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

868
VOLUME II, CHAPTER 30 ,_
telemetered snow sensors) in the Sierra Nevada that are used m (8,200 ft) rarely melt much because temperatures are close
for river forecasting. Basinwide means of April I water equiva- to 0oC (32°F). If snow cover extends to low elevations prior
lence for snow courses above 2,500 m (8,200 ft) suggest that to a warm storm, there can be a substantial snowmelt contripeak
snowpack water equivalence for the high Sierra Nevada bution from those areas. In basins that are largely above 2,000
averages 75 to 85 cm (30 to 33 in), decreases from north to m (6,600 ft), the highest peaks also tend to be caused by rainsouth,
and is lower on the east side of the crest than on the on-snow events. For example, in the Merced River in Yosemite
west side (Kattelmann and Berg 1987). Snow courses between National Park, the four highest floods were caused by rain
1,800 m and 2,500 m (5,900 and 8,200 ft) have an average peak on snow and were 1.5 to 1.8 times greater than the maximum
water equivalence of about 60 cm (24 in). snowmelt peak of record in 1983. In the past sixty years, six
large-magnitude floods (peak flows greater than twice the
Flow Variability mean annual flood) have occurred in almost all rivers draining
the snow zone: December 1937, November 1950, Decem-
Flow in Sierra Nevada rivers is highly variable in time, both ber 1955, February 1963, December 1964, and February 1986.
within and between years. Peak flows can be up to five orders
of magnitude greater than minimum flows. Annual vol-
Specific discharges of these largest floods ranged from 0.2 to
4 m3/s/km2 (18 to 360 ft3/s/mi2). The largest flood in Caliumes
can be twenty times greater in very wet years than in fornia history occurred in January 1862. Following hundreds
very dry years. Some smaller streams cease flowing during of millimeters (tens of inches) of antecedent rainfall and snowprolonged
dry periods, fall down to the floor of the Central Valle)4 250 to 400 mm (10
to 16 in) of rain fell in Sacramento (and undoubtedly higher
Floods
amounts in the Sierra Nevada) between January 9 and 12.
High-water marks on the American River near Folsom were
High water levels are an integral feature of Sierra Nevada 3.5 m (11 ft) above those observed in 1907, the third highest
rivers and have a variety of effects on aquatic biota as well as flood measured on the American. In the eastern Sierra
channel morphology (Erman et al. 1988). Peak flows in the Nevada, Owens Lake rose 3 to 4 m (10 to 13 ft) during that
Sierra Nevada result from snowmelt, warm winter storms° winter.
summer and early-autumn convective storms, and outbursts When subtropical air masses move into the Sierra Nevada
from storage (Kattelmann 1990). In rivers with headwaters in in summer and early autumn, sufficient moisture is available
the snowpack zone, snowmelt floods occur each spring as to generate extreme rainfall. Intense convective storms occurperiods
of sustained high flow, long duration, and large vol- ring over a period of three or four days can generate local
ume. However, they rarely produce the highest instantaneous flooding. These convective storms can generate the greatest
peaks. The magnitude of a snowmelt flood depends on the floods in some alpine basins that are high enough to avoid
spatial distribution of both snow and energy input to the midwinter rain-on-snow events. For example, the four highsnowpack.
The largest volumes occur when all or almost all est floods of Bear Creek (gauged near Lake Thomas A. Edison)
the basin is contributing high rates of snowmelt runoff. In were generated by summer rainfall. The peak discharge was
basins spanning hundreds of meters of elevation with varied
aspects, such situations are rare. Snow usually disappears
more than twice that of the largest snowmelt flood in this basin
of 136 km2 (52 mi2) with a mean elevation of about 2,850 m
from south-facing slopes and low elevations long before melt (9,300 ft). The greatest recorded floods in several east-side
rates peak on north aspects and high elevations. Large snow- streams occurred in late September 1982 when 150 to 200 mm
melt floods occurred in many river basins of the Sierra Ne- (6 to 8 in) of rain fell in two days.
vada during 1906,1938,1952,1969, and 1983. In all cases, snow In limited areas, the greatest floods occur during a sudden
deposition was more than twice average amounts and per- outburst from storage because of avalanche-induced displacesisted
into April and May even at low elevations. In basins of ment of lake water or failure of a natural or man-made dam
less than 100 km2 (39 mi2) within the snow zone, maximumor aqueduct. Peak flows generated by such mechanisms can
specific discharges during snowmelt have ranged from 0.2 to
0.8 m
be several times greater than those produced by meteorolo~-
3 per second per km2 (18 to 73 ft3/s/mi2) on the western
slope and 0.1 to 0.2 m3/s/km
cal events.
2 (9 to 18 ft3/s/mi2) on the eastern
slope.
Midwinter rainfall on snow cover has produced all the high-
Droughts
est flows in major Sierra Nevada rivers during this century At the other extreme, stream flow in Sierra Nevada rivers can
(Kattelmann et al. 1991). The most important factor in rain- become quite low during intense and/or extended droughts.
on-snow floods is probably their large contributing area. For example, during 1977 when average snow water equiva-
During these warm storms, most of a basin receives rain in- lence in early April was only 25% of the long-term mean,
stead of snow, generating short-term runoff from a much stream flow as a proportion of average annual flow ranged
larger proportion of the basin than during cold storms. How- from 0.08 to 0.26. Basins with most of their area at low elevaever,
even during the warmest storms, snowpacks above 2,~’K~ tions generally had the lowest proportions of average voi-
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