Running Water
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Running Water opens with an introduction to the hydrologic cycle and the exchange of water between the
oceans, atmosphere, and land. The factors that control streamflow and their influence on a stream's ability to
erode and transport materials are presented along with discussions of base level, graded streams, and stream
erosion. Erosional and depositional features of streams are followed by a brief look at both narrow and wide
stream valleys. The chapter concludes an examination of drainage patterns, floods, and flood control.
Learning Objectives
After reading, studying, and discussing the chapter, students should be able to:
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Discuss and explain the hydrologic cycle on Earth.
Briefly discuss the concept of streamflow including discharge and gradient.
Explain the changes that occur from the head to the mouth of a stream.
Briefly explain the concept of base level and graded streams.
Compare and contrast the various mechanisms by which streams transport sediment.
Distinguish between the competence and capacity of a stream.
List and briefly describe the various types of stream deposits.
Compare and contrast the characteristics of narrow and wide stream valleys.
Briefly discuss the significance of incised meanders.
List and briefly describe the various types of drainage patterns.
Explain the concept of headward erosion and how it relates to stream piracy.
Briefly discuss flooding and flood control.
Chapter Outline___________________________________________________________________
I.
Hydrologic cycle
A. Illustrates the circulation of Earth's
water supply
B. Processes involved in the cycle
1. Precipitation
2. Evaporation
3. Infiltration
4. Runoff
5. Transpiration
C. Cycle is balanced
II. Running water
A. Begins as sheet flow
1. Infiltration capacity controlled by
a.
Intensity and duration of the
rainfall
b. Prior wetted condition of the soil
c. Soil texture
d. Slope of the land
e. Nature of the vegetative cover
B. Streamflow
1. Two types of flow determined
primarily by velocity
a. Laminar flow
b. Turbulent flow
2. Factors that determine velocity
a. Gradient, or slope
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b.
C.
D.
E.
F.
Channel characteristic
1. Shape
2. Size
3. Roughness
c. Discharge
Changes from upstream to downstream
1. Profile
a. Cross-sectional view of a stream
b. From head (headwaters or source)
to mouth
1. Profile is a smooth curve
2. Gradient decreases
downstream
2. Factors that increase downstream
a. Velocity
b. Discharge
c. Channel size
3. Factors that decrease downstream
a. Gradient, or slope
b. Channel roughness
Base level and graded streams
1. Lowest point a stream can erode to
2. Two general types
a. Ultimate
b. Local or temporary
3. Changing causes readjustment of
stream activities
a. Raising base level causes
deposition
b. Lowering base level causes
erosion
Stream erosion
1. Lifting loosely consolidated particles
by
a. Abrasion
Transport of sediment by streams
1. Transported material is called the
stream's load
a. Types of load
1. Dissolved load
2. Suspended load
3. Bed load
b. Capacity – the maximum load a
stream can transport
2. Competence
a. Indicates the maximum particle
size a stream can transport
b. Determined by the stream’s
velocity
G. Deposition of sediment by a stream
1. Caused by a decrease in velocity
a. Competence is reduced
b. Sediment begins to drop out
2. Stream sediments
a. Well sorted
b. Called alluvium
3. Channel deposits
a. Bars
b. Braided streams
c. Deltas
4. Floodplain deposits
a. Natural levees
1. Form parallel to the stream
channel
2. Built by successive floods over
many years
b. Back swamps
c. Yazoo tributaries
5. Alluvial fans
a. Develop where a high-gradient
stream leaves a narrow valley
b. Slopes outward in a broad arc
6. Deltas
a. Forms when a stream enters an
ocean or lake
b. Consist of three types of beds
1. Foreset beds
2. Topset beds
3. Bottomset beds
c. May develop distributaries
H. Stream valleys
1. The most common landforms on
Earth’s surface
2. Two general types of stream valleys
a. Narrow valleys
1. V-shaped
2. Downcutting toward base level
3. Features often include
a. Rapids, and/or
b. Waterfalls
b. Wide valleys
1. Stream is near base level
2. Downward erosion is less
dominant
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3. Stream energy is directed from
side to side forming a
floodplain
4. Features often include
a. Floodplains
1. Erosional floodplains
2. Depositional floodplains
b. Meanders
1. Cut bank
2. Cutoff
3. Oxbow lakes
4. Meander scar
I. Incised meanders and stream terraces
1. Incised meanders
a. Meanders in steep, narrow valleys
b. Caused by
1. Drop in base level or
2. Uplift of land
2. Terraces
a. Remnants of a former floodplain
b. River has adjusted to a relative
drop in base level by downcutting
J. Drainage networks
1. Drainage basin – land area that
contributes water to a stream
2. Divide – an imaginary line that
separates the drainage basin of one
stream from another
K. Drainage patterns
1. Pattern of the interconnected network
of streams
2. Common drainage patterns
a. Dendritic
b. Radial
c. Rectangular
d. Trellis
L. Headward erosion and stream piracy
1. A stream can lengthen its course by
a. Building a delta
b. Headward erosion
2. Headward erosion may result in
stream piracy – the diversion of the
drainage of one stream into another
M. Formation of a water gap
1. Steep-walled notch where a river cuts
through a ridge that lies in its path
2. Two possible methods of formation
a. Antecedent stream – stream exists
before the ridge
b. Superposed stream – stream let
down upon a preexisting structure
N. Floods and flood control
1. Floods are the most common and
most destructive geologic hazard
2. Causes and types of floods
a. Result from natural-occurring and
human-induced factors
b. Types of floods
1. Regional floods
2. Flash floods
3. Ice-jam floods
4. Dam failure
3. Flood control
a. Engineering efforts
1. Artificial levees
2. Flood-control dams
3. Channelization
b. Nonstructural approach through
sound floodplain management
Answers to the Review Questions
1. The oceans are the main reservoir for water and a good starting point for discussing the hydrologic cycle.
Water from the oceans evaporates and eventually falls as precipitation (rain, snow, sleet, etc.) on land or
into the sea. Precipitation on land can evaporate back into the atmosphere, flow as runoff into streams and
rivers, infiltrate into the soil and bedrock to recharge the groundwater, or be frozen into glacial ice.
Groundwater discharges as springs or seepage flow to perennial streams, and glacial ice eventually melts.
Thus freshwater derived from land areas eventually returns to the ocean, completing the hydrologic cycle.
Glacial ice volume has an important regulatory effect on sea level. As ice caps formed
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and grew larger during the Pleistocene glacial epochs, sea level fell, and as the ice sheets melted and
shrank, sea level rose.
2. Most precipitation originates by evaporation from the oceans. Over time, water evaporated from the
oceans is replenished by inflow of freshwater from rivers and streams. Therefore, sea level does not drop.
3. Textural properties of the surface material, kinds and abundance of vegetation, topography, and delivery
mode of the moisture all have important effects on infiltration capacity.
Permeable, initially unsaturated, highly porous regolith can hold up to 30 percent or more of its volume as
water when fully saturated. Of course, the infiltration capacity of any porous material declines as the
percentage of unsaturated pore space declines. Impermeable, surficial materials such as massive bedrock
and asphalt paving prevent infiltration so moisture runs off or evaporates. Water infiltrates more readily
into moist, unsaturated regolith than into dry regolith.
Dense, vegetative cover enhances infiltration because soils are typically moist and porous, thus runoff is
retarded. In forested areas, trees slow down the rate at which precipitation is delivered to the land surface,
and considerable moisture is temporarily stored in humus and forest litter. On gentle tree-covered slopes
and flat lying terrain, slow runoff enhances infiltration. Runoff is accelerated on steep slopes and in areas
with sparse vegetation, and infiltration decreases accordingly.
The rate at which water is delivered to the land surface has a very important effect on infiltration. Shortlived storms with intense rainfall result in lower infiltration and increased runoff because the water “piles
up” on the surface faster than it can infiltrate. During periods of light to moderate rainfall, runoff is
retarded and infiltration rises accordingly. Special environmental conditions, such as snow melting above
frozen ground, greatly intensify runoff and reduce infiltration; conversely, snow melting above unfrozen
ground can result in a high percentage of the moisture infiltrating the soil. Thus regional and local
climatic factors are also important.
4. The three main zones of a river system are the zone of erosion, a zone of sediment transport, and a zone
of sediment deposition.
5. The gradient is the drop in elevation of the stream divided by the length of the flow path. Thus the
gradient is 2000 meters per250 kilometers or 8 meters per kilometer.
6. The new gradient would be 2000 meters per 500 kilometers or 4 meters per kilometer. If a fairly straight
channel should develop meanders, the flow path would lengthen without any change in the elevation
drop; thus the gradient is lowered. In this example, the length of the stream doubled so the gradient
decreased by 50 percent.
7. The average velocity of a stream is given by the equation V = Q/A where V = velocity, Q = discharge,
and A = area of water cross-section. Therefore as discharge increase, the velocity increases as well.
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8. In stream and river systems with orderly, increasing discharges in the downstream direction, the channel
parameters (width and depth) and average velocity increase gradually to accommodate the increased
discharge moving through the channel. Rivers that flow from wet areas into dry areas and gradually lose
discharge downstream will show decreases in the channel parameters and average velocity in
downstream direction. Over time, the channel shape, dimensions, and gradient adjust to accommodate the
discharge normally passing through the channel.
9. Streams transport sediment as dissolved load, suspended load, and bed load.
10. The suspended load will eventually settle to the bottom, but the dissolved load will remain in solution in
the clear water. Unless some sediment from the stream bottom was scooped up with the water, the bed
load would not have been sampled.
11. These terms describe sediment transport characteristics of a stream. Competence describes or measures
the maximum size of detrital particles (gravel, sand, etc.) that are moved by a stream. The largest particles
in a stream move as bed load. Competency depends directly on velocity, so the largest particles are
moved during flood stage when velocities are highest.
Capacity describes or measures the total amount or weight of sediment (bed, suspended, and dissolved
loads) carried by a stream. The capacity is directly dependent on velocity and discharge, so substantially
more sediment is moved during floods than during periods of low discharge. Large rivers with high
discharges and low gradients have low competency and very high capacities; small mountain streams with
steep gradients have high competencies (they can move boulders) but low capacities because they move a
relatively small volume of sediment.
12. Settling velocity describes the speed at which a particle, acted upon only by gravity, sinks through a
motionless fluid, in this case water. We can assume that the density and viscosity of stream waters are
essentially constant. In general, more massive particles settle faster than less massive ones; and, for
spherical particles of equivalent diameters, settling velocities vary directly with density. Given equal
masses, spherical and equidimensional particles settle faster than rod-like particles, and plate-like
particles such as mica flakes settle even more slowly. Tiny, clay-sized platelets settle so slowly that the
slightest turbulence is enough to keep them in suspension.
13. Stream channels are eroded by abrasion, scouring, and solution. Abrasion results from impacts of
sediment particles with the bottom or with each other. Potholes (circular to elliptical, steep walled
depressions in bedrock channels) are drilled by the abrasive action of sand and pebbles swirling round and
round in turbulent eddies.
Scouring involves dislodging sediment particles from the channel walls and bottom and lifting them into
the water column to be moved downstream. Soluble bedrock, such as limestone and dolostone, can be
slowly dissolved by streams, especially if the water is initially acidic. Such a situation might arise if a
stream originates in a marsh or swamp, or if acidic mine waters discharge into a stream.
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14. Braided channels result from excessive bed load. Glacial outwash streams are good examples. Rivers and
streams that lose discharge downstream also typically become braided because they can no longer
efficiently move bed loads acquired upstream where discharges and competence are higher. In addition,
bed load influx from a highly competent, steeper tributary, an abrupt decreases in gradient, and an abrupt
widening of the channel cross section can result in excessive bed loads and braiding.
15. Base level is the lowest elevation to which a stream can downcut or lower its channel. The elevation of a
major river at a junction with a tributary is the base level elevation (a temporary base level geologically)
for the tributary. Dams and unusually hard bedrock layers function as temporary base levels for the
upstream portion of the drainage basin. Sea level is the ultimate base level for rivers that discharge into
the oceans; thus sea level is the base level for the Mississippi River. In closed, low-elevation basins such
as Death Valley, CA, and the Jordan River Valley-Dead Sea depression, the lowest lake level or land
surface elevation functions as base level for streams in the drainage basin.
16. Two common situations that could potentially trigger the formation of incised meanders are a dramatic
drop in base level or regional uplift of the land.
17. Natural levees are mounds of sandy-to-silty sediment built up on floodplains directly adjacent to rivers
and streams. When a stream is at flood stage, high velocities and turbulence allow silt and finer sand to be
carried in suspension. As sediment laden floodwaters spill onto the floodplain, velocities drop very
quickly; and the coarser, suspended sediments (fine sand and silt usually) are deposited. With many
successive floods, this sediment accumulates to form a natural levee. Thus on a broad floodplain, the
highest ground is typically on the natural levee adjacent to the channel. Backswamps are the lower parts
of the floodplain away from (or “back” from) the channel and natural levee. These areas stay inundated
for longer periods following floods and receive less sediment (mostly clays and fine silt) than the natural
levees; thus the backswamp areas remain at lower elevations than the natural levees.
Tributaries to a main stream with extensive, natural levees may flow for some distance parallel to the
main stream before joining. These are called yazoo tributaries after the Yazoo River, a tributary to the
lower Mississippi River in Mississippi.
18. Alluvial fans and deltas both represent accumulations of stream-transported sediment at sites where
gradients and velocities decrease abruptly. In an ideal sense, both show delta (∆), map-view shapes and
smaller channels (braided, anastomosing channels on alluvial fans and distributaries on deltas) that
diverge outward from the apex of the delta or fan. Channel bottom and overbank sediments become
progressively finer grained in the downstream direction.
Deltas form at a land-water (sea or lake) interface where stream velocities decelerate to zero. Except for
natural levees along major distributaries, deltas are very flat and close to sea level in elevation. Alluvial
fans are entirely terrestrial, they have no relation to sea level (elevations can be above or below), and fan
slopes are fairly steep. Alluvial fans, especially in their upper portions, are typically composed of much
coarser sediments (gravels and coarse sands) than deltas (sands and finer sediments). Although alluvial
fan sediments may locally show fluvial cross bedding, they do not exhibit the large-scale, cross
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stratification that characterizes the entire deltaic accumulation. Finally, alluvial fans are products of
weathering and erosion in dry lands with locally high relief resulting from active or recently active
faulting. Deltas form where sediment laden rivers enter large bodies of water and no particular tectonic or
climatic conditions are required.
19. The artificial levees prevent sediment and freshwater from being dispersed into the wetlands. So the river
is forced to carry its load to the deeper waters at the mouth. At the same time the processes of
compaction, subsidence, and wave erosion continue. Consequently, enough sediment is not added to
offset these forces so the size of the delta and the extent of its wetlands are shrinking.
20. (a) Streams diverging from a central high area such as a dome – radial
(b) Branching, “treelike” pattern – dendritic
(c) A pattern that develops when bedrock is crisscrossed by joints and faults – rectangular
21. Water gaps are common features in mature landscapes formed on tilted or folded strata with varying
resistances to weathering and erosion. The Valley and Ridge Province of the Appalachians is a good
example. With the passage of time, a trellis drainage pattern develops as the landscape is lowered by
erosion. Long, tributary streams to the “master river” erode linear valleys into outcrop areas of weak,
easily eroded strata; linear ridges develop on outcrop areas of the harder strata. However, the courses of
the main (master) rivers in such a region are superposed across the weaker and harder strata alike, their
positions being inherited from a time before the valley and ridge topography was formed. Thus a water
gap describes the short, steep-sided valley segment or gap through which the master stream flows across
the outcrop area of harder, ridge-forming strata.
22. Regional floods are long term events that cover large areas (hundreds or thousands of square miles) and
last for days or weeks. They result from seasonal fluctuations such as rapid snow melt or large, slow
moving storm systems such as hurricanes. Flash floods are much shorter events (generally several hours)
that are confined to much smaller areas. Flash floods are characterized by high discharges, rapid rises in
water levels, and high velocities. They typically occur in narrow canyons or urban areas where runoff is
rapid following intense rainfall episodes such as thunderstorms. Flash floods would generally be deadlier
because they occur with little or no warning and also because they involve rapid rises in water with high
velocities. Regional floods take much longer to develop so there is more lead time for warnings and
velocity and water levels increase much more slowly.
23. Three engineering strategies are channelization, construction of levees, and construction of dams.
Channelization describes an engineering activity wherein a natural stream/river channel is straightened
(meander loops are bypassed), freed of obstructions (fallen trees, large boulders, etc., are removed),
widened in many cases, and smoothed. Thus the modified channel has a steeper gradient and lower
roughness factor than the natural channel. For a given discharge, these changed channel characteristics all
contribute to increased average velocities in the modified channel over those in the natural channel. The
increased average velocity is accompanied by lower water surface elevations for a given discharge, thus
providing for lower flood peaks in the modified channel.
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A major drawback of channelization is that it has the effect of exporting floods to unmodified channel
reaches farther downstream. When the “modified, faster-moving discharge” arrives at the unmodified
channel reach, the lower gradient, smaller cross section, and rougher channel are reestablished. Now,
however, the natural channel can’t efficiently handle the rapidly arriving discharge, so the water “piles
up” and flood stage elevations are higher than they would have been if the whole length of the channel
had been left unmodified and the natural mechanisms for upstream flooding and overbank storage were
unimpeded.
Artificial levees function in essentially the opposite way from channelization. Levees confine the water to
the channel and prevent it from moving onto the floodplain. Thus for a given discharge, river-stage
elevations are raised, not lowered. The discharge is all confined to the cross sectional area of the channel
and the floodplain does not function to “store” excess water during floods and to release it slowly as the
river stage declines following the flood. Artificial levees export floods upstream, downstream, and to
adjacent parts of the river that have no levees. Thus for a given discharge, flood-stage elevations are
raised in upstream and adjacent locations without levees. Downstream, the unlevied natural channel can’t
efficiently convey the fast-moving discharge delivered from the levee-lined channel, so as with
channelized streams, higher flood stages than expected for a given discharge are observed. Levee failures
result in sudden, catastrophic floods that are far more threatening to lives and property than would be the
case for a “natural flood” in an unlevied floodplain. These observations were well documented during the
Upper Mississippi basin floods of 1993.
Dams and reservoirs are the mainstay engineering modifications that allow streams and rivers to be
managed for electrical power generation, water supply, flood control, and navigation. Upstream floodstage discharges can temporarily be stored in reservoirs and released slowly. Thus downstream discharges
are lowered and associated peak flood-stage elevations are lowered or eliminated entirely. Flood control,
however, may not be the number one management consideration. For example, reservoir managers may
take a risk by discounting the possibility of several late summer/early fall heavy rainfall events and store
runoff from a major early summer storm to insure for an adequate supply of water during the normally
drier months. If unexpected late rainfall events materialize or if an unusually long period of above normal
precipitation occurs, the reservoir or reservoirs are filled or nearly filled and have incidental excess
storage capacity if any. At these times, the reservoir managers have no choice but to open the floodgates
and hope for the best. Above all other considerations, a worst-nightmare-failure-of-the-dam scenario has
to be prevented. Thus a filled reservoir pool has no flood control value, and massive releases from the
dam may cause or intensify downstream flooding.
Dams also trap sediment leading to possible adverse environmental effects downstream and sooner or
later, reducing the water storage capacity of the reservoir. Small reservoirs can lose much of their storage
capacity to sedimentation in a short time. A very large reservoir, such as Glen Canyon (Lake Powell), is
estimated to lose about half its initial storage capacity in a few hundred years. Downstream effects of
dams may include scouring and deepening of the channel, loss of natural soil replenishment during
flooding, and severe erosion and loss of delta wetlands, such as has occurred in the Nile delta since
closing of the Aswan High Dam. Well-intentioned flood control dams on small rivers and streams in areas
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with rugged coastal topography, such as southern California, will trap so much sediment that local
beaches are “starved of sand” and beach erosion occurs.
Lecture outline, art-only, and animation PowerPoint presentations for each chapter of Earth,
9e are available on the Instructor’s Resource Center CD (0131566911).
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Classroom Demonstration
Characteristics of Water – Surface Tension
Contributed by: Elizabeth L. Simmons – simmonse@mscd.edu
Metropolitan State College of Denver
Objective: Observe the surface tension of water
Prep Time:
10 minutes
Materials needed:
Pyrex glass pie pan about 6–8” in diameter
Tweezers
Overhead Projector
Needle
DAWN detergent
Part I
Procedure:
1) Put the Pyrex pie pan on the overhead projector. 2) Turn on the light and focus the image on the screen
on the printing on the bottom of the pan. 3) Ask the students to verify that the object is indeed a steel needle
that SHOULD sink when placed on the water. 4) Take a class vote as to how many think the needle will sink
and how many think it will float. 5) Using the tweezers to hold the needle, carefully drop the needle onto the
water. Did it float? (It should have).
Suggestions: Ask questions to involve discussion like: Why does the needle float? What influences surface
tension in the ocean? What creatures are affected by surface tension? What adaptations might organisms have
to overcome surface tension?
Part II
Discuss how could you lessen the surface tension of the water in the pan without touching it?
Option A - By heating the water. (If left long enough on the water, the needle will drop as the water heats
from the bulb in the overhead projector)
Option B - By adding a substance to the water that lessens the surface tension, called a surfactant, or soap.
Procedure:
1) Drop just one drop of DAWN detergent onto the pan of water on the other side of the pan from the needle.
Did the needle drop? How quickly did the needle drop?
Suggestions: Discuss how DAWN detergent was proven to be the best cleanser during the Valdez oil spill
cleanup. It is biodegradable, digestible, and quickly breaks the surface tension to clean the grease and oil off
the otters.