River Erosion and Velocity

River Erosion and Velocity

Shaping the Landscape

Once created, a river of any size has immense power to shape the landscape. In temperate latitudes, rivers are the most important agents of erosion on land. When rivers flow, they erode sediment from the surrounding rock, transport the sediment, and deposit it. The river's velocity (the speed at which water flows in one direction past a given point) is the key to knowing where and when a river erodes, transports, or deposits sediment. The faster a stream flows, the greater its

capacity to erode land and to carry sediment and debris. The slower it flows, the greater its ability to deposit sediment. While factors, such as the specific properties of the water and the shape of the stream channel, also come into play, two main factors determine the speed at which water flows in a stream or river: gravity and friction.

Water flows downhill under the pull of gravity. All else being equal, water flows faster down steeper slopes. Mountain streams flowing down gradients of five degrees or more average speeds of some five to 10 mph (8-16 km/h). Lowland rivers in level plains with a gradient of less than .01 degrees usually idle along at less than 0.5 mph (0.8 km/h), unless the river is in a state of flood or is being strongly affected by tides.

Friction between river water and the sides and bottom of the channel tends to reduce water flow. All else being equal, water flows faster the smoother the surface across which it flows. Warm water is runnier (less viscous) than cool water, and so flows more readily. The shape of the river channel also affects river flow by causing more or less turbulence. Turbulence is uneven or haphazard flow, visible as the swirls and eddies that occur in running water.

Water is capable of two very different types of flow, laminar and turbulent. In laminar flow, water flows smoothly so that stream water in one layer does not mix with that in the layer above or below. In nature, laminar flow is rare and only really occurs in slow-running, very shallow water in a smooth channel. Laminar flow has little power to uplift and move particles.

Turbulent flow is the typical flow of streams and rivers. As the water rushes along, swirls and eddies are created that mix water from different levels. Turbulence picks up and carries particles, and the faster the flow, the greater the turbulence and the greater the river's capacity to erode and transport sediment. In mountain streams, white water is evidence of turbulence; in lowland rivers, swirling eddies at the surface are telltale signs of turbulence.

The velocity of a river varies with the season. It is greatest when rates of precipitation are high (the wet season) and the river contains a large volume of water, and it is least during the dry season, when the watercourse contains correspondingly less water.

Velocity typically changes along a river's length. It is often high for first- and second-order streams, because they have steep gradients, and lower for higher-order streams with their gentler gradients. This is a generalization, however, and in reality situations are not as clear-cut. Although tributaries appear to flow faster than the lowland rivers into which they feed, this is not always the case. A small stream flowing at five mph (8 km/h) looks livelier than a wide river flowing at the same velocity. The effect of friction upstream can account for the water velocity increasing downstream even though the slope there is gentler. Friction between flowing water and the sides and bottom of the river acts as a brake to water flow. Volume for volume, the water in a mountain stream is in touch with a much larger surface area of sides and bottom than is the case for a swollen lowland river. This effect alone can slow a mountain stream much more than a lowland river. When hydrologists put their current meters in the water to measure water velocity, they sometimes find surprising results. The lower Amazon, for example, flows faster than many of the mountain streams that feed it.

Finally, velocity changes across the stream channel. In a moderately straight section of river, velocities are highest near the surface in the middle of the river. This region is farthest from the effects of friction along the riverbed and sides. In a curved section of the river, friction slows water on the inside of the bend while centripetal force (a force that constrains a body to move along a curved path as it moves around a bend) tends to accelerate the water on the outside of the bend. The outside of the bend tends to erode, while the inside tends to collect sediment.

Erosion and Transport

Stream erosion proceeds in three ways: by hydraulic action, by

corrosion, and by abrasion. Hydraulic comes from the Greek words hydôr, meaning"water," and aulos, meaning "pipe." Hydraulic action refers to the movement of liquid; specifically, the action of the stream as it physically uplifts and drags particles from one place to another. Small particles that present a large surface to the river's flow, such as fine sand grains, are those most liable to be eroded in this way. A sand grain the size of the period at the end of this sentence would be eroded by a stream flowing at about 0.75 mph(1.2 km/h). At the other end of the scale, fast-moving water exerts an immense force-comparable to the force exerted by the blast from jet engines-and the blasting effect of water crashing against rock can split bedrock.Strong turbulence can then uplift large chunks of the shattered rock.

Corrosion, or chemical weathering, occurs when the river water dissolves substances in the underlying rock or sediment. This can weaken the rock along joints and cracks, making the rock more liable to be eroded in other ways. In rivers flowing through limestone country, corrosion can account for more than half of stream erosion in terms of mass of material removed.

Abrasion is the action of moving particles-such as sand, pebbles, or boulders-scraping or chipping at the sides and bottom of the river. On some hard river bottoms, swirling eddies laden with sand or pebbles wear potholes in the bedrock. At low water, these potholes-with telltale abrasive particles at the bottom-are obvious even to the casual observer.

Erosion causes a stream channel to deepen, widen, and lengthen over time. How much hydraulic action, corrosion, or abrasion contribute to erosion depends on a variety of factors, such as the steepness of the gradient, the shape of the channel, and the temperature of the water. The nature of the bedrock is particularly significant. For a river flowing through shale, for example, hydraulic action and abrasion predominate and corrosion is relatively unimportant. This is because shale is structurally weak but contains chemical constituents that are fairly insoluble in water.

Vertical erosion, or down-cutting, which erodes the riverbed, initially produces a steep-sided river valley. Although a freshly cut river valley usually begins with steep or near-vertical sides, the valley sides are soon worn back by weathering. The weathered material falls into the river, and the valley sides develop a shallower slope. A gentler V-shaped valley develops. Where the rock is very hard, or where a particularly dry atmosphere slows chemical weathering, the valley sides can remain steep, forming a gorge. The near-vertical sides of the famous Three Gorges section of China's Yangtze River is a testament to dry air and hard rock.

The river's ability to deepen its valley depends, in part, on its height relative to the sea. For a river that flows into the sea, assuming the horizontal distance between source and sea remains the same, the higher the upland reach of the river in relation to the sea, the greater the river's potential to erode the riverbed. Relative height can change markedly over time. If the sea level drops-for example, during an ice age, when precipitation stays frozen on land rather than flowing to the sea-or the land rises-for instance, because of uplift caused by two of Earth's plates colliding in the nearby vicinity-then the river's gradient steepens. This increases the river's rate of flow, and the rate of down-cutting increases. The Grand Canyon has been down cut by the Colorado River in this way as the land has become gradually uplifted several times in the last few million years (experts cannot agree on how many times or the exact time period involved).

Lateral erosion cuts back the sides of the river channel. This is most pronounced on the outer banks of a curve in the river. In still another type of erosion, headward erosion, smaller streams lengthen the river system by gradually cutting into the hillsides from which the streams originate.

Technical terms describe the particle size (measured in diameter) a section of stream or river can carry and the total amount of sediment that can be carried in a given time. A stream's competence is the largest particle size it can carry. Competence grows exponentially with increasing stream velocity. Competence increases by a factor of between the mathematical square and cube of the stream's velocity, depending on the shape and density of the particles concerned. So, a stream that doubles in velocity is able to carry particles with diameters of four times (two squared) to eight times (two cubed) the size of those it could carry before.

A stream's capacity is the total amount of sediment it can transport. Slow-flowing rivers tend to carry much more sediment overall than their fast-flowing smaller tributaries. This is so in part because the volume of water in lower sections of the river system is so much greater.

Once running water picks up sediment, it transports it in three ways. The dissolved (or solute)load represents the chemicals that dissolve in river water. This tends to be made up of ions (charged atoms and molecules) that are small and are attracted to water molecules. The dissolved load may be exceptionally high-reaching some 30 percent of the total sediment load-in rivers crossing limestone regions.

Typically, more than 50 percent of a river's transported sediment is suspended in the water as small particles, often as silt or clay. This portion is called the suspended load. It gives the river water its cloudiness (turbidity). The Mississippi River is coffee-colored in its lower reaches because of its high levels of suspended mud. China's Yellow River is so called because of the yellowish wind-deposited silt (loess) suspended in its waters.

The river's bed load includes all those particles that are too large to be suspended in the water but are rolled, slid, or bumped along the riverbed by the moving water. Some of the bed load only moves when the river is in flood conditions and stream velocities are highest. The table distinguishes sediment particles by size.

Much of the sediment transported by a river arrives from elsewhere and is not eroded by the river system itself. Weathering refers to rock being broken down into fragments or dissolved substances by natural processes-physical, chemical, and biological. Weathered material enters the river system in water flowing off the land.

Material is dumped directly into the watercourse when nearby land collapses. Mass wasting is the term for movement down slope of rock, soil, or sediment under the influence of gravity. All slopes experience it. Steep slopes experience dramatic rockfalls and landslides, but even gentle slopes show gradual slippage of material, so all rivers receive at least some material by mass wasting. In some cases mass wasting can dramatically alter the path of a river or block the channel entirely to create a lake.

A glacier-fed stream in Chugach National Forest, Alaska. Notice the river-smoothed cobbles in the foreground and the temporary "islands" of deposited cobbles on the inside bend of the river.

Deposition and Precipitation

Particles of sediment settle when the stream's velocity slows sufficiently so that the particles are no longer swept along. In a river, two of the most common types of deposition are lateral accretion and over-bank deposition.

Lateral accretion takes place on the inside of bends in the river channel, where the water flow slackens. This deposition, coupled with erosion on the outer edge of the bend, causes the bend to gradually widen farther into a broad meander, or loop.

Over-bank deposition takes place in the floodplain, the broad flat areas between the river channel and the valley's sides. During a flood, water spills over the banks and spreads onto the floodplain. On the floodplain, over-spilling water first deposits coarse sand and gravel, close to the banks of the original channel. With repeated floods, this material forms natural levees (raised riverbanks). On the floodplain and away from the river channel, the over-spilling water tends to be shallow and slow-flowing and deposits finer particles-silt and mud.

Sediment deposited by a river is termed alluvium (from the Latin lavare, meaning "to wash"). The alluvial soils on the floodplains of the world's great rivers, such as the Nile and the Ganges, are among the most fertile soils in the world. Alluvium is usually rich in dissolved nutrients. Despite this benefit, alluvial floodplains are, of course, at risk of flooding.

The dissolved load transported by a river does not settle out in slack water, but it can come out of solution (precipitate) under certain conditions. This can happen when dissolved substances become concentrated in situations where rates of evaporation are high. When the Jordan River empties into the Great Salt Lake in Utah, its dissolved load becomes concentrated by evaporation and contributes to the layer of common salt that crystallizes on the salt flats at the margins of the lake.

Chemical transformation that leads to deposition occurs when carbon dioxide diffuses out of solution and into the air from waters rich in carbonates. Calcite, the most common form of crystalline calcium carbonate, then accumulates on the riverbed or sides of the channel or in isolated pools after flooding.

The map shows the classic triangular delta of the Nile River and the unusual bird's-foot delta of the Mississippi River.

Questions-

1. Summarize the first paragraph.

2. What makes rivers flow faster?

3. What reduces a river’s flow?

4. What is turbulence and what does turbulence do to a river?

5. Describe where velocity is the fastest and slowest in the river. Where is erosion the greatest in a river?

6. What are the three ways rivers erode a stream. Describe each.

a.

b.

c.

7. What are the three ways erosion changes the shape of a river?

8. What are three ways sediments are deposited?

9. What does load refer to?

10. Where does most of the load in a river come from?

11. What are 2 ways sediment are deposited?

12.