Water Flow and Streams and Rivers
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III.
Over 70 % of the Earth’s surface is covered by water
A. Most of the water is in the oceans, close to 97%
B. Glaciers contain about 2.15% of the water
C. The remaining 0.65% is found in the rivers, lakes, groundwater and atmosphere
D. Only a small part of the total water is found in streams and rivers and this is the place where most of
the erosion is seen
E. Running water is the major element in erosion even though it is not common is some areas
1. Deserts have little water but occasional storms cause most erosion
2. Glaciated areas are not affected by water
F. Running water is vital to the civilization that we live in
1. Fresh water source
2. Producing electricity
3. Commerce
The Hydrologic Cycle
A. This is the constant recycling of water that takes it from the oceans to the atmosphere to rivers and
glaciers and back to the oceans
B. This cycle is powered by solar radiation and is a result of the properties of water- that it changes
phases easily in surface conditions on Earth
C. The amount of water evaporated yearly from the oceans is equal to 1m thick from all the oceans
D. 85% of the water in the atmosphere comes from the oceans, 15% from the land
E. The complex process of cloud formation helps move the moisture around the Earth
1. Most of the precipitation falls on the oceans, about 80%, where it start the cycle again
2. 20% falls on the land as rain and snow and is involved in the more complex cycle
a) evaporation, condensation, movement of water vapor, precipitation, runoff and
infiltration
b) Some of the precipitation evaporates and rejoins the cycle
c) This comes from lakes, streams, forests, and vegetation as transpiration
F. Most of the rainfall on the continents returns to the oceans as runoff, the surface flow of the rivers
and streams
G. It eventually returns to the ocean, which is the ultimate reservoir an storage area
H. Some of the rainwater is temporarily stored in lakes, snowpacks, glaciers and groundwater
I. But essentially there is a relatively small amount of water flowing back to the ocean at any one time
1. But this small amount is responsible for a major amount of the work of erosion
2. Running water is the most important process causing land features on the surface
Running Water
A. Water flows over any surface and seeks to reach its lowest level
1. The flow can be characterized by laminar or turbulent
2. Laminar flow the flow lines are parallel with one another with no mixing between layers
3. Turbulent flow the streamlines intertwine and develops a complex mixing of fluids
B. Laminar flow occurs when water flows very slowly
1. As groundwater moving between the pores of soil or rock
2. It cause no erosion
C. Water in streams commonly forms turbulent flow
1. The main control factor is the speed of the water
2. The second most important is the roughness of the surface or bottom of the stream
3. Turbulent flow is energetic and causes considerable erosion
D. The amount of runoff depends on the infiltration capacity of the soil
1. This is the maximum rate that surface material can absorb water
2. This depends on several factors
a) The intensity and duration of rainfall
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b) Loosely packed, dry soils absorb water more quickly than wet packed soils
c) Dry hard packed soils also resist infiltration
If the rain is absorbed as fast as it rains, no runoff occurs
1. If the rain should exceed the infiltration of the soil, the excess collects on the surface and
starts to run downhill
2. As it starts to flow, it will form in a sheet flow, a continuous film of water that moves on the
surface
3. Sheet flow forms over the entire hillside and causes sheet erosion concerning the surface
material so the soil, usually in agricultural areas
In channel flow the water is confined to long continuous trough-like depressions
1. Channels form from small streams and gullies to large rivers like the Mississippi River
2. The many sizes of streams are rill, brook, creek, stream, and river
Streams receive water from many sources from sheet flow to rain falling on the stream itself
1. Much more important is the water from soil and groundwater which flow downhill and
discharge into streams
2. In humid areas where groundwater is abundant the streams are fed continually and maintain
the same level year around
3. In arid regions, the steams are supplied by surface water with little groundwater to
continually supply them
Gradient is the slope that streams flow over
1. If the stream drops 10m in 1km its gradient is 10m/km
2. Gradients vary considerably in a stream or river
a) Generally streams are steeper in the upper headwaters of the region where they can be
10"s of meters per km
b) They are usually shallow in the lower portions of the stream, about cms/km
c) Some streams can have steep gradients in mountainous terrain of hundreds of m/km
Velocity is the measurement of the downstream distance traveled per unit of time
1. Measured in ft/sec or m/sec and can vary widely between streams or even within each stream
a) Variations occur in the length and also within the width and depth of the stream
b) The streams bed and banks cause friction to flow
c) Turbulence to flow is cause at the banks and in the bed which slows the water
2. Channel shape and roughness can affect the velocity
a) Broad shallow channels and narrow deep channels have greater contact with the
channel
b) Water in a semicircular channel flows faster because there is less friction
c) The water in the center of the streams, near the top is the fastest
3. Channel roughness is the measure of the frictional resistance of the channel
4. Vegetation helps slow the flow in channels
5. The most obvious control on velocity is gradient
a) You might think that the steeper the gradient the faster the flow velocity
b) But the average velocity in a channel increases downstream even though the gradient
decreases
c) Three factors contribute to this phenomenon
(1) Velocity increases in response of the acceleration to gravity
(2) In the upstream portions boulders and trees increase the roughness so
resistance is high - Downstream the channels are smoother with more
semicircular shape and have less resistance to flow
(3) The total volume of water downstream increases and with increasing
discharge results in increased velocity
6. Discharge is the total volume of water in a stream moving past a particular point in a given
period of time
a) To determine the discharge you must know the cross sectional area of the stream
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b) Then its flow velocity
c) Calculate using the formula Q=VA
d) Is expressed in cubic feet per second or cubic meter per sec
Stream Erosion
A. Streams have potential and kinetic energy
1. Potential energy is the energy of position
2. The downward movement of water changes potential energy to kinetic- the energy of motion
a) Most of the energy is dissipated by heat in the turbulence of the stream
b) Some is used for erosion and transport of sediment, maybe 5%
c) Erosion involves the physical removal of dissolved substances and loose particles
d) Dissolved load is the minerals in solution
e) Solid particles are the clay to boulders size particles
3. Hydraulic action causes the particles to move
4. Abrasion is the particles hitting each other causing breaking down of material
a) If the stream carries no sediment very little erosion occurs
b) The sand and rock bouncing on the bottom abrades and erodes the rock
c) Potholes form when rocks and gravel are caught in a hole and circulate and cut a
depression in the solid rock
Transportation of sediment by streams
A. Steams carry various type of materials usually classified by size
1. Dissolved load is the ions in solution from chemical weathering of minerals
2. Suspended load is the clay to silt size particles that stay in suspension by turbulence
a) This suspended load only deposits when the turbulence is minimal as in a lake or
lagoon or the ocean
b) Mississippi caries 200 million metric tons of sediment per year
3. Bed load is the sand and gravel that travels along the bottom of the stream
a) Part of the bed load is suspended when the turbulence throws it upwards
b) They flow along then fall to the bottom to be lifted again when the turbulence is great
c) The maximum size particle a stream can carry is determined by its competence or
velocity of water
d) High velocity is needed to transport boulders and cobbles and sand needs less
velocity
e) High velocity is needed to erode clay particles but low velocity will carry it
4. Capacity is the total load a stream can carry
a) It varies with the discharge, the greater the discharge the more sediment a stream can
carry
b) Capacity is dependent on the volume of water while competence is dependent on
velocity
5. Braided streams have a network of dividing and rejoining channels
a) Develops when there is excessive sediment as sand and gravel bars along the stream
b) These are bed load transport streams composed of predominantly sand and gravel
c) Forms in arid and semi-arid regions or where erosion rates are high or there is little
vegetation
d) Melting glaciers supply a lot of sediment and commonly form braided streams
6. Meandering streams have a single channel of looping curves
a) The channels have a semicircular cross-section on straight sections
b) At the curves they form asymmetrical cross-sections that are deepest at the outer
banks
c) This outer bank is called the cut bank because it is being cut or eroded by the higher
turbulence and velocity of the stream
d) The inner bank is the inside curve and is the point of deposition along the stream
e) With the erosion on the outer bank and deposition on the inner bank, the stream
meanders laterally as it progresses, maintaining the same width and depth
f) Deposits formed on the inside are called the point bar, consisting of imbricate sand
and gravel
7. Meandering streams sometimes become so sinuous that a thin neck of land forms between
the meanders
a) This neck of land eventually is eroded through, creating an oxbow lake
b) These lakes are eventually cut off and fill with sediment forming a meander scar
c) The stream has decreased the distance it flows, but travels at the higher velocity, thus
increasing the gradient at that point
d) This increase in gradient causes greater erosion downstream and the stream forms
more meanders and greater curvature in the existing ones
VI.
Floodplains are formed when streams receive flood waters which overflow the channel and spreads out
across flat terrain
A. Floodplains vary with the size of the stream, the larger the stream or river the larger the floodplain
B. Streams with deep channels don’t form floodplains, but sometimes terraces are made alongside the
channel
C. Meandering floodplains are made of sand and gravel from meandering streams migrating back and
forth across the area
1. These deposits cut and re-cut the previous deposits forming point bar gravel beds developed
by lateral accretion
D. Some floodplains from large rivers with lots of sediment form silt and sand deposits
1. These floodplains experience vertical accretion of fine grained sediments from the river
flooding over its banks
2. When the water flows over the bank it loses velocity and deposits its load
a) This forms ridges of sandy alluvium along the banks called natural levees along the
margins of the stream
b) These natural levees separate the floodplain from the channels
c) During flood stages, the water flows out over the floodplain, slowing down and
eventually settling the silt and clay, building up the soil layers and levees
VII. Deltas form when the stream flows into a body of water, either a lake or ocean and drops it sediment
load rapidly in the lower energy environment
A. In this environment, the shoreline gradually builds outward in prograde fashion
B. Develops a vertical sequence of
1. bottomset beds of silt
2. middle layers of forest beds with inclined layers
3. topset beds of layered sand and gravel
C. Three types of deltas form
1. Stream-dominated deltas form long finger like deposits that build seaward
2. Waves-dominated deltas like the Nile have reworked sediments that form a series of barrier
islands and the whole delta develops seaward
3. Tide-dominated deltas develop patterns that are parallel to the direction of the tidal flow, ie
The Ganges River in India
D. Oil and gas can form in delta deposits when organic rich layers get buried
VIII. Alluvial Fans are deposits formed on land
A. Usually form in arid and semiarid regions with little vegetation with low areas next to highlands
B. When flash floods and heavy rains wash sediment down from mountains it is funneled through steep
canyons onto the valley floor
C. When it discharges from the canyon it spreads out and slows down, flowing downward until it loses
energy, and drops its load
D. Mudflows, flash floods, and stream deposits make up alluvial fans
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Drainage Basin is an area that drains into one outlet, also called watershed
A. These are separated by ridges and crest of hills called divides
B. Drainage patterns are developed that have particular shapes
1. Dendritic- network of channels that resemble the branches of a tree
a) Developed from areas of uniform erosion surfaces
b) Flat lying areas of sedimentary rocks or homogeneous igneous or meta rocks
2. Rectangular drainage has channels with right angle turns
a) Controlled by geologic structures in the bedrock
b) Typically regional jointed patterns
3. Trellis drainage forms in areas of parallel structural development
4. Radial drainage forms in areas of domes, basins, or ancient volcanic mountains
5. Deranged or unorganized in areas of undeveloped drainage patterns
Base Level is the lowest point in the drainage area of a stream
A. A stream cannot erode lower than its base level
B. The ultimate base level is sea level
C. A steam cannot erode itself to the base level because it has to have some gradient to flow
downstream
D. Streams develop local or temporary base levels like a lake, river, or bedrock area. A waterfall can act
as a temporary base level, usually seen as a riffle or rapid
E. When the sea level rises or falls or the land uplifts or down-drops, the base level changes
1. During glacial periods the sea level was 200 to 300 feet lower and the base level rose and fell
with it
2. Streams would cut deeper into the valleys and extend their channel deep into the continental
shelves
3. Rising sea levels caused the base levels to rise and the streams deposited sediment into the
channels and stream valleys in response to base level changes
F. Streams adjust to human intervention like dams and levees
1. Building a dam changes the base level of a stream
2. When a stream enters the lake it deposits its sediment and will fill the reservoir
3. Another problem is that the water coming out of the dam is sediment free but still has the
energy necessary to transport sediment
4. Consequently, downstream the stream erodes the banks, terraces, and channel sediment
along the bottom and sides
A stream will seek an ideal equilibrium profile called a graded stream
A. This is formed when a balance develops between the gradient, discharge, flow velocity, channel
characteristics, and sediment load
B. This forms a profile that neither erosion or deposition occurs and has ideally smooth angles
C. This forms in response to the conditions of the stream
1. If the rainfall amounts change it must adjust to the increased runoff
2. Any change in base level would cause a change in stream characteristics
3. Vegetation changes can influence the erosion patterns
a) Decreased vegetation causes higher erosion rates
b) This results in more sediment in stream than it can carry
c) Deposition occurs, increasing the gradient which will allow it to carry more sediment
load
Development of Stream Valleys
A. Many types of valleys are formed, gullies, broad canyons, and deep gorges
B. These evolve as a result of the stream processes at work
1. Downcutting, lateral erosion, sheet wash, headward erosion, and mass wasting
2. Combination of these processes develop the valley
C. When a stream has more energy than it needs to carry its sediment load, the excess energy is used to
downcut the valley
1. Streams also have tributaries that bring in sediment
2. Undercutting and lateral erosion create unstable situations along streams
3. Mass wasting from the sides of the valley widens the valley as the stream carries the material
away
D. Headward erosion lengthens the stream valley by eroding the headwaters portion of the drainage
basin or watershed.
1. This will eventually erode through a divide and capture more watershed
2. Steam piracy is the process where headward erosion captures a stream that drained into
another watershed
3. Adjustments must be made after piracy occurs because one stream now has more drainage
and the other has less
E. In an area where the land has been uplifted, the process of stream development starts out as eroding
downward forming deep narrow V-shaped canyons
1. Over time the stream starts to erode laterally creating a broad floodplain with meandering
streams
2. Finally a large flat featureless plain develops from the lateral erosion
3. The age of a stream used to be determined by these profiles but this concept has been
abandoned
4. This is because any stream could develop any of these stages depending on the landscape or
position near the base level or ocean
5. If an area of land continues to uplift like the Grand Canyon, the stream will continue to erode
downward
F. Stream terraces are erosional remnants of ancient floodplains that were formed when the river was
higher
1. They have a flat upper surface and a steep slope that ends in another flat surface
2. This makes a step like structure and can show the pre existence of previous higher
floodplains
3. Terraces are usually attributed to renewed erosion, either from a lowered base level or uplift
of the land itself
4. If the sea level lowers, the erosion cuts the floodplain down to another level and once it
reaches the graded level again it forms a flat plain
G. Incised Meanders are meandering streams cut into bedrock (Pg. 437)
1. These streams cut downward and have little ability to cut laterally
2. They do erode laterally by mass wasting along the sides
3. These areas are thought to have been a meandering stream on a floodplain before the land
began to rise, changing the process from lateral erosion to vertical erosion
4. This is in response to the change of base level
H. Superposed Streams are ones that seem to defy the topography
1. These streams have had preexisting pathways over a floodplain or drainage area
2. As the land began to uplift the streams were able to keep up their downcutting as fast as the
land was rising
3. In this way, it seems that the stream cross cuts the mountain ranges or structural features
creating water gaps- steep walled canyons that cut through rock