Surface Water and Stream Developmentbedfordfms.sharpschool.net/UserFiles/Servers/Server... ·...
Transcript of Surface Water and Stream Developmentbedfordfms.sharpschool.net/UserFiles/Servers/Server... ·...
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Surface Water
and Stream
Development
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Surface Water
The moment a raindrop falls to earth it begins its return to the sea.
Once water reaches Earth’s surface it may evaporate back into the atmosphere, soak into the ground, or flow across the surface of Earth.
Runoff- the movement of water across the surface of the earth
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Factors Affecting the Rate of Runoff
Rate of Precipitation – more precipitation per unit time increases run-off.
Topography – The steeper the slope the greater the run-off.
Level of Soil Saturation – As soil pores get closer to saturation the level of run-off increases.
Type of Soil – Soils that have greater porosity and permeability will decrease the rate of run-off.
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Factors Affecting the Rate of Runoff
(continued)
Amount of Vegetation – Two effects Vegetation physically blocks the flow of surface
water decreasing the rate of run-off.
Vegetation, especially during the spring and summer months, absorbs much of the precipitation decreasing the rate of run-off.
Presence of Man-made Structures –Pavement and other developed surfaces prevent the absorption of surface water increasing the rate of run-off.
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Watersheds / Drainage Basins
All of the land area whose water drains into a stream system is called the streams watershed.
Watersheds can be very small (<2 Km2) to extremely large (The Mississippi watershed drains ~40% of the US land area.
Watersheds are composed primary streams and their tributaries.
Tributaries are smaller streams that contribute water to a larger stream.
Watersheds are separated from one another by highland areas called Divides
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Common Drainage Patterns
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Watersheds of Virginia
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Five Drainage PatternsDendritic – Form in regions in which the rocks have a uniform
resistance to weathering. The pattern is very tree-like with tributaries making up the “branches” of the tree.
Radial – A pattern that forms on the slopes of large mountains
Rectangular – A pattern found where bedrock has been extensively faulted. The pattern has tributaries that meet at right angles.
Trellis – A pattern found where there has been extensive folding of rock strata. The tributaries flow along synclinal valleys. The pattern is typified by many tributaries that run parallel with one another.
Braided – A pattern found where water flows through a region of unconsolidated materials. Also found where changes in elevation are so minute as to produce no directional bias. The pattern resembles a braid in which stream channels intertwine and are punctuated with many islands or bars.
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Characteristics of Moving Water
Moving water is the single most important factor in shaping the Earth’s surface
Water is set in motion by gravity
Water shapes the topography of Earth by:
1. Weathering the soil and rocks through which it passes
2. Transporting the resulting sediments
3. Depositing those sediments at some remote distance
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Mechanisms of Stream Erosion
There are three mechanisms of stream erosion: Hydraulic Action - is due to the force of moving
water. Moving water can move both loose and consolidated materials along the bed of the stream. If this process removes material below the waterline on a stream bank, the bank may collapse in a process known as calving.
Abrasion - Sediments carried along by the stream can impact on the streambed or with other sediment. The resulting abrasion gradually reduces the size of the sediment or removes material from the streambed if the channel is cut in bedrock
Corrosion - Chemical erosion of rock and sediments due to acids in the stream water
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Gradient and Potential Energy
The amount of erosion that any moving body of water can accomplish is controlled by the gradient of the stream bed and the resulting potential energy.
Gradient is a measure of the vertical drop over a defined lateral distance. In mathematics gradient is known as slope and is define as follows:
Slope (m) = Vertical Rise/Horizontal Run
The steeper the gradient the greater the potential energy of the stream.
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Kinetic Energy and Competence
The steeper the gradient of a stream the faster the rate at which the potential energy of the stream is converted to kinetic energy.
Kinetic Energy – Energy of motion KE = ½ (mass)(velocity)2
The greater the kinetic energy of the water the larger the particle size that the water is capable of carrying.
Competence -The largest particle size that water can carry; velocity determines the competence of a stream
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Discharge and Capacity
Discharge – the volume of water that passes a point on the stream per unit of time
Discharge = Astream x VStream
Capacity – the maximum quantity of load that a stream can carry; controlled by the discharge of a stream
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Types of Stream Load
Stream Load – the mechanisms by which sediments are moved through a stream system. There are three types of stream load:
1. Bed Load – Material that is rolled or pushed along the stream bed. This load typically includes all sediments larger than silt. Sand typically moves by saltation, while large sediments are pushed or rolled along the stream bed.
2. Suspension Load – Material that is small enough to be physically suspended in the water of the stream for long distances. It typically includes silts and clay sized sediments.
3. Solution Load – minerals that have dissolved out of rocks due to chemical weathering.
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Stream Profile
The profile of a stream is a cross-sectional view of a streambed as it flows from its head (highest point) to its mouth (lowest point), or, where it enters another body of water.
Base Level – The lowest level to which a stream can down cut. Most base level are temporary.
Ultimate Base-Level = Sea Level. No stream erosion can take place below sea level.
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Formation of Stream Valleys
All stream valleys share the following characteristics:
1. They are V-shaped.
2. They are formed through a combination of down- and lateral-(side-to-side) cutting (erosion).
3. The amount of down- versus lateral-cutting is determined primarily by the slope of the stream bed.
4. There are three types of Stream Beds
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Young River Valley
Characteristics:
1. Deep, Narrow, V-shape
2. “Straight”
3. Fast Flow
4. Down-Cutting Predominates
5. Rapids and Waterfalls Common
6. Occurs on Steep Slopes
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Mature Stream Valley
Characteristics:
1. Broad V-shape
2. Intermediate Flow
3. Down- and Lateral Cutting Occur
4. Meanders Develop
5. Flood-Plain begins to develop
6. Occurs on Intermediate Slopes
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Floodplain Structures
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Cut-bank and Point-Bar
Development
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Stream Bed Shape
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Old River ValleyCharacteristics:
1. Extremely Broad V-shape
2. Low Flow
3. Lateral-Cutting Predominates
4. Well Developed Flood Plain
5. Extreme Meandering
6. Oxbows and Oxbow Lakes Present
7. Occurs on Very Low Slopes
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Formation of an Oxbow
and Oxbow Lake
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What Type of Valley Is It?
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What Type of Valley Is It?
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What Type is a Valley is It?
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Deltas
Watersheds/Drainage Basin
Watersheds of Virginia
Additional Stream Features
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Deltas
Named for their triangular shape which resembles the Greek capital letter D
Formed when sediment laden stream enters a still body of water.
Because slope is non-existent at a base level the water of the stream begins to carve random channels through the deposited sediment.
These random channels are known as Distributaries
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Further Examples of Deltas
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Alluvial Fans
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