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OCR for page 1
Executive Summar~r
The Mono Basin National Forest Scenic Area lies in
eastern California about 300 mi north of Los Angeles and
190 mi east of San Francisco. The basin is walled in by
the eastern escarpment of the Sierra Nevada to the west
and by Great Basin ranges to the north, south, and east.
Consequently, no water naturally flows out of the basin.
The only loss of water occurs through evaporation and,
since 1941, diversions of fresh water by the city of Los
Angeles.
The Mono Basin
id
s the hydrologic drainage basin for
Mono Lake. As a result of millennia of evaporation from
its surface, the lake (at 500~000 Years one of the oldest in
~ ,
North America) has gradually increased in salinity, and it
is now about 2.5 times as saline as the Pacific Ocean. The
salinity--and the particular chemistry of the dissolved
ions--has a large effect on the biota of the lake and the
ecology of the basin.
The unusual chemistry of the lake is
also responsible for one of its distinctive scenic
attractions, the tufa towers.
Mono Lake has a simple but productive ecosystem.
Benthic and planktonic algae provide the major base of the
food chain in the lake. The primary consumers are aquatic
arthropods, mainly the pelagic brine shrimp (Artemia mon-
ica) and the benthic brine fly (Ephydra hians), which are
present in immense numbers. Both brine flies and brine
shrimp depend on algae for their food. In addition, the
brine fly requires shallow habitats for feeding, and shallow,
hard surfaces for reproduction.
1
OCR for page 2
2
The Mono Basin Ecosystem
The great productivity of these aquatic invertebrates
permits hundreds of thousands of birds to use Mono Lake.
In addition, no fish or other aquatic predators live in the
lake to compete with the birds for the abundant food
supply. The most abundant birds on the lake (with esti-
mates of maximal population numbers using the lake annu-
ally) are the eared grebe, Podticeps nigricollis (900,000~;
Wilson's phalarope, Phalaropus tricolor ( 125,000~; the red-
necked phalarope, Phalarop?cs lobatus (54,000~; and the
California gull, Larus californicus (50,000~. The populations
represent one-quarter to one-third of the North American
population of eared grebes and 15 to 25 percent of the
North American population of California gulls. Eared
grebes and red-necked phalaropes use the lake as a stop-
over during migration; Wilson's phalaropes use it as a
major staging area before undertaking a long, possibly in-
tercontinental, flight; and California gulls nest on the is-
lands in the lake.
The Los Angeles Department of Water and Power
(LADWP) has been diverting fresh water from streams that
feed Mono Lake since 1941. As a result, the lake level has
dropped about 40 ft to approximately 6380 ft above sea
level in 1986. Because of the potential ecological effects
of water diversion, there has been much concern over the
future of the lake. In 1984, the Congress passed legisla-
tion designating the area as a National Forest Scenic Area,
the only such area in the country. The same legislation
mandated this study by the National Research Council to
review the available scientific information and to assess
the potential effects of changing lake levels on the ecosys-
tem of the Mono Basin.
Various resources of the Mono Basin ecosystem--aquatic
biology, bird populations, and shoreline and upland
environments--are here assessed (see figure on the follow-
ing page). The scientific background for these assessments
is presented in chapters 2 through 5 of the report, and the
effects of changes in lake level are discussed in detail in
chapter 6.
Because the various resources would be affected differ-
ently by different lake levels, and because the effects
occur gradually over a range of levels rather than at one
specific level, the consequences of changing lake level are
OCR for page 3
Executive Summary
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6380
6370
6360
6350
6340
6330
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resource maintained
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resource eliminated
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185
assessed for a series of lake levels. Lake levels from the
elevations of 6430 to 6330 ft above sea level, in intervals
of 10 ft. are considered. The upper level of 6430 ft was
chosen because it is close to the historic high stand of
6428 ft reached in 1919. This level could conceivably be
reached again after a series of wet years if no water were
exported from the basin. The lower level of 6330 ft was
it is the approximate stabilization level
(i.e., inflows of water would equal loss through evapora-
tion), assuming exports of 100,000 acre-ft/yr of water from
chosen Because
OCR for page 4
4
The Mono Basin Ecosystem
the basin and climatic conditions similar to those of the
past 40 years. Most of the ecological consequences of
water levels below 6330 ft would presumably not be sig-
nificantly different from those of a water level of 6330 ft.
Because there are no natural outlets from Mono Lake,
ions dissolved in the lake would become diluted i ~ En_
_ _ _, · · .
. ~ ~ . . . . ~
Unity would be reduced, it- water level rose and increased
the volume of water in the lake. Salinity would increase if
lake level fell. Approximate salinities for lake levels be-
tween 6330 and 6430 ft are given in Table 6.2. The salin-
ity of lake water critically affects the ability of aquatic
organisms to thrive, and therefore salinity is a crucial fac-
tor in the lake's ecosystem.
The depth and salinity of Mono Lake encourage stratifi-
cation of its waters, which in turn affects chemical and
biotic processes. The wet winters of 1983 through 1986
have resulted in large influxes of fresh water, which has
only partially mixed into the heavier saline lake water.
The result has been meromixis, an incomplete mixing of the
lake's waters. Meromixis could have profound effects on
the chemistry and the biology of the lake by trapping nu-
trients in the bottom layer and could be expected to inten-
sify if inputs of fresh water are large in relation to the
volume of saline water. If salinity increased above approx-
imately 125 g/1 of total dissolved solids (TDS), minerals
that contain sodium would begin to precipitate and a per-
sistent stratified layer would be likely to occur near the
bottom of the lake. The precise effects of meromixis and
precipitation of minerals on the salinity cannot be quan-
tified with the current understanding of the geochemistry
of Mono Lake. Meromixis, with a less saline surface layer,
could to some extent alleviate the effects of increasing
salinity for the lake's biota.
The algae in Mono Lake are fairly resistant to increased
salinity, although their productivity is likely to decrease
gradually at salinities above about 100 g/1 and decrease
more rapidly above 150 g/1 (corresponding to lake levels of
approximately 6370 ft and 6350 ft above sea level). Brine
shrimp are expected to gradually decrease in abundance if
salinity exceeds 120 g/1 (corresponding to a lake level of
approximately 6360 ft) because of bioenergetic demands
placed on larval growth and development and reductions in
OCR for page 5
Executive Summary
primary productivity. The effects on brine shrimp popula-
tions would be severe if salinity reached 150 g/1 (lake level
of approximately 6350 ft). Reduction in brine fly popula-
tions would be large at salinities greater than 130 g/1
(lake level of approximately 6356 ft).
Changes in lake level would also affect the habitat
available for brine flies to reproduce and feed. If the lake
level declined to approximately 6370 ft. the acreage of
shallow, hard substrate would be reduced from current val-
ues by about 40 percent, leaving considerably less habitat
area. The brine fly population would consequently be
affected, although it is not possible to predict the precise
impact.
Although increased salinity does not appear to physio-
logically affect birds that use the lake, a decreased food
supply would certainly limit their numbers.
The critical
food resources for aquatic birds using Mono Lake are brine
shrimp and brine flies. If the lake fell to levels at which
the birds' food sources were adversely affected, the bird
populations would be reduced. The decrease in availability
of brine shrimp for food would begin to affect those birds
relying on them--eared grebes and California gulls--at a
salinity of 120 g/1 (lake level of 6360 ft). The impacts
would be acute at salinities above 150 g/1 (6350 ft). For
those birds relying on brine flies--the phalaropes--impacts
would begin at a lake level of 6370 ft and would become
acute at levels below 6360 ft.
Lower lake levels would also reduce the surface area of
islands that are free from predators and hence available to
California gulls for nesting sites. At a lake level of ap-
proximately 6350 ft. virtually all islands would be connect-
ed to the shore, and those gulls remaining despite the loss
of lake-supported food would not be able to nest at the
lake.
Changes in lake level would also influence the shoreline
environment, notably the vegetation, snowy plover habitat,
tufa formations, and air quality. In general, changes in
shoreline vegetation would be controlled by changes in the
availability of fresh water and inundation of suitable habi-
tat as lake level fell or rose. A rise above the present
level would inundate shoreline vegetation. A drop in lake
level would expose additional barren areas of playa, and
OCR for page 6
6
The Mono Basin Ecosystem
vegetation would be established only where springs and
seeps provide fresh water. If lake level fell below the
current level, streams would incise, reducing or eliminating
established vegetation and denuding their banks.
The snowy plover (Charadrius alexandrius), the only
shorebird that would be affected by changes in lake level,
numbers about 350 birds at Mono Lake, about 11 percent of
the California population. The plovers nest primarily on
the exposed playa and pumice dunes on the lake's eastern
shore and have probably benefited from recent drops in
lake level because additional areas of playa have become
available for nesting. If lake level rose, the playa area
would be reduced. However, even complete inundation
would probably not destroy all their nesting, because there
is nesting area available above the playa. Lower lake lev-
els would expand the availability of habitat; a lake level of
approximately 6360 ft would probably maximize the nesting
population.
A drop in lake level would expose currently submerged
tufa towers and permit public access to towers that are
now offshore. There would be the potential for increased
visibility of the tufa for visitors; there would also be the
potential for increased vandalism. A rise in lake level
would reduce access to tufa towers. Wave action could
damage some of the tufa towers and would start to destroy
some of the delicate sand tufas if lake level rose above
6390 ft.
Falling lake levels would expose more of the lake bed
and would increase the supply of alkaline dust to form dust
storms during high winds, thus increasing the frequency
and severity of dust storms. Conversely, a rise in lake
level would decrease the frequency and severity of dust
storms.
Most features of the upland environment of the Mono
Basin would not be affected by changes in lake level.
However, riparian (streamside) plants and animals, although
not directly affected by changes in lake level, would be
affected by changes in streamflows associated with changes
in lake level. Minimal flows of 19 cubic feet per second
(cfs) and 10 cfs are currently being maintained by court
order in lower Rush Creek and lower Lee Vining Creek,
respectively. The two hydrologic models of the basin
OCR for page 7
Executive Summary
7
predict different lake levels, 6360 and 6330 ft. that would
be maintained by these flows. These flows should generally
adequate to maintain riparian stands of vegetation
equivalent to those present in 1941, before diversions
began. On the lower part of the alluvial fan, however,
downcutting has destroyed the riparian community where
recruitment might otherwise have occurred. These flows
are also adequate to maintain breeding populations of
brown trout. Thus, all flows necessary to maintain the
lake level above approximately 6360 ft. regardless of which
model is used, should maintain riparian vegetation and fish
populations in and alongside ~
~ . ~ . ~ ~ .
be adequate to maintain rinarian
lower Rush and Lee Vining
creeks. Periodic heavy releases in spring are necessary to
enhance the recruitment of riparian vegetation.
Responses of various resources to changes in lake level
will, for the most part, occur gradually over a range of
levels. (These consequences of changes in lake level are
summarized in Table 6. 13, and the range of levels over
which they are predicted to occur is summarized, with
three salinities added for reference, in Figure 6.3, which is
reproduced on page 3 of this executive summary.) Deci-
sions about optimal lake levels to protect the ecosystem
will require decisions about which resources or combina-
tions of resources are most important. Undoubtedly, trade-
offs between preservation of different resources will have
to be made.
Climatic fluctuations and long-term changes will produce
fluctuations in lake levels. Because the predictions in this
report are based on data and understanding that are in-
complete or uncertain in some cases, the committee cannot
pinpoint precise lake levels at which particular effects will
occur. For all these reasons, if a maintenance lake level
is selected, the committee strongly recommends that a buf-
fer be built into that level to protect against such uncertainties.
Representative terms from entire chapter:
lake levels
