Lecture 7: Water

Water is impt because life depends on it.

TodayÕs Top Ten List

The top 10 reasons why streams are important
drum role please...

Streams are important because they provide
10) (maintain saltyness of oceans).
9) erode the land; form many geomorphic features.
8) carry sediment from the mountains to the sea -- billions of tons -- rock cycle.
7) essential part of the hydro cycle.
6) floods -- a natural hazard; muchu $$ is put into preventing/preparing for them
5) asthetic value -- people like to look at, play in them
4) transportation -- cities often built near them.
3) nutrients for agricultural land.
2) irrigation water.
1) We like to drink water.

Therefore important to understand streams, g-water, and the hydrologic cycle.

What makes a stream tick

1) gradient -- how steep the stream is --- the slope of the stream -- generally decreases
downstream, -- (h1-h2)/length -- dimensionless
2) cross-sectional area (A) -- width X avg depth. -- how big the stream is -- units of length2
3) average velocity (V) -- how fast the stream is moving -- units of l/t -- often deceptive
4) discharge (Q) -- how much water is flowing in the river per unit time -- Q=A X V -- important
concept -- units of volume (length3)/time

5) load
	a) dissolved load -- salts that keep the sea salty -- little effect on river shape
	b) suspended load -- material (usually relatively fine grained) that is suspended by the
water due to turbulence -- makes the river muddy.
c) bed load -- material (relatively courser grained) that moves along the bed of the river due
to force of the moving water -- makes placer deposits.

-different velocities are needed to erode, transport, deposit material of different sizes (or
density) [draw graph here, discuss implications].
alluvial fans

6) Base Level -- the limiting level below which a stream cannot erode. 
Streams have kinetic E derived from their potential E.
Take away kinetic E, no more erosion (or movement) -- all deposition.
Base levels can be sea level, lake level, or basins below sea level (e.g., Death Valley).
Entire stream may not ÒfeelÓ ultimate base level.
Many features related to river geomorph and topographic features can be related rivers and
their base level, as we will see.
What happens when base level is lowered, like from a fault? see fig. 12.8
Or raised, like with a dam?

Why Rivers Look the Way They Do

Stream shape depends on rever evolution from high in mountains to low in valleys.

1) Straight channels These are rare.
More often sinuous.
Deepest part of stream (thalweg) is rarely in center. It migrates back and forth.
This causes erosion on outside bank,
and deposition on inside bank, to make a point bar.
[draw picture]
Thus, straight channels evolve into .....

2) Meandering streams form where
	the gradient and sed load is low
channels formed in easily erodable fine-graind sediments
effort by stream to reduce the resistance to flow.
Oxbow Lakes form where stream meanders get cut off, as water tries to find the shortest
distance between elevations.
3) Braided channels form where
sediment load is high,
discharge is highly variable,
and the banks are easily erodable.
Large amounts af material are often dropped anywhere in the channel as discharge changes,
thus single channels are difficult to establish.
Common at the foot of glaciers (daily and seasonal change in discharge; high sed load).

Geologic Features Related to Streams

Terraces -- a remnant of an abandoned flood plain. 
Eroded into bedrock or constructed with alluvial (laid down by rivers) deposits.
Bedrock only means base level has dropped sporadically over Quaternary time.
Alluvial deposit terraces means that valley must have filled up with alluvium at one time in
past.
All related to base level -- visible terraces means base level has dropped in Q.
Useful tool for reading Quaternary history, -- commonly for rise and fall of sea level.

Alluvial Fans -- deposits formed where streams leave mountains and enter broad valleys.
Gradient drops, so does velocity, and so does sediment -- abruptly.
Abrupt drop of sed causes bed to shift one way and another often,
Thus, -- fan shape.
Size of fan related to the size of the drainage basin, i.e., the area from which water flows
into the stream that feeds the fan.
Most Common in arid regions (wet mountains, dry valleys)
Thus a good paleoclimate indicator.

Deltas -- Similar to alluvial fans, but in the water.
	Marshy areas.

Stream Pattern as Related to Structure of Regional Geology

Consequent -- Course of stream determined by slope of land surface.
Subsequent -- Determined by geologic structure,
Antecedent -- Retains its course as structures are raised in its path; ex: appalachians
Superposed -- River that maintains its course after cutting down through upper beds that
determined its pattern, ex: Colorado plateau.

-relate to Earth History

Floodplains

-Areas where fine-grained sediments are deposited -- good for agriculture.
-Natural levees can form where river initially loses velocity as it overflows banks.

Floods

10-year flood -- read 1 in 10 probablility of happenning in any year.
No meteorological reason two 50- or 100-year floods cannot happen in successive years. 
Introduce concept of recurrance interval.
Flooding is a bad example of Òrecurrance intervalÓ
Wait Ôtill EQs to find out why.

 Slides

Groundwater

Incredibally impt source of drinking and irrigation water.

Of all water on planet...

Oceans			97.3%
Glaciers and polar ice	  2.1%
Groundwater		 0.6%
Lakes and Rivers	 0.01%
Atmosphere		0.001%

By far most freshwater (excluding polar ice, which we canÕt exploit) is in the ground

part of H2O cycle.
precipitation
infiltration -- soaking into ground
transpiration -- release by plants
runoff -- over the surface

Where is the groundwater?
-The rocks or sediments that hold the water are called aquifers

In order to hold water, a rock or sediment must be porous
In order for that water to flow, the rock or sediment must by permeable
[spend time explaining the difference]
We will use the coefficient of permeability, K, as a measure of permeability; wait till later

Draw picture
be sure to include

Unsaturated Zone
Saturated Zone
Water Table
Hydrologic Gradient = slope of the water table; V is propotional to hydro grad
Recharge
Discharge
Unconfined Aquifer

This is is an Unconfined Aquifer its characteristics include,
-water table is a muted expression of topography
-recharge is local
-upper surface equals the water table
-thickness varies with seasons
-easily contaminated

draw new picture with confined aquifer
be sure to include

confining layer (=aquiclude)
potentiometric surface (called Òartesian pressure surfaceÓ in book!!??)
artesian well or spring
perched water table

DarcyÕs Law Stuff

Concerned with groundwater flow

Remember velocity propotional to hydrologic gradient (h1-h2)/length
Darcy suggested that the propotionalility constant was related to permeability.

V=K(h1-h2)/length

where K is the coefficient of permeability a measure of the ease with which water flows through
rocks. -- Units of l/t

ig rx 		10-10 cm/s
fractured ig rx 	10-3 cm/s
sandstone	10-6 cm/s

Remember Q= V X A

therefore, 

Q = AK(h1-h2)/length

This is known as DarcyÕs Law
If we take A as a constant for any given situation, we can calculate any of the variables if we know
the other two. We can often measure K and the hydrologic gradient.

Do example. 
Make clear that in many situations (like in the homework) A is a constant 1 cm2. 

Example

Triangle 3 km high and 9 km long.
hypotenuse 9.5 km from SQRT(a2+b2)

if K=1X10-3 cm/s, A=1 cm2 because we are looking at one particular cross section.
what is Q??

Q = K(dh/dl)A

Q = (1X10-3 cm/s)(3km/9km)(1 cm2 ) = 0.33X10-3 cm3/s = 3.3X10-4 cm3/s

(3.3X10-4 cm3/s)(60s/min)(60min/hour) = 1.2 cm3/hour

How much time for water to move from one end to the other?

Q=VA
V=Q/A=3.3X10-4 cm/s

V=L/t; t=L/V

now use hypotenuse (note difference between ÒlÓ and ÒLÓ)

t = L/V = 9.5km/(3.3X10-4 cm/s) = 9.5X105cm/(3.3X10-4 cm/s) = 2.9X109 seconds

(2.9X109 seconds)(year/piX107seconds) = 92 years

(pi = 3.14; there are actually about 3.15 seconds in a year)

If time permits --Karst topography-- erosion by groundwater

book has a very nice section on groundwater. fig 14.15

acidic groundwater dissolving CaCO3 -- limestone or marble

Normal rainwater becomes acidic as it percolates through the soil and picks up carbon dioxide 
(CO2) produced by organisms in the soil.

The CO2 dissolves in the water and forms carbonic acid

CO2 + H2O = H2CO3 (carbonic acid)

which disassociates into a hydrogen cation and bicarbonate anion to form carbonic acid

H2CO3 = H+ + HCO3- (bicarbonate anion)

The hydrogen of the carbonic acid then attacks the calcium carbonate of
which the marble is composed:

CaCO3 (calcium carbonate) + 2H+ = Ca++ + 2HCO3-

All this permits Erosion by groundwater

Various karst features

Erosion along joints or fractures fig 14.14
evolution from sinkholes to Chinas mountinous terrain.
stalagtites and stalagmites

Saltwater intrusion

fresh water floats on salt water
if water table goes down, amount and weight of fresh water go down
salt water moves inland fig 14.24
[draw picture]