meandering Stream
Fines of Floodplain
Crossbeds of Point Bar
Gravel of bed
Erosion
Formation of natural levees by flooding
Meandering Stream
Flood over banks, sediment carrying water into wide area, greater drag, lower velocity, load drops
• Incised meanders
Delaware Water Gap
• Meanders in steep, narrow valleys
– Caused by a drop in base level or uplift of region
Stream Terraces
River meanders across
floodplain.
Base level drops,
or region uplifts.
River cuts into bed, cutoffs
abandon large loops,
leaving oxbows
Stream less wide, uses
narrower floodplain
Old floodplain is a terrace
A flight of river terraces
3. Braided Streams
• Occur where bed load is very high. Often
big boulders in the stream. At headwaters
and near glaciers
• Many channels because flow is
intermittent/seasonal - old channel banks
collapse in dry season , redirect flow next
flood season
• "mid-channel bars" between channels
Deltas
• Occur where stream hits slow water
• Velocity, competence, bedload drop
• Deposits build out into lake or sheltered
sea, extending the length of the river
• Consists of three types of beds
Topset beds
Foreset beds
Bottomset beds
Deltas have strata deposited in a characteristic geometry
Topsets, Foresets and Bottomsets
Actually much more complex: many distributary channel deposits
River delta distributary channels change location with time
General trend: Delta builds Seaward.
If Floodwaters happen to reach
the sea via a new shortcut, they flow by
the shorter (steeper) path cutting a
new channel, abandon old
A fan-delta in a lake
Part 2 - Groundwater
• water found in sediment, plus
narrow fractures in bedrock
• Groundwater is the largest usable
reservoir of fresh water available to
humans, but much more in glacier ice
Distribution of Groundwater
•Water table is the
upper limit of
saturation
•Variations in the
water table
Depth is
highly
variable
•Varies
season
ally and
from
year to
year
•Shape is
usually
similar to
surface
topography
Groundwater table
generally follows topography
Factors influencing movement of
groundwater - 1
• Porosity – Volume of Pore Space
• Permeability – the ability of a material
to transmit a fluid through pore space
High Permeability
Low Permeability
Factors influencing movement of
groundwater - 2
• Aquifer – a permeable rock layer or a
sediment that transmits groundwater
freely (such as sands and gravels)
• Aquitard – an impermeable layer that
hinders water movement (such as
mudstone - clay)
Springs may result from a perched water table
Rollie’s Well
Aquifer
GEYSERS
•Intermittent hot
springs
•Water erupts with
great force
•Occur where
extensive underground
chambers exist near
hot igneous rock
•Groundwater heats,
expands, changes to
steam, and erupts
Yellowstone Geyser erupting
Wells
To obtain a continuous supply of
water, we must cut down below the
water table
Sometimes a pump is not needed
Demo: Artesian
Discussion:
Quicksand
Hydraulic head and urban water supplies
Just
Towns
like an
pump
artesian,
the water
savesupmoney
to their water tank at night, when
electricity is cheap. Artificial artesian.
Excessive Pumping of wells can cause
Drawdown (lowering) of the water table
Cone of depression in the water table
May cause dry well
Discharge exceeds
recharge
Saltwater contamination due
to excessive well pumping
Wildwood Crest
Well design requires knowledge of local geology
Don’t fire the town engineer!
Geologic work of groundwater
• Acidic groundwater dissolves limestone
• Groundwater is often mildly acidic
– Contains weak carbonic acid, H2CO3 which
forms when carbon dioxide from the air dissolves
in cloud water droplets
– Groundwater becomes more acidic from
decaying plants "Humic Acid"
• Acid dissolves calcite in limestone, releasing
CO2 gas
CaCO3 + 2H+ ↔ Ca++ + H2O + CO2 (g)
The reaction reverses, and calcite is deposited, if CO2 gas concentration gets high
Geologic work of groundwater
• Caverns
• Features found within caverns
– Composed of dripstone (travertine)
• Calcite deposited as dripping water
evaporates
• Includes stalactites (hanging from the
ceiling) and stalagmites (form on the floor of
a cavern)
CaCO3 + 2H+ ↔ Ca++ + H2O + CO2 (g)
Reaction reverses, and calcite is deposited, if CO2 gas concentration gets high
– Sinkhole or sinks (formed by
groundwater slowly
dissolving the bedrock often
accompanied by collapse)
– Disappearing (aka sinking)
streams
Karst Topography
Stream incision, cave
development and karst
topography
In Xanadu did Kubla Khan
A stately pleasure-dome decree:
Where Alph, the sacred river, ran
Through caverns measureless to man
Down to a sunless sea.
Xanadu, by Samuel Taylor Coleridge
Solution and Collapse Sinkholes
Winter Park, Florida. 1981
Porsche Dealership and Pool
Don’t fire the town’s engineer!
Water weighs 8.34 pounds per gallon,
and is incompressible
Limestone weighs 5744 pounds/ m3
Pool 50x25x2 meters