Powerpoint Presentation Physical Geology, 10e

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Running Water

• Running water is the most important geologic
  agent in eroding, transporting and depositing
  sediment
• Nearly every landscape on Earth shows the results
  of stream erosion or deposition

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Hydrologic Cycle

• Hydrologic cycle - the movement and interchange
  of water between the sea, air, and land
• Evaporation
• Solar radiation provides energy
• Precipitation
• Rain or snow
• Transpiration
• Evaporation from plants
• Runoff
• Water flowing over land surface
• Infiltration
• Water soaking into the ground

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Running Water

• Stream - a body of running water, confined to a
  channel, that runs downhill under the influence
  of gravity
• Headwaters - upper part of stream near its source
  in the mountains
• Mouth - place where a stream enters sea, lake
  or larger stream
• Channel - a long, narrow depression eroded by a
  stream into rock or sediment
• Stream banks - sides of channel
• Streambed - bottom of the channel
• Floodplain - flat valley floor composed of
  sediment deposited by the stream

Insert revised Fig. 10.2
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Drainage Basins

• Drainage basin - the total area drained by a
  stream and its tributaries
• Tributary - a small stream flowing into
• a larger one
• Divide - ridge or high ground that divides one
  drainage basin from another
• Continental Divide separates the streams that
  flow into the Pacific from those that flow into
  the Atlantic and Gulf of Mexico

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Mississippi River Drainage Basin
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Missouri River Drainage Basin
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Yellowstone River Drainage Basin
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Powder River Drainage Basin
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Drainage Patterns

• Drainage pattern - the arrangement, in map view,
  of a stream and its tributaries
• Most tributaries join the main stream at an acute
  angle, forming a V or Y pointing downstream
• Dendritic - drainage pattern resembling the
  branches of a tree
• Radial pattern - streams diverge outward like
  the spokes of a wheel
• Typically form on conical mountains (volcanoes)
• Rectangular pattern - tributaries have frequent
  90 bends and join other streams at right angles
• Trellis pattern - parallel streams with short
  tributaries meeting at right angles

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The Parana River, Brazil rectangular drainage?
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The Big Sandy a meandering stream
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Sediment Deposition

• Meandering streams flow faster along the outside
  of bends and more slowly along the inside,
  depositing point bars on the insides of the
  meanders
• Meander cutoffs may form when a new, shorter
  channel is cut through the narrow neck of a
  meander (as during a flood)

Insert Fig. 10.20
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Deposition of sand bars, formation of cross
bedding and Ox-bows (not related)
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Stream Erosion

• Stream erosion (and deposition) controlled by
  flow velocity and discharge
• Stream velocity controlled by stream gradient
  (slope), channel shape and channel roughness
• Maximum velocity near center of channel
• Floods involve increased velocity and discharge
  (volume of water passing a particular point in a
  stream over time)
• Higher stream velocities promote erosion and
  transport of coarser sediments
• Erosion of very small particles difficult due
  to molecular binding forces

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Stream Erosion

• Stream gradient is the downhill slope of the
  streambed
• Typically measured in feet per mile in the U.S.,
  and in meters per kilometer elsewhere
• Usually decreases downstream
• Channel shape and roughness
• Both effect stream velocity due to drag
• Narrower, deeper channels allow faster flow
• Smoother channels allow faster flow
• Wider, shallower channels decrease flow speed
• Rougher channels decrease flow speed
• Stream discharge is the volume of water flowing
  past a given point in a unit of time

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Stream Erosion

• Streams cut their own valleys, deepening and
  widening them over time and carrying away the
  sediment
• Stream erosion occurs by three mechanisms
  hydraulic action, solution, and abrasion
• Hydraulic action - ability of flowing water to
  pick up and move rock and sediment
• Solution - dissolving of rocks (e.g., limestone)
• Abrasion - grinding away of stream channel by the
  friction and impact of the sediment load
• Potholes are eroded into streambed by the
  abrasive action of the sediment load in the
  stream

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Sediment Transportation

• Sediment load transported by a stream can be
  subdivided into bed load, suspended load, and
  dissolved load
• Bed load - large or heavy particles that travel
  on the streambed
• Traction load - large particles that travel along
  the streambed by rolling, sliding or dragging
• Saltation load - medium particles (typically
  sand-sized) that travel downstream by bouncing
  along - sometimes in contact with the streambed
  and sometimes suspended in the flowing water
• Suspended load - sediment that is small/light
  enough to remain above the stream bottom by
  turbulent flow for an indefinite period of time
• Dissolved load - dissolved ions produced by
  chemical weathering of soluble minerals upstream

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Sediment Deposition

• Sediments are temporarily deposited along stream
  course as bars and floodplain deposits, and
  at/near its end as deltas or alluvial fans
• Bars - ridges of sediment (usually sand or
  gravel) deposited in the middle or along the
  sides of a stream
• Braided streams contain sediment deposited as
  numerous bars around which water flows in highly
  interconnected rivulets

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A Braided River typical of rivers just
disgorging from mountainous areas. A rapid drop
in current velocity and too much stuff to carry
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The North Platte River A Braided Stream
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Sediment Deposition

• Floodplains are broad strips of land built up by
  sedimentation on either side of a stream channel
• Floodplain sediments are left behind as flood
  waters slow and recede at the end of flood events
• Main channel has slightly raised banks with
  respect to the floodplain known as natural levees

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Sediment Deposition

• Delta - body of sediment deposited at the mouth
  of a river when flow velocity decreases
• Surface marked by shifting distributary channels
• Shape of a delta depends on whether its
  wave-dominated, tide-dominated, or
  stream-dominated

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The Ganges River Delta
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Sediment Deposition

• Alluvial fan - large, fan- or cone-shaped pile of
  sediment that forms where stream velocity
  decreases as it emerges from a narrow mountain
  canyon onto a flat plain
• Well-developed in desert regions, such as the
  southwestern U.S.
• Larger fans show grading from large sediments
  nearest the mountains to finer sediments farther
  away

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Flooding

• When water levels rise and overtop the banks of a
  river, flooding occurs
• Natural process on all rivers
• Described by recurrence intervals
• A 100-year flood is, on average, the size of the
  largest flood within a 100-year period of time
• Can cause great damage in heavily populated areas
• High velocity and large volume of water causes
  flood erosion
• Slowing of waters as flood ends causes flood
  deposits (usually of silt or clay-sized
  particles) to be deposited in the floodplain

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Flooding

• Urban flooding
• Paved areas and storm sewers increase runoff by
  inhibiting infiltration
• Rapid delivery of water to streams increases peak
  discharge and hastens occurrence of flood
• Flash floods
• Local, sudden floods of large volume and short
  duration
• Typically triggered by heavy thunderstorms

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Flooding

• Flood control
• Dams designed to trap flood waters in reservoirs
  upstream and release it gradually over time
• Artificial levees designed to increase capacity
  of river channel
• Works well until stream overtops artificially
  raised levees, leading to extremely rapid
  flooding and erosion
• Wise land-use planning, including prevention of
  building within 100-year floodplains, is most
  effective

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Stream Valley Development

• Downcutting
• Process of deepening a valley by erosion of the
  streambed
• V-shaped valleys typically form from downcutting
  combined with mass wasting and sheet erosion
• Streams cannot erode below their base level
• Basel level can be sea level, a lake, or the
  bottom of a closed basin (e.g., Death Valley, CA)
• Downcutting rate can be rapid if a stream is well
  above base level (e.g., Grand Canyon, AZ)

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Stream Valley Development

• Graded streams
• Characteristic concave-up longitudinal profile
• Rapids and waterfalls have been smoothed out
  by extensive erosion over a long period of time
• Delicate balance between available sediment
  load and transport capacity
• Lateral erosion widens stream valleys by
  undercutting of stream banks and valley walls as
  stream swings from side to side across the valley
  floor
• Headward erosion is the slow
  uphill growth of a valley above
  its original source by gullying,
  mass wasting, and sheet erosion

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Stream Valley Development

• Stream terraces
• Step-like landforms found above a stream and its
  floodplain
• Occurs when river rapidly cuts downward into its
  own floodplain
• Represents relatively sudden change in rate of
  erosion
• Can be caused by rapid uplift, drops in base
  level, changes in underlying lithology or climate
  changes

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The Colorado a meandering stream
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The Colorado downcutting due to tectonic uplift
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Stream Valley Development

• Incised meanders
• Retain sinuous pattern as they cut vertically
  downward
• May be produced by profound base level changes,
  as when rapid tectonic uplift occurs

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Stream Valleys on Mars

• Evidence of different climate in past
• Liquid water not stable on surface of Mars under
  present conditions
• Too cold
• Atmospheric pressure too low
• Stream channels and terraces suggest long-term
  erosion by flowing water
• Lack of smaller tributaries is puzzling, but
  these do exist for channels networks in more
  ancient terrains on Mars
• Requires warmer, wetter Mars
• NASA missions targeting such locations

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Lecture OutlinesPhysical Geology, 11/e

• Plummer, McGeary Carlson

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Mass Wasting

• Mass wasting is downhill movement of masses of
  bedrock, rock debris or soil, driven by the pull
  of gravity
• Landslides have been far more costly in the
  U.S., in terms of both lives and dollars, than
  all other geologic and weather hazards combined
• Mass wasting is, with proper planning, perhaps
  the most easily avoidable of all major geologic
  hazards

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Classification of Mass Wasting

• Types of mass wasting are classified based on
• Rate of movement
• Wide range from lt 1cm/year to gt100 km/hour
• Type of material
• Did moving mass start out as solid bedrock or as
  debris (unconsolidated material at Earths
  surface)
• Type of movement
• Flow, slide, or fall

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Classification of Mass Wasting

• Types of movement
• Flow
• Descending mass moves downhill
  as a viscous fluid
• Slide
• Descending mass remains
  relatively intact, and descends
  along well-defined surfaces
• Translational slide - movement
  along plane parallel to motion
• Rotational slide (slump) - movement along
  a curved surface
• Fall
• Material free-falls or bounces down a cliff

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Factors Controlling Mass Wasting

• Factors making mass wasting likely
• Steep slopes
• Shear forces maximized by gravity
• Large relief
• (large elevation change from top of
• mountains/hills to valley floor)
• Thick layer(s) of loose rock,
• debris, soil
• Presence of water
• Lubricates moving rocks/debris/soil
• Lack of vegetation
• No roots to hold rock/soil in place
• Seismic (earthquake) activity

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Factors Controlling Mass Wasting
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Common Types of Mass Wasting

• Creep (or soil creep)
• Very slow downslope movement of soil
• Major contributing factors include water in soil
  and daily freeze-thaw cycles
• Can be costly to maintain homes, etc., on
  creeping ground as foundations, walls, pipes and
  driveways crack and shift downslope over time

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Common Types of Mass Wasting

• Debris flow - mass wasting in which motion takes
  place throughout the moving mass (flow)
• Earthflow - debris moves downslope, slowly or
  rapidly, as a viscous fluid
• Commonly occurs on steep hills, with thick debris
  cover, after heavy rains
• Solifluction is an example
• Mudflow - flowing mixture of debris and water,
  usually down a channel
• Most likely to occur on steep unvegetated slopes
  with thick debris cover
• Heavy rains on the slopes of stratocone volcanoes
  with fresh ash layers often triggers
• Debris avalanches are very rapid and turbulent
• Can reach speeds of several hundred km/hr

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Types of Mass Wasting

• Rockfall - when a block of bedrock breaks free
  and falls or bounces down a cliff
• Commonly an apron of fallen rock fragments
  (talus) accumulates at cliff base
• Rockslide - the rapid sliding of a mass of
  bedrock along an inclined surface of weakness
• Rock avalanche - a very rapidly moving, turbulent
  mass of broken-up bedrock
• Debris slide - a coherent mass of debris moving
  along a well-defined surface
• Debris fall - a free-falling mass of debris

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Preventing Landslides

• Preventing mass wasting of debris
• Construct retaining wall with drains
• Dont oversteepen slopes during construction
• Preventing rockfalls and rockslides on highways
• Remove all rock that is prone to sliding
• Stitch together outcrop
• Important to know the susceptibility of land to
  mass wasting before building any road or
  structure!