D--048966 - CALFED Bay-Delta Program - State of California

888
VOLUME II, CHAPTER 30
16
14
12
Snow ~nd Glacier Melt Only: Maclure Croek,~
3500 m, 0.9 .sq. km.,1967
contamination of the enclosed water and provides greater
operational flexibility. However, a finding by the staff of the
State Water Resources Control Board held that improvements
to effectively constitute a new diversion that the ditch owner
~ does not hold rights to. Leakage currently provides water for
6
improvement of wildlife habitat and other uses. A decision is
4 pending on this case involving the Crawford Ditch of the E1
2
0 ~~ ,
Dorado Irrigation District (Borcalli and Associates 1993). Occasional
failures of these (and more modern) canals result in
serious erosion or debris flows. Four flume failures occurred
4000
3~
Mostly Seowmelt a,~,~uturn,~ Rain: Merce,~ River A
at Happy Isles. 1200 m. 460 sq. km..1967 II
along the Tule River just between 1962 and 1965. In 1992, the
Cleveland fire in the South Fork of the American River Basin
destroyed a large portion of the El Dorado Canal, which sup-
2so0
2000
I I plies about a third of the total water to the E1 Dorado Irrigalion
District. In November 1994, a fallen oak blocked the ~ger
150o
10oo 1 J/I 1 , II
Creek Canal, diverting water to the slope below and eroding
hundreds of cubic meters of soil.
o Environmental Consequences
3500
3000
2~oo
2ooo
Rainfall and Snowmelt: South Fork Merced River.
1200 m, 250 sq. km., 1967
The construction, existence, and operation of dams and diversions
have a variety of environmental effects. Inundation
of a section of stream is the most basic impact. A river is transformed
into a lake. The continuity of riverine and riparian
habitat is interrupted. To creatures that migrate along such
~soo corridors, this fragmentation has consequences ranging from
~000 altering behavior of individuals to devastating populations.
500 Dams have the potential to alters downstream flows by oro
-- ders of magnitude and, at the extreme, can simply turn off
the water and dry up a channel. Changing the natural transs0o
port of water and sediment fundamentally alters conditions
4~o
~
~o
~00
200
200
lso
too
60
o ~
R~n~, O~y: Ma~e, Creek.
500 rn, 44 sq. kin., 1967
|
~J~ for aquatic and riparian species. Changing stream flow also
has dramatic impacts on chemical and thermal attributes of
downstream water. The abundance of impoundments in the
Sierra Nevada is impressive when one realizes that virtually
all flat water at the lower elevations of the west slope is man-
made. The terrain is simply not conducive to the formation
of natural lakes below about 1,500 m (5,000 ft).
An obvious impact of water management is alteration of
the natural hydrograph (temporal pattern of stream flow). For
example, during the snowmelt season, the daily cycle of run-
14000
~o00
t0000
~o00
Combined Sources: Merced River Near Briceburg. 360 m, 1,800 sq. km., 1967
~
]/ ,.
off and recession may be transformed into a constant flow. A
series of hydrographs from streams in and near Yosemite
National Park illustrate natural stream flow patterns generated
under various watershed and climatic conditions at dill000
ferent elevations (figure 30.5). Dams are built to change those
~00o
~oo
~~v~ ~~,~a~
patterns (figure 30.6). Diversions not associated with large
impoundments change the volume without much effect on
o ~
O N D J F M A M J J A S
timing (figure 30.7). Large projects usually alter both volume
and timing (figure 30.8).
FIGURE 30.5
Watersheds with different elevation ranges and sources of
High Flows
runoff have different patterns of stream flow over a water
year (October to September).
The most obvious alterations in formerly natural hydrographs
are decreases in peak flows. The size of an impoundment and
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