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Powerpoint Presentation Physical Geology, 10/e


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 ... – PowerPoint PPT presentation

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Title: Powerpoint Presentation Physical Geology, 10/e

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

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
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

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

The Parana River, Brazil rectangular drainage?
The Big Sandy a meandering stream
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
  • 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
Deposition of sand bars, formation of cross
bedding and Ox-bows (not related)
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

Stream Erosion
  • Stream gradient is the downhill slope of the
  • 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

Stream Erosion
  • Streams cut their own valleys, deepening and
    widening them over time and carrying away the
  • 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

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

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

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

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

The Ganges River Delta
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

  • 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

  • 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
  • Typically triggered by heavy thunderstorms

  • 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

Stream Valley Development
  • Downcutting
  • Process of deepening a valley by erosion of the
  • 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)

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
  • Headward erosion is the slow
    uphill growth of a valley above
    its original source by gullying,
    mass wasting, and sheet erosion

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

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The Colorado a meandering stream
The Colorado downcutting due to tectonic uplift
Stream Valley Development
  • Incised meanders
  • Retain sinuous pattern as they cut vertically
  • May be produced by profound base level changes,
    as when rapid tectonic uplift occurs

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

Lecture OutlinesPhysical Geology, 11/e
  • Plummer, McGeary Carlson

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

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
  • Type of movement
  • Flow, slide, or fall

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

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

Factors Controlling Mass Wasting
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

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

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

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
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