Geology 1001-section 4
Dr. Dutrow
Partial version of overheads
 
 

  Return to syllabus                                                last updated: 04/08/98

RIVERS

Streams - any flowing body of water, irrespective of size, flowing downhill in a natural passageway

Rivers - major branches of a large stream system

Channel - natural passageway of a stream

Load - detritus that a stream moves

Worldwide streams carry 16 billion tons of clastic sediment/year and 2-4 billion tons of dissolved material Discharge - quantity of water passing a point along a stream/unit time
Hydrologic Cycle
Amount of water,

1.36 Billion cubic km

97.2% in oceans, 2.15% in ice caps,           0.65% in lakes,
 

water cycles

cycles driven by
 
 

Anatomy of a STREAM
watershed - are from which water

drainage basin

Drainage divide - bounds each
 

        e.g. Continental Divide

Headwaters - origin of the

    rills
    trunk stream
    tributaries
 

Mouth - distall portion of a

Distributaries - small channels that
 

STREAM GRADIENTS - Stream flow and discharge

Flow of stream driven by

Longitudinal Gradient - slope of a river from

 

determined by plotting elevation vs. distance

- produces

- steep near

 

streams all have this profile because

 

 

Base Level - elevation at which stream enters a standing body of water controls

- cannot cut below this point

 

 Graded Stream - a stream at - i.e. no sedimentation or erosion

- produces a

 Alluvial Fans - cone-shaped accumulations
HOW STREAMS FLOW

Motions of flowing fluids

- depicted by Streamlines - lines of fluid motion

-streamlines do not

-characteristic of

   -streamlines

-characteristic of

-most watercourses in Nature

-e.g. eddies

 Type of flow dependent on:
1. average velocity of flow (m/sec)
      as velocity increases
2. geometry of flow;
depth increases -

decreases -
3. physical properties of fluid
      e.g. viscosity, temperature

      -viscosity - measure of fluid's

      - high viscosity -

4. stream may have both flow types
- turbulent in center, laminar along edges
  5. Velocity     depends on
  6. Discharge = volume of water passing
 
  typically given in cfs

- if  discharge increases, then                       or

  
EROSION BY RUNNING WATER
Geologic work of a stream
Stream Erosion - stream cuts down to

Processes:

    Downcutting - creation and deepening of

    Abrasion - scouring of

                    potholes

    Hydraulic lifting - erosion by
 

    Dissolution - dissolves
 

    Drainage networks - pattern of

            Dendritic
            Rectangular
            Trellis
            Radial

            -often provide clues to

Stream Piracy - when a stream breaks through
 

 
 Additional Erosion by running water:

1. pick up unconsolidated materials from

2. undercut banks causing

3. make gullies or valleys by eroding material

- headward erosion 4.

5. swirling eddies creates

6. 

7.
 

STREAM VALLEYS, CHANNELS and FLOODPLAINS

PHYSIOGRAPHIC FEATURES OF A STREAM:

1. STREAM VALLEY - area between tops of slopes on both sides of river

- often 2. CHANNEL - bottom of valley through which water runs

3. FLOODPLAIN - flat area ca. level with top of channel that is

4. size dependent on tectonic activity and location

   
CHANNEL PATTERNS

-vary due to changing gradient, discharge, load

1. Straight Channels - run straight, usually for

2. Meandering Channels - smooth bends

(a)meanders - curves or bends in a channel
  e.g Mississippi River - 20 m/yr  
-placer deposits in higher velocity region i.e. like panning for gold
(b) point bar - sediment accumulation on
(c)oxbow lake - cutoff

 

e.g. False River, LA 3. Braided Channels- many channels

 

-form where rivers have

 

 FLOODPLAINS - created by migration of

 

- when river spills over banks, water

 

  natural levees - buildup of

 

- confine stream

 
7. STREAM LOAD and SEDIMENT MOVEMENT

Stream Load carried as:

1. Solid portion

(a) suspended load = particles suspended in the water,

- commonly carried due to

(b) bed load - particles moving along

(c) Competence - ability of a flow to

 

  e.g. faster current,

- increase velocity,

 (d) Capacity - total sediment e.g. larger volume flow,

MS - moderate velocities, fine-medium particles, but

 (e) Placer deposits - created when e.g. Gold, diamonds

 

 2. Dissolved load - primarily due to e.g. important ions: HCO3-, Ca2+, SO42-, Cl-, Na+, K+
 
Settling from Suspension

Sediment suspension dependent on competing forces of

    1.                             and
    2.  
Settling velocity - rate with which suspended particles of various sizes settle to bottom

e.g. silt and clays,              ; sand

Saltation - movement of sand grains

 

e.g. sucked in by turbulent eddies, move, drop back to bed
  Point Bar - sediment accumulation on the

Floodplain Deposition

        when river spills over banks, water velocity rapidly

        natural levees

Deposition into  Valley

    Alluvial Fans -    shaped accumulations of

                base of

Deposition into standing body of water

    Delta - depostional platform where a stream enters
                        and deposits its
 

        Composed of
            1. Topset
            2. Foreset
            3. Bottomset beds -

        Migrate,

                e.g. MS delta (Bird's Foot) began ca. 150 MY near OH/IL
 

BEDFORMS

1. Ripples - low, narrow ridges separated by wider troughs.

 

2. Dunes - same general form as ripples but larger - up to meters in height

generally form in 3. Migration causes grains to be deposited at angles of 30-45° on downslope slide creates

cross-beds - proportion to size of dunes.
- can estimate relative velocity
 

FLOODS
     

FLOOD PREDICTION

1. impossible to predict months in advance

2. use probabilities that

e.g. 20 % probability

 

20 % chance of flood, i.e.

 

use flood frequency curves

 

 Problem: we are dealing with

 

Other factors effect results of prediction.

  1. the climatic observational base
  2. climate change is
 

 

However, we know that floods occur because

 

 

Catastrophic flood - very infrequent (several hundred years)

e.g. Channeled Scablands of Washington, due to breaking of an ice dam in MT during the last glaciation
    - e.g. base of concave upward - upper reaches have

 

Terraces - flat benches that

 

 

 
DRAINAGE NETWORKS

Divide - a line along which runoff is

 

e.g. Continental Divide Drainage basin - all streams and tributaries that are

 

 

Stream Piracy - when stream breaks thru

 

Drainage Patterns

Drainage networks - patterns of connections between tributaries

    1. Dendritic
    2. Rectangular
    3. Trellis
    4. Radial
DELTAS

Delta - depositional platform where a stream enters

 

Composed of:

(a)topset beds - materials dropped
 
(b) foreset beds - downcurrent,
 
(c) bottomset - thin,
 

Migrates, grow with time to build up ,