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