Distribution of H₂0 on Earth
-1.36 billion km³ H₂0 near surface
Volume
Oceans 97.3%
Glacier & Polar Ice 2.1-3.5%
Underground Aquifers 0.6%
Lakes & Rivers 0.0160%
Atmosphere 0.001%
Biosphere 0.00004%

Streamflow
Hydrologic Cycle: describes quantity and distribution of H₂0 on Earth's surface.
channelized flow of any size
Stream- dominant agent of landscape alteration = 40% of rainfall "runs off" (=60% infiltrates or soaks into the ground)
Discharge = (avg. current velocity *cross-sect. area of stream)

A stream's discharge, velocity and depth all increase downstream
Laminar vs turbulant flow
Stream cross-section and longitudinal profile
local (temporary) Base level
- ultimate base level= sea level

Graded stream is in "equilibrium"
"lifting"
Stream erosion
abrasion
dissolution (limited)

In the U.S. the erosion rate is 461 tons/yr per square mile.
Sediment Transport
-suspended load: clay and silt
-bed load: sand, pebbles and boulders
-dissolved load (average is about 200 ppm)

Stream Deposits are made of alluvium
-channel bars
-channel deposits
-point bars
-braided patterns
-deltas
-alluvial fans
-fan deltas
-depositional flood plains
-natural levees
-meander scars

Meanders
-oxbow lakes
-artificial cutoffs

Stages of stream development
youthful
mature
old age

Drainage Networks
-drainage basins
-divide

drainage patterns
-dendritic
-radial
-rectangular
-trellis

Antecedent vs. Superposed streams
water gaps
Headward Erosion and Stream Piracy
wind gaps
Stream rejuvination -> entrenched meanders
Stream Terraces -erosional vs. depositional
-paired = same age
-unpaired = different ages

Flood hazard
-most common and costly hazard in U.S.
-most human activity aggravates flood hazard

Bankful Stage = Bankful discharge

Flood Frequency - statistical prediction
Hydrograph: fluctuation in stream discharge over time -- info. on "normal" stream behavior
Recurrence Interval (RI) = (N+1)/m
Annual Exceedence Probability = 1/R.I.
Flood Frequency Curve
Effect of Urbanization on discharge:
-magnitude of flooding increases
-frenquency of flooding increases
-shorten lag time between rainfall and flood

Adjustments to flooding
1. Control flood - dams, levees, cloud seeding
2. Decrease physical vulnerability - forecasting warning system & evacuation
3. Spread costs - govt. relief (subsidized insurance; in 1981, the annual subsidy for those homes located in floodplains was $231).

20's and 30's - structural solutions
40's and 50's - societal adjustments (insurance)
1968 Natl. Flood Ins. Act negligible participation
1973 Flood Disaster protection Act -"mandatory" -River Flood Mapping

1. Explain how artificial levees contribute to flooding in the area where they are built.
2. Would a river flowing in an old-age valley make a good boundary between two countries? Explain.
3. Explain the interrelationship between stream gradient and base level (temporary and ultimate) and discuss how both would be affected by the construction of a dam across a river. Draw the longitudinal profile of a stream and illustrate its relationship to temporary and ultimate baselevel.
4. During the last ice age, sea level was considerably lower than it is today. How would this have affected the world's rivers? What happened to the rivers as sea level rose after deglaciation?
5. Why does stream velocity generally increase downstream, despite a decrease in stream gradient?
6. What are the principle characteristics of a graded stream?
7. What is meant by a "100-year flood"? Two years ago the city that you live in suffered the most disastrous flood in its 100-year history, and the city council now says that they need not worry about contingency planning for another major flood for the next few years. What is your response to the council?
8. How are meanders and oxbow lakes related? Use a labeled diagram to help you explain how meanders develop and why they migrate.
9. Describe how stream piracy occurs.
10. Both alluvial fans and deltas have drainage patterns in which a channel breaks up into a number of distributaries. Is there a common cause for this behavior? If so, what is it? If not, what are the causes in each case?

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