Streams and energy
Sediment load: an evolving balance
Floods and floodplains
The 3 stages of stream development
Linkage with groundwater
REVIEW: Hydro cycle
Erosion, and now...
What is a river? A stream? A creek?
Is a brook bigger or smaller than a rill?
And if so, by how much?
Are there exact values for each, and if so, what are they based on?
Channel size? DEFINE
Drainage basin size DEFINE
How about daily and seasonal variations?
All in all, a seemingly nebulous classification scheme!
We'll use the term "stream" for all bodies of moving water which are confined in a definable channel
Which way does a stream flow? (downhill)
How far does a stream flow? (until it reaches the bottom)
DEFINE: Base level: Temporary vs. Permanent
Need to get the mechanical & chemical sediments to the ocean
What is the transportation system?
In most cases moving water
The purpose of a stream is to transport the weathered and eroded materials to the beach
Energy is the determining factor
Need to get the sediments moving
There are 3 interconnected formulas dealing with how this is accomplished
Formula #1: Need to impart an "acceleration" to the sediment
Newton defined this in the 1600's in his 2nd Law of Motion
a = F / m (acceleration = force / mass of the sediment)
DIGRESS: Fractions (numerator vs. denominator)
In this case, mass of the sediment is critical
Greater mass requires greater force
Where do we get the force?
Water, wind, gravity, geologists
Formula #2: Moving water has energy
Kinetic energy: Ek = 1/2 mv2
Where "m" is the mass of the water, and "v" is its velocity
DIGRESS: velocity is a vector
But since water usually moves in the same direction we can concentrate on the speed
What happens if we double the mass (discharge) of the stream?
What happens if we double the velocity of the stream?
Velocity is the clearly most important variable
So if we are trying to maximize sediment transport, it is important to get the water moving as fast as possible
Formula #3: What affects the velocity of a stream?
Gradient: how steep the stream bed is
Discharge: how much water is in the stream
Flood stage - lots of water, usually moving fast
Channel configuration: wide vs. narrow; shallow vs. deep
V = Q / A (velocity = discharge / cross-sectional area)
If we assume that discharge in a constant locally
Then any change in channel morphology has to affect the stream's velocity
Lots of examples of this
Meander vs. point bar
Several types of load are possible
Material moving downstream while dissolved in the water
Generally produced by chemical weathering processes
Cannot be seen, and will not settle out of the water
Can usually be separated from the water by evaporation of the water
Resulting in the precipitation of the dissolved material
These are actual clasts of weathered rock
Generally produced by mechanical weathering processes
But chemically weathered clay also falls into this category
Can be seen, and will settle out of the water
There are 2 varieties
Generally based on the size of the clast relative to the velocity of the stream
"Larger" material moving downstream without losing contact with the river bed
"Smaller" material moving downstream while suspended in the water
Bedload vs. suspended load
There are dynamic terms
Change as the velocity changes
Many level to this
Local variations in channel shape and size
Riffles, meanders, holes, etc.
Normal seasonal variations in discharge
Wet season vs. dry season
Not always winter vs. summer!
Longer term variations in discharge
Bed load can become suspended in times of flood
Floods occur when the amount of discharge exceeds the size of the "normal" channel
Given names based on frequency
50 year flood, 100 year flood, 500 year flood, and so on
Floodplains: Sediment-filled areas along the borders of most streams
Basically serve two (2) purposed:
Store excess water in times of high discharge
Store excess sediment in times of low discharge
Unfortunately, they are also convenient areas for settlement and agriculture
Humans have historically built on floodplains
And regretted it when the water rises
Only to rebuild as soon as the floodwaters recede!
Remember: the floodplain is part of the stream
And Water Always Wins
Youthful : generally in the mountains
V-shaped valleys with steep canyon walls
Relatively lower discharge
Small to non-existent floodplains
Rounded hills and valleys
Small to locally well developed floodplains
Old age: generally found near the mouths of river systems
Very low relief topography (essentially flat)
Very slow velocity
Generally extensive floodplains
Meanders and point bars common
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