rivers presentation

What is River or Stream?

• A river is nothing more than surface water finding its way over land from a higher altitude to a lower altitude, all due to gravity.

How they begin?

• Rivers and streams begin their lives as smaller creeks, often called "the headwaters".

• These small tributaries run downhill until they merge to form bigger tributaries, which continue merging to form larger rivers.

The Blue Nile Falls fed by Lake Tana near the city of Bahar Dar, Ethiopia

Tributaries

River Rhine and its tributaries

Work of Streams:

The work of streams includes

Erosion Transportation Deposition

Erosion:

Causes of Erosion Attrition Corrasion Hydraulic

Action Chemical

Solution

a cohesive river bank being vertically eroded

• The way that rocks in rivers are worn down by rubbing against each other

Attrition

Corrasion

• Corrasion- Grinding away of solid rock surfaces by particles carried by water, ice, and wind. It is generally held to be the most significant form of erosion in rivers.

Hydraulic action

• Hydraulic action is a form of mechanical weathering caused by the force of moving water currents rushing into a crack in the rockface. The water compresses the air in the crack, pushing it right to the back. As the wave retreats, the highly pressurised air is suddenly released with explosive force

Chemical Solution

• Chemical Solution - dissolving of minerals into solution. Most minerals have low solubility in pure water, but rain contains carbonic acid, so that carbonate minerals dissolve readily in acidic solutions.

Ways of Transportation

Stream transport their load of sediment in three ways.

– In solution (dissolved load)– In Suspension (Suspended load)– Scooting or rolling along the bottom (bed

load)

Stream ability to carry load

Streams vary in their ability to carry there load.

Their abilities are determined by two criteria• Competence

• Capacity

Competence and Capacity

• The competence of a river is the maximum particle size that it is able to transport.

• As the velocity of the river increases it is able to transport larger particles and so its competence increases.

• The capacity is the maximum load a river can transport.

• Capacity will also increase with increasing discharge and velocity.

Large angular bedload upstream requires high flow conditions for transportation and has travelled only a short distance.

Smaller sub-rounded and rounded bedload downstream, the size has been reduced by attrition.

Stream Channel Types

Within a single Stream we can often recognize three different Channel types.

These unique channel types develop in response to changes in stream due to following reasons.

1. Velocity

2. Sediments texture

3. Stream grade.

Stream Channel Types

1.Narrow Channels

Channel located in the

upper reaches of many

streams tend to be

narrow with flow

moving at high velocities

Upper reach of a stream in the Rocky Mountains, Canada.

Braided channel

Streams with high sediment loads that encounter a sudden reduction in flow velocity generally have a Braided channel type.

Braided stream channel.

Braided river at junction of Gakona and Copper River, Alaska

Braided channel

• This type of stream channel often occurs further down the Stream profile where the grade changes from being steep to gently sloping.

• In a braided stream, the main channel divides into a number of smaller, interlocking or braided channels.

• Braided channels tend to be wide and shallow because bedload materials are often coarse (Sands & gravels) and non-cohesive.

3.Meandering channels

• Meandering channels form where streams are flowing over a relatively flat landscape with a broad floodplain.

Meandering stream channel

• Technically, a stream is said to be meandering when the ratio of actual channel length to the straight line distance between two points on the stream channel is greater than 1.5.

• Channels in these streams are characteristically U-shaped and actively migrate over the extensive floodplain.

Stream Channel FeaturesOR

Fluvial Landforms Within the stream channel are a variety of sedimentarybeds and structures. Many of these features are dependentupon the complex interaction between stream velocity andsediments size.

1. Bars2. Bluff 3. Point Bars4. The Floodplain5. Levees6. Alluvium7. Aggradation8. Crevasses9. Depressions10. Oxbow Lakes11. Alluvial Fans and Deltas 11. Delta

Bars:

• Streams carrying coarse sediments develop sand and gravel bars. These types of bars seen often in braided streams which are common in elevated areas.

• Bars develop in braided streams because of reductions in discharge.

• Cause of reduction in discharge:1. Reduction in the gradient of the stream and/or 2. The reduction of flow after a precipitation event or

spring melting of snow and ice.

Figure d: Braided stream channel with gravel bars.

Bluff or Cliff

• A very steep hill or small cliff (often of limestone), frequently next to a river or ocean

McIntyre Bluff and Vaseux Lake

Point bars

• Point bars develop where stream flow is locally reduced because of friction and reduced water depth.

• In a meandering stream, point bars tend to be common on the inside of a channel bend.

The Floodplain:

• Alongside stream channels are relatively flat areas known as floodplains.

Missouri River

Levees:

• Floodplains develop when streams over-top their levees spreading discharge and suspended sediments over the land surface during floods.

• Levees are ridges found along the sides of the stream channel composed of sand or gravel.

• Levees are approximately one half to four times the channel width in diameter.

New overbank sand from the 1993 flood on upper Mississippi River

Alluvium:

• Upon retreat of the flood waters, stream velocities are reduced causing the deposition of alluvium. Repeated flood cycles over time can result in the deposition of many successive layers of alluvial material.

Aggradation:

• Floodplain deposits can raise the elevation of the stream bed. This process is called aggradation.

• Braided channels produce horizontal deposits of sand during times of reduced discharge.

• In meandering streams, channel migration leads to the vertical deposition of point bar deposits.

Crevasses:

• A number of other geomorphic features can be found on the floodplain. Intersecting the levees are narrow gaps called crevasses.

• These features allow for the movement of water to the floodplain and back during floods.

Depressions:

• Topographical depressions are found scattered about the floodplain. Depressions contain the some of the finest deposits on the floodplain because of their elevation.

Oxbow Lakes:

• Oxbow lakes are the abandoned channels created when meanders are cut off from the rest of the channel because of lateral stream erosion

Alluvial Fans and Deltas

• Streams flowing into standing water normally create a delta

Nile Delta (Source: NASA).

What is Delta?

• A delta is body of sediment that contains numerous horizontal and vertical layers.

• Most deltas contain three different types of deposits:

1. Foreset bedsForeset beds make up the main body of deltas. They are deposited at the outer edge of the delta at an angle of 5 to 25 degrees. Steeper angles develop in finer sediments.

2. Topset beds On top of the foreset beds are the nearly horizontal topset beds. These beds are of varying grain sizes and are formed from deposits of the small shifting channels found on the delta surface.

3. Bottomset beds.

• In front and beneath the foreset beds are the bottomset beds. These beds are composed of fine silt and clay. Bottom set beds are formed when the finest material is carried out to sea by stream flow.

Mississippi Birdfoot Delta (Source: NASA).

Alluvial Fan

• An alluvial fan is a large fan-shaped deposit of sediment on which a braided stream flows over.

Montana, near Bozeman