www'geo'utep'edu pub hurtado2412

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www.geo.utep.edu/pub/hurtado/2412
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Running Water

• Introduction the hydrologic cycle
• Hydrology
• Stream erosion, sediment transport, deposition
• Floods
• Fluvial geomorphology
• Base level and graded streams
• Drainage basins and stream valley development

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Water

• Earth is a water world. Unlike any of the
  other terrestrial planets (Mercury, Venus, Mars),
  we have lots of liquid water.
• 71 of Earth is covered by water and a small
  amount is in the atmosphere. The crust and
  mantle also contain water, inside minerals.

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Water

• Most water is in the ocean (1.3 billion cubic
  kilometers of it)!
• Only 0.8 of the Earths water is in streams,
  atmosphere, lakes, and groundwater.
• This small percentage contributes profoundly to
  fluvial erosion and modification of the Earths
  topography.

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Water

• Water is a critical resource. And it is
  critically scarce in many parts of the world. It
  is similarly all too abundant sometimes!
• Understanding the behavior of running water on
  the Earths surface is useful for describing
  geomorphology AND for practical matters.
• Streams are complex, dynamic systems continually
  responding to change in order to attain
  equilibrium.

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Hydrologic Cycle

• Like the rock cycle, it describes the pathways by
  which water is constantly recycled between the
  crust, atmosphere, and hydrosphere.
• Powered by solar radiation.
• Enabled by the ability of water to change phase
  easily (solid, liquid, gas) under common Earth
  surface conditions.

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Hydrologic Cycle

• Evaporation 85 from ocean, 15 from land
• Condensation cloud formation in the atmosphere
• Precipitation 80 falls in the ocean, 20 falls
  on land
• Runoff, transpiration, evaporation, temporary
  storage.

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Running Water

• Introduction the hydrologic cycle
• Hydrology
• Stream erosion, sediment transport, deposition
• Floods
• Fluvial geomorphology
• Base level and graded streams
• Drainage basins and stream valley development

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Hydrology

• Water (like most low-viscosity fluids) has no
  shear stress. It will flow.
• Flow can be
• Laminar smooth flow with parallel streamlines
• slow flow or viscous fluid
• little mixing
• little erosion or sediment transport.

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Hydrology

• Water (like most low-viscosity fluids) has no
  shear stress. It will flow.
• Flow can be
• Turbulent complex flow with intertwined
  streamlines
• fast flow or low viscosity fluid
• substantial mixing
• lots of erosion and sediment transport.

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Hydrology

• Infiltration capacity maximum rate at which soil
  or other material can absorb water. Depends on
• Intensity and duration of rainfall
• Nature of soil (packing and moisture content)
• If infiltration capacity is exceeded (or surface
  material is saturated), water cannot be absorbed.
  It will collect on the surface and flow
  downhill. Overland flow.

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Hydrology

• Sheet flow overland flow of water in a
  semi-continuous film (sheet). Not confined to
  depressions. Results in sheet erosion over a
  wide area.
• Channel flow overland flow confined to
  depressions (streams, rivers). Fed by sheet
  flow, rainfall, groundwater.

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Hydrology

• Streams flow downhill from the source to
  baselevel.
• The slope of the path the stream takes is the
  stream gradient.
• Streams are steeper in the upper reaches (m/km)
  and less steep in lower reaches (cm/km).
  Concave-up shape

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Hydrology

• Stream velocity downstream distance traveled per
  unit time (in m/s).
• Velocity can vary (velocity gradients)
• in time
• with downstream distance
• over the width of a stream
• with depth in the stream
• Flow velocity is slowest (zero) at the stream bed
  and along the banks because of friction with the
  stationary boundaries.

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Hydrology

• Channel shape affects flow velocity (friction).
• Broad, shallow channels narrow, deep channels
• more water contacts boundaries ? more friction ?
  slower flow
• Semi-circular channel
• less water contacts boundaries ? less friction ?
  faster flow

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Hydrology

• Channel roughness affects flow velocity
  (friction).
• Rough channel with lots of boulders or vegetation
  ? slow flow
• Smooth channel with fine grained sediments and/or
  little vegetation ? fast flow

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Hydrology

• Stream gradient has some effect on flow velocity
  (fast flow where channel is steep).
• BUTaverage flow velocity also generally
  increases downstream (note that gradient
  decreases downstream).
• Flow accelerates downhill, regardless of slope
  due to gravity
• Upstream reaches commonly rough, downstream
  smooth
• Downstream reaches more semi-circular
• Volume of water increases downslope

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Hydrology

• Discharge total volume of water in a stream
  moving past a particular point in a given amount
  of time (m3/s)
• Q V A
• Q is discharge
• V is flow velocity
• A is cross-sectional area of the channel

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Hydrology

• Back to energy again! How much power to move
  stuff does the flow have?
• Stream velocity erosive power
• Capacity amount of sediment carried by the
  stream
• Competence the largest size material a stream
  can carry

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Running Water

• Introduction the hydrologic cycle
• Hydrology
• Stream erosion, sediment transport, deposition
• Floods
• Fluvial geomorphology
• Base level and graded streams
• Drainage basins and stream valley development

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Stream Erosion

• Streams have potential energy and kinetic energy.
• Potential energy energy due to position of mass
  above some datum gravity wants to pull the mass
  down.
• Kinetic energy energy due to motion of a mass
• Kinetic energy not dissipated as heat in
  turbulent flow (5) gives flowing water erosive
  power

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Stream Erosion

• Erosion involves
• Physical removal abrasion (wearing away) and
  hydraulic action (lifting)
• Chemical removal dissolution
• Erosion produces
• Solid load particles of sediment (wide range of
  sizes)
• Dissolved load dissolved ions

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Sediment Transport

• The dissolved load includes ions liberated during
  chemical weathering and transported in solution.
• The solid load includes detrital (clastic)
  particles carried in two ways
• Suspended load clay and silt size particles kept
  in suspension (floating) in the flow by fluid
  turbulence
• Bed load coarser particles (sand and gravel
  size) that move along the stream bed.

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Sediment Transport

• Material in the bed load is too large to be
  continually suspended in the flow by turbulence.
• Bed load moves in two ways
• Saltation
• Rolling and sliding

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Saltation
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Sediment Transport

• The maximum particle size that a stream can carry
  is the streams competence. The greater the flow
  velocity, the greater the competence.
• Capacity is the total amount load a stream
  carries. The greater the discharge, the greater
  the capacity.
• A small, fast stream has large competence but
  small capacity. A large, slow river has low
  competence but large capacity.

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Deposition

• Streams can move sediment very long distances.
• Along the course of a stream, there can be
  deposition in a variety of environments
  (channels, banks, floodplains, etc.)
• Streams do most of their work when they flood.
  Floods of various sizes happen periodically, each
  size with a different recurrence interval.
• Stream deposits (alluvium) therefore represent
  periodic sedimentation during floods, not the
  day-to-day activity.

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Running Water

• Introduction the hydrologic cycle
• Hydrology
• Stream erosion, sediment transport, deposition
• Floods
• Fluvial geomorphology
• Base level and graded streams
• Drainage basins and stream valley development

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Floods

• Floods occur when a stream receives more water
  than the channel can handle. The water will
  occupy some or all of the floodplain.
• Floods are costly in lives and property. Floods
  are important and natural parts of the hydrologic
  cycle and landscape development.
• Floods are given a rank based on peak discharge.
  Largest are rank of 1.

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Floods

• Recurrence interval time period during which a
  flood of a given size or large can be expected to
  occur.
• R (N 1)m
• R Recurrence interval (years)
• N number of years on record
• m magnitude

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Running Water

• Introduction the hydrologic cycle
• Hydrology
• Stream erosion, sediment transport, deposition
• Floods
• Fluvial geomorphology
• Base level and graded streams
• Drainage basins and stream valley development

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Fluvial Geomorphology

• Braided streams networks of dividing and
  rejoining channels.
• Broad and shallow.
• Carry bedload and deposit sheets of gravel and
  sand.
• Common in arid/semiarid regions and in glaciated
  areas places where there is a lot of sediment
  supply.
• Stream has too much sediment. Sediment creates
  temporary and shifting gravel and sand bars that
  divert the water.

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Fluvial Geomorphology

• Meandering streams a single, sinuous (winding)
  channel with broad, looping curves (meanders)
• Semicircular cross-section along straight
  reaches
• asymmetric in the meanders

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Deepest part (thalweg) is about in the center of
the channel
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Thalweg is at the outer bank (cut bank) where
flow velocity is greatest. Erosion. Point bars
form in the inner bank where flow is slowest.
Deposition.
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Oxbow lakes form when meanders get so twisty
that a meander gets cut off from the main part of
a river during a flood. Eventually, the oxbow
lake may dry up and get filled by sediment ?
meander scar.
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Fluvial Geomorphology

• Floodplain low-lying, relatively flat areas
  adjacent to the main channel of a river.
• El Paso and Juarez are built around the
  floodplain of the Rio Grande (the Mesilla
  Valley).
• Flat ground
• Fertile soil
• Butincreased potential for flooding

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Fluvial Geomorphology

• Floodplain deposits include sand and gravel
  deposited as point bars that grew as the meanders
  migrated laterally.
• Floodplains dominated by fine-grained sediments
  deposited during floods.
• Natural levees are deposited along the banks of
  the river during floods.

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During floods, water spills into the floodplain
and slows down due to greater friction (the river
gets wider ? more surface area). In response to
this, large grained material gets deposited
first, closest to the channel ? levees. Mud and
silt (clay-sized material) spread all over the
floodplain.
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Fluvial Geomorphology

• Deltas large, fan-shaped sedimentary deposits
  that form at the mouth of a stream when it flows
  into a larger body of water and the flow velocity
  drops.
• Deltas grow outward from the shoreline (prograde)
  over time as more and more sediment gets added.
• Deltas in lakes are simple. Deltas in marine
  settings are complicated because of interactions
  with ocean tides and wave action.

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Fluvial Geomorphology

• Simple deltas have a characteristic vertical
  sequence of sediments with unconformities between
  them.
• Bottomset beds at the bottom
• Foreset beds in between
• Topset beds at the top.

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Fluvial Geomorphology

• Bottomset fine sediments that are carried far
  beyond the streams mouth in suspension.
• Foreset inclined beds of sand and silt near the
  mouth.
• Topset coarse-grained sediments deposited in a
  braided (distributary) network of channels on top
  of the delta
• lengthening of the stream on top of the new
  land

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Fluvial Geomorphology

• Alluvial fans lobate, fan-shaped deposits of
  alluvium (mostly coarse sand and gravel with some
  mud flow deposits) on land.
• Form in all environments but are best in dry
  regions next to high mountains (like the El Paso
  area!).

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Fluvial Geomorphology

• Alluvial fans form when periodic rainstorms occur
  and runoff begins in the mountains.
• The flow is confined to canyons leading to the
  lowlands.
• When the flow gets to the lowlands, it spreads
  out, slows down, and deposits all the material it
  is carrying.

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Running Water

• Introduction the hydrologic cycle
• Hydrology
• Stream erosion, sediment transport, deposition
• Floods
• Fluvial geomorphology
• Base level and graded streams
• Drainage basins and stream valley development

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Base Level

• Streams can only erode down (incise) to the level
  of the body of water they flow to. This lower
  limit is the base level.
• Ultimate base level sea level
• Affected by rise and fall of sea level and
  tectonics
• Local base level lakes, larger river,
  waterfalls
• Affected by tectonics, geology, and landscape
  development
• A stream needs a slope. Therefore, it cannot
  lower its entire channel to base level. Streams
  will always try to though...

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Graded Streams

• Longitudinal profile the elevation of a channel
  along its length as viewed in cross section.
• Ideally, the longitudinal profile should be a
  smooth curve with a convex up shape and that can
  be described as an exponential function.
• In reality, there are irregularities
  (knickpoints) due to tectonics, geology, local
  base levels, etc.

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Graded Streams

• Where the profile is too steep, there will be
  more erosive power available to erode the stream
  bed (erasing knickpoints).
• Where the profile is too shallow, there is more
  deposition (filling-in the channel).
• Streams work to erase irregularities and attain
  their equilibrium shape (a graded profile).

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Graded Streams

• The equilibrium, graded river profile is attained
  by a delicate balance between gradient,
  discharge, flow velocity, sediment load, base
  level, etc.
• At equilibrium, neither deposition nor erosion
  occurs. This is never attained. Something
  always disturbs equilibrium But rivers work to
  adjust themselves.

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Running Water

• Introduction the hydrologic cycle
• Hydrology
• Stream erosion, sediment transport, deposition
• Floods
• Fluvial geomorphology
• Base level and graded streams
• Drainage basins and stream valley development

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Drainage Basins

• Streams form networks or systems.
• Large trunk streams have smaller tributaries that
  feed into them. Those tributaries have their own
  smaller tributaries, etc.
• All the streams in a particular network (or part
  of a network) carry surface runoff from an area
  called the drainage basin. All water in that
  basin will flow into that network and no other.
• Drainage basins are separated by topographic
  highs called divides.

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Drainage Patterns

• The arrangement of tributary streams in a
  drainage network can vary depending on the nature
  of the geology and topography in the drainage
  basin.

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Stream Valley Development

• Valleys form as a consequence of stream incision.
  The river sets the slopes of the sides of the
  valley and cause mass wasting. Overall the
  valley gets deeper and wider.
• Valleys can get longer by headward erosion.
• Valleys vary in size, depth, and shape due to
  geology, tectonics, and erosion process

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(a) A gorge formed mostly by down cutting. Rapid
incision may be due to rapid surface uplift. (b)
A valley formed by both down cutting and
valley-widening. Can happen if uplift is slow.
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Stream Capture

• One consequence of headward erosion (erosion of
  drainage divides).
• Headward erosion can breach a drainage divide and
  divert water from one drainage basin into another
  one. The bigger drainage basin will assimilate
  the smaller one.
• Stream capture causes very drastic and relatively
  rapid changes to the system. Will result in
  disequilibirum in discharge, gradient, flow
  direction, erosion, deposition, etc.

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Superposed Streams

• Stream courses are determined by topography.
  Water will flow down the steepest part of the
  topography. Water takes the easy way.
• But there are many examples of rivers cutting
  through mountains! Why would the stream want to
  do this?

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Superposed Streams

• Stream courses are also affected by geologic
  history and subsurface geology.
• Streams may start out cutting through soft rock
  only to find harder rock beneath. By that time,
  their courses are well-established and they keep
  incising.
• Also rapid tectonic events (uplfit, faulting,
  folds) can force rivers to incise into
  topographic highs.

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River Terraces

• Erosional remnants of now abandoned floodplains
  that have been incised into.
• The terrace surfaces formed when the stream was
  flowing at a higher level in the past.
• There are often several levels, sometimes very
  high above the present river level.
• Indicates episodic incision of the river due to
  uplift, baselevel changes, climate changes, etc.

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Incised Meanders

• Streams in areas of rapid surface uplift may
  become restricted to deep, meandering canyons cut
  into solid bedrock.
• The stream cannot effectively erode laterally.
  So it must cut straight down.
• The fact that meanders form due to lateral
  migration means that rivers with incised meanders
  must have originally flowed over alluvium. Later
  on the river was disturbed by rapid uplift.

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www.geo.utep.edu/pub/hurtado/2412