Chapter 6.2 The Work of Streams

• Streams are the most important agents of erosion.• cut and erode their channels• carry enormous amounts of sediment• drop the material to create new features

Streams (remember “stream” mean all flowing bodies of water) erode their channels by

• abrasion (scraping)• grinding (sand and gravel act like sandpaper)• dissolving (water dissolves soluble materials)

The glaciers move through the mountains grinding rock and ice against the mountains and a fine dust called glacial flour (as fine as the flour in your kitchen) stays in suspension in the water, giving it that muddy appearance.

Large amounts of coarse gravel and boulders carried along underneath glaciers provide abrasive power to cut grooves. Finer sediments also in the base of the moving glacier further scour and polish the bedrock.

SEDIMENT TRANSPORT Streams transport sediment in 3 ways:

SEDIMENT TRANSPORT #1: DISSOLVED LOAD (in solution)

• Most dissolved load enters the stream through groundwater.

• ppm = parts of dissolved materials per million parts of water.

• Average ppm = 115-120 ppm. Can reach 1000 ppm +.

SEDIMENT TRANSPORT #1: SUSPENDED LOAD (in suspension)

• Most streams carry the largest part of their load as suspended load.

• Most visible part of load.

• Usually carry sand, silt and clay in suspension.

• during floods can carry larger materials.

• Bed load is the streams load of solid material that contains sediment too large to suspend.

• Travels along the bottom of the stream.• Requires more force from the water.

SEDIMENT TRANSPORT #1: BED LOAD (scooting and rolling)

SEDIMENT TRANSPORT #1: COMPETENCE and CAPACITY

The ability of rivers to carry a load depends on two factors:

Competence = the largest size particles that the stream can carry.

Faster stream = stronger water = larger particles.

More water = larger particles.

Lake Lure, NC

Capacity = Maximum load the stream can carry.

Larger stream = more water = higher maximum load.

DEPOSITIONWhen a river slows down it dumps it’s load!

Alluvium = sorted material that is deposited by streams.

Alluvial Fan = fan-shaped deposit where streams meet larger bodies of water.

Amazon River Delta

Delta = accumulation of sediment formed where a stream enters a lake or ocean.

Nile River Delta

Mississippi River Deltas over the past 5000 years.

Today 2/3 of the sediment is deposited at the Bird Foot delta and 1/3 at the Atchafalaya delta.

Colorado River Delta

Natural Levees = landform that parallels some streams.

• Forms when a stream overflows its banks.

• When the stream overflows, its velocity decreases rapidly and sediment is deposited.

STREAM VALLEYSNarrow Valleys• primary cutting is downward• lots of rapids and waterfalls

Wide Valleys• channel is closer to base level• erosion is less dominant• energy directed from side to side• produces a flat valley floor called a FLOODPLAIN

Narrow Valleys

Wide Valleys

DRAINAGE BASINS = the area of land in which all rain that falls contributes to the same stream.

Flash: Rivers Shapers of Earth

FLOOD!Melbourne, AUSTRALIAFlash Flood, AZ

Florence, MA

Most floods are caused by rapid spring snow melt or storms that bring heavy rain over a large region.

FLOOD CONTROLArtificial Levees = earthen or synthetic mounds built on the banks of a river

Flood-Control Dam = store floodwater and then let it out slowly

Limited Development = Floodplain management, protection of wetlands.

Levee construction. New Orleans 1727

Oklahoma

Riverside forests help to suppress overflowing water during floods

What happens when Flood Controls fail?

http://www.nola.com/katrina/graphics/credits.swf

Some of the following photos are graphic.

Katrina + Poor Levees = Fatal Failure