Surface Processes
The Hydrologic Cycle
Streams

Hydrologic Cycle
=> solar powered circulation of Earth's water supply
=> Precipitation (meteoric water) on land leads to runoff and infiltration
=> runoff =  streams
=> infiltration =  groundwater

Streams
=> streams = the most important agent of erosion and transport

=> surface features produced by internal processes but are modified and sculpted by streams.........
Surface Runoff ==> Streams
•     => gravity driven
*Stream = channelized flow of any size

    => parts = channel (bed and banks) & floodplain

Stream Formation
•     Sheetwash
•     Downcutting & headward erosion = channel
•     Trunk streams & tributaries = drainage network
Drainage Patterns
•     Controlled by geologic setting

•     Dendritic, radial, rectangular, trellis

Drainage Basins and Divides
- A stream system occupies a drainage basin
- Stream systems are separated by divides

Streamflow: characterized by
1.  Gradient: = slope of stream channel (ft/mi, m/km)

2.  Channel characteristics: shape, size, bed conditions (friction)

3.  Discharge: = volume/unit time (m3/sec)
         => cross-sectional area X velocity

Work of Streams - as a surface process
•     1) Erosion; 2) Transport; 3) Deposition

•     Fluvial landforms
 Erosion
•     Turbulent flow

•     mechanisms = abrasion, hydraulic action, solution

Types of Stream Erosion
1. headward erosion

2. Downcutting

3. lateral erosion

Transport
=> accomplished in three ways - Loads:

    1) dissolved load - ions in solution
    2) suspended load - silt and clay
    3) bed load (saltation and sliding) - sand and gravel

 Deposition
•     takes place as velocity decreases and competence is reduced

=> 3 major depositional environments (fluvial)

1) Delta; 2) Alluvial Fan; 3) Channel and Flood deposits

Delta
•     velocity decreases as stream enters lake or ocean

•     Deposition => distributary channels

•     Delta Growth => channel migration

•     depositional features = 1) Prodelta, 2) Delta front, 3) Delta plain
•     Progradation

•     deltas shaped by fluvial processes plus tides and waves..

EX: Mississippi River Delta - a river-dominated delta

•     Channel migration => the fate of New Orleans

Alluvial Fan
•     velocity decreases due to abrupt change in stream gradient......

=> stream leaves mountains and enters broad flat valley
EX: Basin and Range Province
Channel and Flood Deposits
    => channel deposits = bars (sand and gravel)

•     2 types of streams: a) meandering, and b) braided
           
Meandering Streams
-> velocity greatest on outside of bend = erosion: cutbank
       
-> velocity is less on inside of bend= deposition: point bar

•     meanders migrate and grow larger over time, if too large they are cut off.........oxbow lake......

braided streams
->more sediment available than stream can carry

->amount of sediment reduces velocity = deposition in channel
 = mid-channel bars

Flood Deposits
=> as floodwaters recede, velocity reduced = deposition on floodplain (silt and clay)

=>during flood, current is slowest along edge of channel
    produces=> natural levee
Flooding
•     Seasonal floods
•     Flash flood

•     Can be very costly

Recurrence of Floods
•     2 - year floods = 50% chance => small events
•     10 - year flood = 10% chance => moderate size
•     100 - year flood = 1% chance => very large floods

Flood Control
•     Levees, bypass spillways, dams, channel modifications
•     Greenbelts or floodways

•     Downstream effects
Stream erosion has limits
Base Level

=> the downward limit of stream erosion:(downcutting)

=> two types       1) local; and 2) ultimate
   
Local or Temporary base level = lakes, waterfalls
   
Ultimate Baselevel = sea level

Local base level - waterfalls
=> streams erode to baselevel, but baselevel changes over time
Spectacular valleys and canyons represent major changes in base level

Tectonic Uplift = lower base level = valley incision
Changes in ultimate base level
•     Changes in global sea level
•     Valley incision
•     Incised valley fill
•     Terraces

Evolution of a Stream System
•     Uplift & rejuvenation
•     Downcutting and headward erosion - narrow floodplain
•     Lateral erosion - wide floodplain
Stream Systems - downstream changes

—How could flooding be prevented or the damage from flooding be reduced in the F/M area?

F/M Geology & Flooding

• Geology complicates the solution
• Lake Agassiz muds = 100 feet of mud