Surface Water

and Stream

Development

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

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.

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.

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

Common Drainage Patterns

Watersheds of Virginia

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.

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

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

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.

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

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

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.

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.

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

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

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

Floodplain Structures

Cut-bank and Point-Bar

Development

Stream Bed Shape

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

Formation of an Oxbow

and Oxbow Lake

What Type of Valley Is It?

What Type of Valley Is It?

What Type is a Valley is It?

Deltas

Watersheds/Drainage Basin

Watersheds of Virginia

Additional Stream Features

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

Further Examples of Deltas

Alluvial Fans