Stream Erosion & DepositionChapter 6 sections 1 and 2

What is a Stream?

• A stream is a moving body of water that eventually connects to an ocean

Note: Today we will be discussing large streams called rivers!

River Characteristics

1. Velocity – how fast or slow the river is moving; this determines the rivers ability to erode sediments

Which river has a greater velocity?

2. Gradient – the slope or steepness of the river; the steeper the gradient, the more energy

River Characteristics

3. Discharge – the amount of water moving by a certain point of the river in a set time

The Amazon River in South America has greatest discharge 212,400 m3/s

Mississippi River in USA has 7th greatest discharge at 17,300 m3/s

River Characteristics

4. Stream Channel – the course that the water flows in a river

River Characteristics

Following a River from Source to Mouth

• A river begins at the source/headwaters

– This is usually where you find the steepest gradient

– You may also find V-shaped valleys here

Young Streams:

http://pinker.wjh.harvard.edu/photos/american_west/pages/Yosemite%20Falls.htm

-steep slope/gradient-high velocity-down cutting erosion

Mature Streams

• flood plain develops

• lower slope/gradient

• meanders (bends) formhttp://dynamic.ou.edu/notes/running_water/running_water.html

NIAGARA RIVER

-low slope/gradient-very wide channel and floodplain-abundant meanders

OLD Streams

Following a River from Source to Mouth

• Alluvial fans – fan-shaped sediment deposit that forms when a high gradient stream leaves a narrow valley

Following a River from Source to Mouth

• As the river continues, you will also find many tributaries – this is where one stream empties into another

As more tributaries add water to the main river, the amount of discharge increases

Following a River from Source to Mouth

• As the river moves downhill, it will erode its channel until it hits base level

• Base level is the lowest point to which a river can erode its channel

Following a River from Source to Mouth

• As a river approaches base level, it can develop meanders – bends in the stream channel

Following a River from Source to Mouth

• Delta - a fan-shaped sediment deposit where a river empties into an ocean at mouth

New Orleans: Built on Mississippi River Delta

Note: Meanders can turn into Oxbow lakes…here’s how

Sediment Transport in Rivers

• Streams carry sediment in 3 ways:– In solution (dissolved load)– In suspension (suspended load)– Scooting or rolling along bottom (bed load)

1. Dissolved Load• Sediments that are dissolved into river water

2. Suspended Load• Fine-grained sediment (like sand, silt, clay) that

remains in the water during transportation– Usually what you see that makes a river look dirty– Most sediment carried by a stream is suspended– This amount increases dramatically during flood

3. Bed Load• Heavier, coarse grained sediments that travel

along the bottom of a stream. – Causes the most stream erosion! – Sediment may not move frequently

Meander Velocity

• Higher velocities on outside of meanders causes erosion (cut bank)

• Lower velocities on inside of meanders causes deposition (point bar)

• Fig. 10.6

Channel Shape and Roughness

A. Narrow and Deep– Less resistance– Faster flow

B. Wide and Shallow– More resistance – Slower flow

C. Rough Streambed – More resistance – Slower flow

Stream Velocity Controls:

• How much and what grainsize of sediment is – Eroded and– Transported

• Where and what grainsize size will be sediment

Drainage PatternsGeology controls stream patternsA. Uniformly Erodible

(e.g., flat-lying sedimentary rocks of the Midwest)

B. Conical Mountains (e.g., Volcanoes)

C. Fractured bedrock(shallow bedrock)

D. Resistant ridges of tilted sedimentary rocks(e.g., Valley and Ridge Province of Pennsylvania)

A. Dendritic

B. Radial

C. Rectangular

D. Trellis