Text Reading: Pages 39-49; 59-116
Terminology: abrasion, alluvial fan, alluvium, base level, bed load, braided stream, capacity, competence, cut bank, delta, dendritic drainage pattern, discharge, dissolved load, divide, drainage basin, floodplain, gauging station, graded stream, gradient, head (headwaters), headward erosion, incised meanders, laminar flow, local (temporary) base level, longitudinal profile, meander, mouth, natural levee, oxbow lake, point bar, pothole, rating curve, rills, river stage, saltation, slope retreat, stream piracy, stream terraces, suspended load, trellis drainage pattern, tributary, turbulent flow, ultimate base level, yazoo stream
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)
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