Fluvial Processes
Streams and Rivers
Atmosphere
Evaporation
Evaporation
iratio
n
( ) = Storage
Eva
Liquid
Flow
potr
a n sp
Vapor
Precipitation
(water vapor)
The Hydrologic Cycle
surface
Land
Streams
Lakes
etc.
Surface
Infiltration
(Surface water)
Subsea outflow
(Ground water)
(Seawater)
Runoff = Streamflow
Groundwater flow
and Baseflow
Water Table
Oceans
Overland flow =
Direct Runoff
Vadose Zone
Gravity
drainage
Hydrologic
Cycle
(Ice, snow,
depression
storage)
Dendritic stream dissection
Streams
• Any body of flowing water on the surface.
• Perennial - always filled with water.
• gaining
• humid regions
• Ephemeral - usually dry
• losing
• arid regions
• Tend to occupy a well-defined channel.
• Slope = stream gradient.
• Base level - the lower limit to which a stream can flow and erode.
• Local base level
• Sea level
Gaining Stream
•Groundwater discharges beneath streambed.
•Water table slopes toward the stream channel.
•Baseflow
Gaining Stream
Losing Stream
•Water drains from the stream into the
groundwater system.
•Water table slopes away from the stream
channel.
Losing Stream
Stream channel
Discharge (Q) = Velocity x Area
Discharge tends to increase along a stream as baseflow and
tributaries contribute water to the channel.
Stream channel
Discharge (Q) = Velocity x Area
Cross Sectional Area
Stream channel
Discharge (Q) = Velocity x Area
Velocity
Neglecting baseflow and tributaries, Q is constant along
the course of the stream channel.
Stream channel
Discharge (Q) = Velocity x Area
Velocity
If A increases, V must decrease.
Deep reaches flow slowly.
Stream channel
Discharge (Q) = Velocity x Area
Velocity
If A decreases, V must increase.
Shallow reaches flow rapidly.
Load - the sediment carried by a stream.
Suspended load
Dissolved load
Bedload
Bed Load - moves by rolling, sliding and saltation.
Saltation = jumping or bouncing motion
rolling
sliding
Carrying Capacity
• Amount of sediment load a stream is capable of moving.
• Related directly to discharge and velocity = total energy.
• Increase in velocity = increase in capacity = net erosion
• Decrease in velocity = decrease in capacity = net deposition
• Increase in discharge = increase in capacity = net erosion
• Streams constantly erode and redeposit sediment.
• Below capacity - net erosion, stream enlarges channel.
• Above capacity - net deposition, stream fills in channel.
• At capacity - equilibrium between erosion and deposition.
Headwater
Net erosion
Net deposition
Equilibrium
Base level
Stream Profile
Streams flowing below
capacity with steep
gradients tend to erode
straight, V-shaped valleys.
Delta
Streams flowing above capacity
• Excess sediment load results in net deposition.
• Sediment forms sand and gravel bars that choke the
channel.
• Braided stream - many small, anastomosing channels.
• Often seen in streams with strongly seasonal flow.
• Decrease in discharge during dry season lowers
capacity.
• Streams in semi-arid regions.
• Glacial outflow streams.
Anastomosing channels
Yukon River, Alaska
Streams flowing at capacity
• Erosion and deposition are in equilibrium, but only
averaged across the entire channel.
• Channel curves, forming bends.
• Outer edge of channel - velocity increases, net erosion.
• Inner edge of channel - velocity decreases, net
deposition.
• Stream will meander within their flood plain.
• Cut banks
• Point bars
• Meander loops
• Cutoffs
• Oxbow lakes (abandoned meanders)
Cross section of a flood plain
natural levees
flood plain
channel
thalweg
Cut bank
Point bar
Meander bend
Point bar in Columbia River
Flood megaripples
Meander formation
Initial bend in irregular
course of a river channel
Meander formation
Deposition of
point bar on
inside of bend.
Formation of cut bank on
outside of bend.
Meander formation
Deposition of
point bar on
inside of bend.
Formation of cut bank on
outside of bend.
Meander formation
Vegetation of
old point bar
deposits
Migration of
meander loop
Meander formation
Vegetation of
old point bar
deposits
Migration of
meander loop
Meander formation
New point bar
New cut bank
New cut bank
New point bar
Meander formation
Meander formation
Meander formation
Meander
cutoff
Meander formation
Meander
cutoff
River meanders and oxbow lakes
Oxbow Lake
Mississippi River
Memphis
Colville River, Alaska
Colville River, Alaska
Colville Oxbow, Alaska
Equilibrium in Streams
• Streams are dynamic systems that trend toward an
equilibrium over time.
• Discharge
• Load
• Gradient
• Channel form
• Variables are interdependent.
• Stream adjusts until it is carrying maximum load with
minimum energy.
• Negative feedback loops.
• Stream in equilibrium is a Graded Stream.
Equilibrium in Streams
• Streams can adjust their gradient.
• Depositing sediment raises the stream bed and increases
the gradient.
• Increased gradient = higher velocity = more load
deposition
New gradient
Original gradient
Equilibrium in Streams
• Eroding sediment lowers the stream bed and decreases the
gradient.
• Decreased gradient = lower velocity = less load
Original gradient
Equilibrium in Streams
• Eroding sediment lowers the stream bed and decreases the
gradient.
• Decreased gradient = lower velocity = less load
Erosion
Original gradient
New gradient
Equilibrium in Streams
• Some variables are imposed on the stream.
• Total vertical distance from headwaters to base level.
• Total sediment load delivered to the stream.
Graded Streams
• Any change imposed on a graded stream will cause it to try
to readjust back to equilibrium.
• Altering one parameter results in changes to other
parameters.
• Some examples….
Thompson Creek, Iowa
• Gradient was artificially steepened by channel
straightening (same change in elevation over a shorter
distance).
• Steeper gradient = faster velocity = greater load capacity
• Stream began to erode channel sides and bottom.
• Erosion decreased velocity until equilibrium was reestablished.
• Bridges were undermined.
• Deep gullies eroded in tributaries.
• Water table was lowered.
Problems with dams
• Sediment load settles out in reservoir.
• Water exiting spillway has very little load.
• Stream below dam is under capacity.
• Stream erodes channel until gradient and sediment load are
balanced.
Increasing sediment load
• Adding sediment to a stream will cause it to become over
capacity.
• Fires, clear cutting can denude landscape, resulting in
increased sediment load to rivers.
• Stream will deposit sediment on channel bottom to
increase gradient and velocity until load and gradient are
balanced.
• Change in gradient can cause tributaries to aggrade,
blocking channels and raising water tables.