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 forcecomparable 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.
Questions1. 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.