Water Cycle - Running Water

Northwest Geology

Chapter 15

http://www.kidzone.ws/water/

Streams

• Streams are the major geological agentsoperating on the surface of the land. The chief

• focus of streams are-How water flows in currents-How currents carry sediment-How streams break up and erode solid rock-How streams carve valleys and assume a variety of forms as they channel water downstream

Stream Water Flow

Begins as sheetflow Infiltration capacity is controlled by

• Intensity and duration of rainfall• Prior wetted condition of the soil• Soil texture• Slope of the land• Nature of the vegetative cover

Sheetflow develops into tiny channelscalled rills

Two types of flow determined primarily

by velocity– Laminar flow– Turbulent flow

Factors that determine velocity

– Gradient, or slope

– Channel characteristics including shape, size,

and roughness

Streamflow

• Laminar – water particles flow in straight paths giving flat, gentle surface

• Turbulent – water moves in an erratic fashion characterized by swirling eddies and whirlpools (white water)

Laminar vs. Turbulent

Streamflow

Factors that determine velocity

– Discharge – the volume of water moving past

a given point in a certain amount of time

- Changes from upstream to downstream

- Profile

* Cross-sectional view of a stream

*Viewed from the head (headwaters or source)

to the mouth of a stream

• Changes from upstream to downstream

-Factors that decrease downstream

• Gradient

• Channel roughness

-Base level and graded streams

(Base level is the lowest point to which a stream can erode

• Two general types of base level

– Ultimate (sea level)

– Local or temporary• Changing conditions causes readjustment

of stream activities• – Raising base level causes deposition• – Lowering base level causes erosion

• • Factors that increase downstream

-Velocity

-Discharge

-Channel size

• Stream erosion

-Lifting loosely consolidated particles by

abrasion

-Dissolution

(Stronger currents lift particles more

effectively)

Streamflow Discharge• Discharge is the volume of water that flows past a

given point in a given time. • The volume is calculated by multiplying the area water

in the stream channel (depth x width) by the velocity (distance/time) of the stream flow.

• The dimensions of the stream channel are expressed in feet or meters and velocity is measured in feet or meters per second. Consequently the units of discharge are cubic meters (or feet) per second, i.e. discharge is the volume (cubic meters or cubic feet) of water that passes a given point in one second.

Discharge (m3/sec) =channel width (m) x channel depth (m)

x velocity (m/sec)

Discharge can be calculated by multiplying the area of the stream channel

in cross section (width x depth) by the distance traveled in a given time (second).

Stream Transport

• Stream “load”– Dissolved load – dissolved and carried by the

water– Suspended load – suspended in and moved

by the water– Bed load – moved along the bottom (bed)– Flotation load – light material floating on

surface

Stream Capacity

• Maximum load a stream can support (total sediment load from previous slide)

Sediment Deposition

• Caused by a decrease in velocity

-Competence is reduced

• Sediment begins to drop out

• Stream sediments generally well sorted

• Stream sediments are known as alluvium

• Deposition of sediment by a stream• • Channel deposits• • Bars• • Braided streams• • Deltas• • Floodplain deposits• • Natural levees – form parallel to the stream• channel by successive floods over many years

• Deposition of sediment by a stream• • Floodplain deposits• • Back swamps• • Yazoo tributaries• • Alluvial fans• • Develop where a high-gradient stream leaves• a narrow valley• • Slopes outward in a broad arc

Stream Valleys

• Most common landforms

• Downcutting toward base level

• Often include falls and rapids

• Two general types– Narrow and steep– V-shaped

Drainage Networks

• The pattern of the interconnected networks of streams in an area

• Common drainage patterns– Dendritic– Radial– Rectangular– trellis

http://id.waterdata.usgs.gov/nwis/current?type

=flow

Streamflow Summary

• Discharge is the volume of water that flows past a given point in a given time.

• The USGS has over 7,000 stream gaging stations.  Data from over half of which are available on-line

• Stream gages measure the depth of water in a stream channel and the velocity of flow

• The recurrence interval is the average time in years between floods of the same size

• A hydrograph  illustrates discharge over time

Stream velocity may be measured quite simply.   The technique is to

drop a floating object in the stream, time its travel along a meter stick

several times, and average the readings

Stream velocity varies within a

stream channel.  Maximum velocity

occurs on the outside of channel bends; minimum

velocity occurs on the inside of bends.

Erosion occurs on the outer banks of streams (cut-banks) where velocity is greatest and deposition

occurs on inner banks (forming point bars) where velocity is least. The stream channel slowly migrates

across the valley floor in the direction of the erosion to form long, looping bends termed meanders.

Fluvial features

Oxbow lake

Factors of stream velocity that control erosion

• Gradient

• Channel shape, size roughness

• discharge

Summary of Stream Velocity• Stream velocity varies around curves in stream channels • Velocity is greatest on the outside of curves, least on the

inside • Erosion occurs in areas of higher velocity forming cut-

banks on the outside of stream curves • Deposition in regions of low stream velocity forms point

bars on the inside of curves • Erosion and deposition cause stream channels to

migrate laterally • Velocity decreases as the  length of the wetted perimeter

(channel banks and bed) increases • A floodplain is a broad, flat plain adjacent to a stream

channel