Revised 8 / 06 (Monroe 6th ed.)
Including...
Introduction
The Hydrologic Cycle
Streams and Energy
Velocity, Channel Morphology, and Discharge
Transportation of Sediments
Deposition of Sediments
Graded Streams
Evolution of Drainage Systems
Linkage to Groundwater
Humans and the Rational Use of Streams
Click here for a list of vocabulary associated with this topic
Rivers and water are pretty important
"Givers of life"
Nile, Tigris & Euphrates, Congo
Some of the earliest human construction projects were associated with the transportation and storage of water
Dams and canals
Also some of the earliest legal disputes
Code of Hammurabi - 1700 B.C.
Natural boundaries throughout history
Communication routes
Routes into the interiors of the continents
Most of the world's leading cities are either on rivers or at their mouths
Act as the transportation systems to get sediments to the ocean
Rivers and river systems are intensely studied
In an effort to "understand and control" the movement of water
"Understand?" Maybe. "Control?" I doubt it!
Refer to Strickler's 4th Law of GeoFantasy
Actually, the concept of controlling ANY earth process gives me trouble
The overall volume of water on earth, while clearly not static in the long term, can be considered constant at human time scales
However, the spatial distribution of water can and does change
Constantly moving between numerous "temporary storage units"
Both "long" and "short" term fluctuations
Ice ages vs. Pepsi cans
This movement is called the Hydrologic Cycle (Monroe: Fig. 15-3, pg. 460)
The cycle has 4 main parts
Each involves a change of state or the interaction of water and gravity
Evaporation: liquid to vapor
Condensation: vapor to liquid
Precipitation: liquid and gravity
Collection: liquid and gravity
The water is usually purified as it changes state
This is good, since we tend to mess it up, no matter what state it's in
Click here for more information on the hydrologic cycle
Need to get weathered sediments to the ocean (Strickler's 3rd Law of GeoFantasy)
What is the transportation system?
In most cases moving water
Energy is the determining factor
Need to get the sediments moving, but inertia tries to keep them stationary
Acceleration = force / mass
In this case, mass of the sediment is critical
Greater mass requires greater force to achieve an acceleration
Click here for more on Newton and the Laws of Motion
Where do we get the force?
Several sources: Water, wind, gravity, geologists and other bozos
Moving water has kinetic energy (the energy of motion)
Kinetic energy: Ek = 1/2 mv2
Velocity is the most important variable (EXPLAIN)
So if we are trying to maximize sediment transport, it is important to get the water moving as fast as possible
Start with the basic equation of flow
Discharge = Area of channel X Average velocity of flow (Q=AV)
We'll find that all 3 variables are almost impossible to define accurately
And if any one variable is suspect, real values for all 3 are in question
Streams are active systems, and don't stop long enough to be measured
And it's even worse than that:
When they're doing the most work it can be downright unsafe to be anywhere near them!
Stream flow is mainly turbulent (Monroe: Fig. 15-4, pg. 461)
As opposed to laminar (like glaciers)
Water molecules go every which way
Up and down
Side to side
Even back upstream
This makes any attempt to determine average velocity difficult
Stream velocity... a definition
"The direction and magnitude of a portion of the stream per unit of time"
Translation - how fast and in what direction
Quite a range of values
Up to 20 miles per hour
Most are less than 4 mph
This is obviously very erratic
And seasonal!
Velocity is VERY important (see above)
Directly related to the stream's energy and its ability to transport sediments
In general terms, the faster the water is moving, the more energy it has
REVIEW: the basic equation for kinetic energy (Ek = 1/2 mv2)
Average velocity: where is it?
Friction is important here
Contact with the sides and bottom slows down the flow
Also contact with the atmosphere!
In most cases, the fastest velocity is near the center of the channel, just below the surface (Monroe: Fig. 15-6, pg. 463)
But this is not necessarily the average velocity
May be impossible to measure due to the turbulent nature of the flow
Therefore, how can we ever trust discharge calculations?
Many factors affect stream velocity
Including...
Gradient (Monroe: Fig. 15-5, pg. 462)
Channel shape and geometry (Monroe: Fig. 15-6, pg. 463)
Roughness of channel
Discharge of the stream
Sediment load
Each of these is highly variable, both regionally and locally
And are definitely inter-related, each working with the others to regulate stream flow
Gradient - the down-valley slope (Monroe: Fig. 15-5, pg. 462)
In general, increased gradient increases the velocity
Gradient is usually measured in feet per mile
Can range from waterfalls to very flat sections
Gradient usually decreases as the stream descends to lower elevations
But can be highly variable locally
Directly affected by lithology
What type of bedrock the stream is flowing over
EXAMPLE: the Rogue River
Describe general gradient from Crater Lake to Gold Beach
DEFINE: Base level (Monroe: Fig. 15-24, pg. 482), Temp. (local) base level (Monroe: Fig. 15-25, pg. 483)
Channel shape and geometry
Sides and bottom cause friction
Wide, shallow channels tend to slow the flow
Semi-circular channels allow the fastest flow
Roughness of channel
Smooth channel results in relatively non-turbulent and laminar flow
Rough channel results in relatively turbulent flow
Also tends to increase the total surface area of the channel
Therefore increases friction and decreases velocity
DIGRESS TO: Hippo's Teeth along the Grapevine (I-5)
Discharge
"The quantity of water which passes a point in a given interval of time"
Rarely constant due to seasonal fluctuations
Also daily fluctuations
High latitudes - daytime highs increase snowmelt and discharge in the afternoon and evening
Global discharge varies greatly
Amazon clearly the largest
Fresh water up to 100 miles out to sea
Definitely affects velocity
In general, increased discharge results in increased velocity
Sounds like a no-brainer, but...
This can be a bit confusing
This is a local variation in a given section of a stream
Usually, increased discharge in the lower portions of a river result in a lower velocity than in the headwaters
Generally related to the decreased gradient
Clearly, the other factors also have an effect
Sediment load
Increased sediment load results in a relative decrease in the amount of water
But, due to density, a relative increase in mass
Therefore, velocity should decrease as sediment load increases
Again, the inter-relationships are beautiful
Increased discharge during flooding results in increased velocity and energy
Results in more sediment in the water, which slows it back down!
Streams are the conveyor belts which move the weathered and eroded sediments to the ocean
The actual amounts can be rather impressive
Mississippi River: average 1,000,000 metric tons per day
Obviously, can be much higher during flood stage
"Removes 2" of soil every 1000 years"
How (and when) did they get this figure?
Obviously, human interaction in the Mississippi basin has a significant impact on this amount
If this is a recent figure based on recent erosional rates, then the actual lowering of the land is much slower
The reality of the process is relatively simple
But difficult to study
The vast majority of transportation takes place during flood
A dangerous time to sample!
DIGRESS TO: Classification of floods (100 year floods, etc.)
Two general categories
Dissolved load
Solid load (suspended load vs. bed load)
Three types of load are possible
Related to the type and size of the material and velocity of the stream
REVIEW: Chemical vs. mechanical weathering
Click here for more on:
The relative proportion of load type can vary locally and regionally within a drainage system
Dissolved load
Material moving downstream "in solution"
Generally produced by chemical weathering processes
Cannot be seen or felt
Can be tasted and smelled
Will not settle out of the water
The dissolved load has a chance of making it to the ocean rather quickly
It's a part of the water and will move with it until it stops
Or a chemical imbalance causes precipitation
Can usually be separated by evaporation of the water
Resulting in the precipitation of the dissolved material
And the formation of chemical sedimentary rocks
Suspended load
"Smaller" solid material moving downstream while suspended in the water
Put dirt in a jug, shake it up, and watch the sediments settle
Pretty much covers the basics of suspended sediments
Generally produced by mechanical weathering processes
Individual particles are called clasts, and result in clastic sedimentary rocks
Can be seen, and will settle out of the water
The size, amount, and length of time clasts remain suspended depends on:
Physical characteristics of the material
Size: clay vs. sand
Shape: clay/mica vs. sand
Specific gravity: gold vs. sand
Velocity of the water
Faster water has greater energy
Can suspend more stuff for a longer time
Turbulence of the water
Keep shaking the jug of dirt and the stuff will never settle
An increase in velocity and/or turbulence will result in more, and larger, clasts in suspension
In general, the bigger stuff (commonly sand) is carried closer to the bottom
And will settle first (a real no-brainer!)
Silt and clay are more evenly distributed throughout the water
Form the majority of the suspended load
Relatively easy to measure
Get some water and let it settle
Weigh the solids
This stuff may make it to the beach on its first try
But no guarantee!
Bed load
"Larger" solid material moving downstream without losing contact with the river bed
Generally produced by mechanical weathering processes
Can be seen, and will settle out of the water
Does most of the work in down-cutting and widening stream channels
DIGRESS TO: potholes (Monroe: Fig. 15-8b, pg. 465)
Particles are moved in different ways
Sliding and rolling
The big stuff which is too heavy for a given volume of water to move easily
Saltation
A "leap-frog" motion of material as it bounces downstream
Transition between true bed load and suspended load
Bed load can be difficult to impossible to measure
Most is moved during floods
REVIEW: Classification of floods (100 year floods, etc.)
The 1964 flood as reported at Agness, Oregon
The bed load has little or no chance of reaching the ocean quickly
The bigger the piece, the longer it will probably take
Additional weathering is required to reduce the particle size
Dissolved vs. solid load (suspended load and bed load)
Ratio between dissolved to solid can vary for many reasons
"Generally, 50% of the total load is in solution"
Basically, the land is dissolving (Strickler's 4th Law of GeoFantasy)
This will obviously vary both locally and regionally
Increased chemical weathering results in increased dissolved load
Tropical climates
Warm, humid, lots of chemical weathering
Increased vegetation to hold clasts (soil) in place
Also greater precipitation so larger river systems
More water reaching the ocean which will skew the global average in favor of the dissolved load
DIGRESS TO: efficiency of chemical weathering in various climates
Increased mechanical weathering results in increased solid load
Colder climates
Mostly mechanical weathering
Sparse vegetation so solid clasts are relatively free to move
Accumulations of sediments are common along the sides of all rivers and streams
Local accumulations of sand, gravel, and boulders to immense "floodplains" covering thousands of square miles
Monroe: Fig. 15-7, pg. 464
Monroe: Fig. 15-29, pg. 486
Floodplains
The debris which forms the banks of any and all streams
By definition, floodplains are very active depositional/erosional environments
Prone to floods and shifting materials
Not a good place to build!
Floodplains store excess sediments at times of low water
And excess water at times of high water
In reality, the edges of rivers are common sites of human activity
But, prone to flooding at all levels of intensity
Both good and bad effects
Bad are relatively obvious
Good: EXAMPLE - the Nile and the impact of the Aswan Dam
Lots of features associated with floodplains and the deposition of sediments
Meanders - probably the most obvious and recognizable feature
Large, curving bends in a river (Monroe: Fig. 15-11, pg. 468)
Common in areas where the river has cut almost to base level (REVIEW)
Generally form when a river has too much energy, and it needs to slow down
Meandering lengthens the channel and reduces the gradient
Generally results in a lower velocity
Energy drops, and the river stops down-cutting into its bed
Meanders do migrate and cut side-to-side
Water velocity is fastest on the outside of the bend
Results in differences in energy across the channel
Cut bank vs. point bar (Monroe: Fig. 15-14, pg. 446)
Cross-over: where fast water crosses the channel between meanders
Also the best place to ford a river
Relatively shallow depths compared to the cut bank / point bar
The river uses up its energy moving sediments from side to side
The meander will migrate in the direction of the cut bank
Oxbow lakes (Monroe: Fig. 15-12, pg. 469)
Meanders tend to form in areas with strong and cohesive materials
Clay and silt rich deposits
Common in temperate and tropical climates where there is a large amount of chemical weathering
Braided streams (Monroe: Fig. 15-10, pg. 467)
A lot like meanders
Form in low gradient streams as a means of using up excess energy
Without cutting deeper into bed (that base level thing again!)
Common in areas where the stream deposits are loose and non-cohesive
Cannot maintain resistant banks
Sand and gravel are the common floodplain deposits
Arid lands and cold climates where mechanical weathering predominates
SUMMARIZE: meanders vs. braids
Deltas (Monroe: Fig. 15-15/16, pg. 471/472)
Because Q=AV streams drop their load when they enter still water (ocean or lake)
Alluvial fans (Monroe: Fig. 15-17, pg. 473)
Basically dry land deltas
Common to arid regions where there is insufficient flow to completely remove the sediments
Stream terraces (Monroe: Fig. 15-29, pg. 486)
Usually uplifted floodplains
Underlain by floodplain deposits
Can also be cut into bedrock
DIGRESS TO: terrace vs. peneplain
Represent times when the stream was at a higher level
Stabilized long enough to create a relatively flat surface
Results from a change in relative base level
Either the land goes up, or the base level goes down
Stream rejuvenation occurs and the stream begins to cut deeper into its bed
Until equilibrium with the new base level is achieved
Can result in incised meanders (Monroe: Fig. 15-30, pg. 487)
Misfit streams
A big valley with a small stream
Common in cases of stream piracy (Monroe: Fig. 15-28, pg. 486)
The earth tries for a balance in all things, but...
Rapidly evolving local stream conditions make it tough for nature to keep up
Several factors are relevant here
And, as usual, they are inter-related
Sediment yield
How much material is being transported
Rapidly changing as humans are causing increased weathering and erosion
Competence
The size of material that a stream can transport
Depends primarily on velocity
Get the water moving fast enough, and some pretty big things can move
Island Mountain bridge
Crescent City jetty core
Capacity
The potential load a stream can carry
Again, velocity is important
Also discharge - more water can move more stuff
In general:
Capacity: what a stream theoretically can do
Load: what a stream is actually doing
Why are these different?
Many factors contribute to increasing the yield beyond capacity
As well as reducing the amount of material available to the stream
Put it all together, and we end up with streams which are usually out of balance
Aggrading streams
Too much load, so deposition will occur
Degrading streams
Too little load, so erosion will occur
At grade streams
Equilibrium, where the sediment load is balanced to the stream's capacity
Being "at grade" is the goal and natural end result of stream dynamics
Any disruption or change in local/regional conditions will force the stream to re-adjust in an attempt to restore equilibrium
These re-adjustments result in deposition or removal of sediments
Both are tough on human developments in the vicinity of the stream
Lots of possible disruptions to a balanced stream
Too much load (load greater than capacity)
Landslides, logging, hydraulic mining
An interesting sequence of events as the stream attempts to deal with the additional material
Dump it
Increase the local gradient
Increase velocity and energy
Erode the sediments again
Sooner or later (hopefully) reach equilibrium
Too little load (load less than capacity)
Rapid increase in discharge, loss of sediments (like below a dam)
In this case the erosional ability increases, and the stream picks up additional material
DIGRESS TO: dams in rivers (ultimately self-defeating)
Axiom: all dams are short term features because the lakes fill in and/or the dam is under-cut at the base
Obviously, streams have a lot to do with the shape of the land
Cut the valleys they flow within
Work with weathering and erosion to reduce relief (DEFINE)
The ultimate goal of all rivers is the creation of Kansas!
In most cases this is not a short-term process
Drainage basin (Monroe: Fig. 15-22, pg. 480)
Land surface included in the area a stream drains
Drainage patterns (Monroe: Fig. 15-23, pg. 481)
Often controlled by lithology and tectonics
The 3 stages of stream development
As usual - lots of intermediate steps and shades of gray
Involve two basic processes (Monroe: Fig. 15-32, pg. 488)
Down-cutting into stream bed
Deepens the valley
Back-wasting of the sides of the valley
Widens the valley
Youthful : generally in the mountains
V-shaped valleys with steep canyon walls
Steeper gradient
Higher velocity
Relatively lower discharge
Down-cutting the dominant process
Mature
Rounded hills and valleys
Moderate gradient
Moderate velocity
Moderate discharge
Old age: generally found near the mouths of river systems
Very low relief topography (essentially flat)
Minimal gradient
Very slow velocity
Increased discharge
Meanders and oxbow lakes common
Back-wasting the dominant process
Groundwater and surface water are part of the same system
Lots of factors can force the water to leak onto the surface
Climates where there is too much precipitation for the ground to hold
Tropics vs. arid lands
Climates where the weathering and/or erosion processes are incomplete
Lack of fractures and/or soil
Places where there is some sort of blockage which forces the water out
Springs - places where water flows or seeps onto the surface
Occur where the water table intersects the surface
Can be caused by many different sub-surface conditions
Effluent stream - Gets its water from the water table
Common to temperate climates
Actually, effluent streams are just springs with a lot of water!
Associated with relatively stable water tables (in a natural setting)
Directly reflects the water table
Deer Creek bridge example
Influent stream - Adds water to the groundwater supply
Common in arid regions
The water is usually from more humid areas upstream which are destined to flow down into a desert
EXAMPLE: the Colorado River and the Nile
Where do we start? Or stop!
The earth doesn't need or want our help! (Strickler's 5th Law of GeoFantasy?)
The reality is that we will continue to use (and abuse) running water
We need water and it's so damn useful
Back to that energy thing again
Tough to balance development with preservation
Economic, political, and cultural needs and desires at work here
Examples of irrational use abound
Dams in general
DIGRESS TO: I hate to trash the Army Corp. of Engineers, but...
Economic, political, and cultural needs and desires at work here, too
Dams are ultimately self-defeating
Q=AV so their impoundments (and the dams themselves) are temporary
Disruption of sediment transport
Fill in lake
Now clean water below dam with all this extra energy
Increased erosion
Levees and flood control
Levees work great when you don't need them
But, next time the water rises...
Restrict floodplains at times of flood
Increased velocity erodes levee
River gets behind the levee
Levee is now a dam!
Development in urban areas
Make the streams straight and pave them to reduce erosion
Both tend to upset the natural balance
Next time the water rises...
Fishermen are a real problem
Not to mention the mess...
In many parts of the world, the best fishing streams are cleaned of debris
So the fishermen can save money on snagged tackle
But the removed debris was a part of the balance
Next time the water rises...
Used as depositories for many (ultimately ALL) waste products
Open, running sewers in many parts of the world
Would YOU like to swim in the lower Gangees?
In combination with groundwater this can be a real long-term problem
Hanford nuclear facility
How many in the Pacific Northwest depend upon a clean Columbia?
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