STREAM VELOCITY CONTROLLED BY

 

Gradient or slope of stream channel

Cross-sectional shape, size and roughness of channel

Stream discharge = volume of water passing point per unit time

 

STREAM GRADIENT

 

Gradient or slope of a stream is the vertical drop over a fixed distance

 

Lower Mississippi River gradient ~ 10 cm/km

Mountain stream channels ~ 40 m/km

 

If all other variables are the same (size, shape, roughness and discharge), steeper gradient streams would have higher velocities

 

CROSS-SECTIONAL SHAPE

 

Water velocities are affected by frictional drag on the channel bottom

 

A semi-circular channel will have the smallest wetted perimeter (surface area for frictional drag) and the highest velocity (if all other variables are the same)

 

In wide, shallow channels – more frictional surface – lower velocities than in a semicircular channel

 

In a straight channel – velocity is highest in center of channel just below the surface

 

In a curved channel – velocity is highest in outside of curve

 

ROUGHNESS

 

Roughness of the channel bottom and sides will affect water velocity

 

Smooth bottom channels (concrete) will have higher flows than same sized and shape boulder-strewn channel bottoms

 

 

 

STREAM DISCHARGE

 

Measured at stream gauging stations

 

Measure stream velocity [length/time] along a transect across the stream

 

Stream discharge [units of length^cubed or volume/time] is calculated by multiplying stream velocity [units of length/time] by the stream’s cross-sectional area [width x depth (units of length^squared)]

 

Monitor water level in channel (river stage) during a wide range of stream discharges to obtain the gauging station’s rating curve

 

With a rating curve, water level can be monitored by an automatic logger to obtain stream discharge

 

 

 

WORLD’S LARGEST RIVERS BY DISCHARGE

 

1. Amazon, Brazil         2,231,000 square miles drainage    

                                      7,500,000 cubic feet per second (cfs)

 

2. Congo, Zaire             1,550,000 square miles drainage

                                      1,400,000 cfs

 

3. Yangtze, China          750,000 square miles drainage

                                      770,000 cfs

 

4. Brahmaputra,             361,000 sq miles drainage

   Bangladesh                700,000 cfs

 

5. Ganges, India             409,000 sq miles drainage

                                       660,000 cfs

 

6. Yenisei, Russia          1,000,000 sq miles drainage

                                       614,000 cfs

 

7. Mississippi, US           1,244,000 sq miles drainage

                                        611,000 cfs

 

8. Orinoco,                   340,000 sq miles drainage

     Venezuela               600,000 cfs

 

9. Lena, Russia            936,000 sq miles drainage

                                      547,000 cfs

 

10. Parana,                   890,000 sq miles drainage

      Argentina                 526,000 cfs

STREAM LONGITUDINAL PROFILE

 

Overall profile is a smooth concave, upward curve (local irregularities may exist)

 

Gradient decreases downstream

 

Discharge increases downstream as more and more tributaries contribute to flow

 

As discharge increases, in order to handle the additional water either

           Stream velocity increases or

            Width and/depth of channel increases or

            Combination of both as enlarged channels typically have less                frictional drag

 

 

 

 

MOVING DOWNGRADIENT FROM HEADWATERS TO MOUTH

 

Stream gradient and roughness decrease

Stream channel width, depth, discharge and velocity increase

 

 

However, in steep, rough mountainous channels of headwards, turbulent flow dominates and may appear faster moving

 

In wide, placid rivers of the lowlands – large discharges and velocities with smooth, laminar flow

 

BASE LEVEL

 

Lowest point to which a stream may erode its channel

Ultimate base level = sea level

Temporary or local base level = lakes, main streams, resistant rock

 

GRADED STREAM

 

Stream with the correct slope and channel characteristics to maintain the velocity required to transport the material supplied to it

 

Graded stream on average is neither eroding nor depositing material but is simply transporting it = equilibrium state

 

Changes in the base level, discharge, channel shape or size, or sediment load will result in erosion or deposition until a new equilibrium state is found

 

CHANGES IN BASE LEVEL

 

Change in sea level

Build dam

 

è cause a stream to adjust and try to establish a new equilibrium

 

 

STREAM EROSION

 

Water picks up and moves loose material (sediments) if sufficient velocity for the sediment size

 

Abrasion – force of loose material hitting against sides of channel

 

Abrasion contributes to channel cutting

 

Potholes – swirling eddies abrade holes in channel bedrock bottom

 

 

SEDIMENT TRANSPORT

 

Bed Load – large sediments moving intermittently along the channel bottom

                  Saltation – sediments hop along bottom

                   Traction – sediments are dragged along bottom

 

Suspended Load – sediments carried in suspension above channel bottom.  Usually the largest percent of the total sediment load.

 

Dissolved Load – minerals (salts) dissolved in stream water

 

SEDIMENT TRANSPORT ABILITIES

 

Capacity – maximum load of solid particles a stream can carry

 

Competence – maximum particle size a stream can transport

 

Competence increases as the square of stream velocity è as velocity doubles, water’s force increases fourfold

 

 

SEDIMENT DEPOSITION

 

Streams deposit sediment when velocity decreases and competence is reduced

 

Sorting occurs as competence decreases and larger sized sediment will be deposited first.  Progressively smaller and smaller grain-sized sediment will be deposited as velocity decreases further.

 

Sediment deposits from stream action (alluvium) occur in channels (bars), along edges of stream channel (natural levees), on floodplains, and at the mouth of streams (deltas and alluvial fans)

 

NATURAL LEVEES

Wedge-shaped deposits of fine sand, silt and mud tapering away from stream banks toward the flood plain

Form during flood stages, water overflows banks, velocity reduced, larger sediments deposited

Levees grow higher over time and river can be higher than surrounding floodplains

STREAM VALLEY TYPES

 

Narrow, V-shaped valleys – primarily downcutting toward base level.  Steep sided walls undergo mass wasting (landslides) and sediment removed by stream.  Contain waterfalls and rapids due to local base levels (from resistant rock) and bottom roughness (boulders in channel).

 

Wide valleys with flat floors – channel is close to base level and stream energy is directed from side-to-side.  Erosion produces a side flat valley floor with floodplain.  Many streams have wide sweeping curves or meanders with oxbow lakes.

 

MEANDERING STREAM CHANNELS

 

Water velocity is highest on the outside curve causing erosion which can produce a cut bank

 

Water velocity is lowest on the inside curve causing deposition of point bars

 

Progressive erosion and deposition results in cutoff meanders forming oxbow lakes

 

BRAIDED STREAM CHANNELS

 

Interwoven channels separated by midchannel bars

 

Occurs when the sediment load to channel exceeds its competence or capacity

 

 

DRAINAGE BASINS OR WATERSHEDS

 

Divide – imaginary line separating drainage basins.  On one side of the line, all water will drain into the drainage basin.  On the other side of the line, all water will drain into a different drainage basin.

 

 

 

Drainage Patterns – depends on rock type and pattern

 

Dendritic – irregularly branching tributaries resembling branching pattern of a tree.  Forms where underlying rock is uniform

 

Radial – streams diverge from a central area like spokes on a wheel.  Forms on isolated uplifted areas such as volcanoes and domes

 

Rectangular – right-angle bends form due to patterns of rock fractures. 

 

Trellis – tributary streams are nearly parallel to one another and look like a garden trellis.  Forms where underlying rock is alternating bands of low and high resistant rock found typically in folded rock areas

 

 

STREAM ORDER

 

1st order stream has no tributaries

2nd order stream formed by joining of two 1st order tributaries

3rd order stream formed by joining of two 2nd order tributaries

progresses until mouth of stream

 

 

 

HEADWARD EROSION

 

Lengthens a stream by extending the head of its valley upslope

 

Sheet flow converges into rills resulting in increased water velocity and erosion at the stream head

 

Can result in stream piracy in which the head of the stream extends upslope and erodes into another streams path – capturing the water drainage

 

 

 

INCISED MEANDERS

 

Meandering channels flowing in steep, narrow valleys

 

1.    Meanders on floodplain of stream near its base level

2.    Drop in base level or land uplifted, stream starts downcutting

 

Example of drop in base level = start of Ice Age, water locked in ice sheets, sea level dropped

 

Example of land uplift = Colorado Plateau of southwestern US

 

STREAM TERRACES

 

Drop in base level or land uplifted è flood plain uplifted è stream terrace

 

Other changes can cause erosion into flood plain deposits to produce a high ledge of sediments (terrace) along a channel.  Possibilities are:

1.    Increase in stream discharge – caused by climate change or successful stream piracy

2.    Decrease in sediment load – downstream of dams

 

During last Ice Age,  hydrology of most rivers changed significantly and produced many stream terraces

 

Glacial meltwaters and sediment load increased to many streams

 

End of Ice Age – erosion of channel into deposited sediment

 

Western US Stream Terraces

·        Most western alluvial valleys show three terrace levels

·        All developed within last 5,000 to 7,000 years (Holocene)

o      Ice Age ends about 10,000 years ago

o      Climate changed to drier and warmer from about 8,000 to 5,000 years before present è less vegetation, more intense rainfalls (not spread out over entire year), more erosion è produce terrace

o      Another dry period at 800 to 1100 AD (900 to 1200 years ago)

 

EPHEMERAL STREAMS

·        Carries water only during storms

·        Common in arid and semi-arid parts of the world

·        All relatively wide channels with small depth of flow

·        Generally sand bottoms, sometimes cobbles

·        Called a rambla in Spain

·        Called a wadi in Middle East

·        Called an arroyo in southwestern US