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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
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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
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Executive Summary 6420 6410 6400 Ad 6390 o U] by 6380 6370 6360 6350 6340 6330 3 resource maintained --—resource silghtly affected .... resource severely affected resource eliminated _ j : 1 1 1 - 1 · 1 ~ · · : ~ _ : : 1 · 1 : 1 1 l C. . . . .. Q ~ ~ at s ~ In , ~ m ._ m . In vat A) = ~ 3 a) a) a) Q Q O O _ _ CL _ a) ~ y O ~ a) l · I · ! . . · C\5 C' ~ . ~ . O Or U) l 1 , l 1 1 l al : a, 0 0 .m ~ . ~ — ·— os CO Cal a) a, Its O ~ Q ~ al ._ a) an ~ lo: ~ In - 89 As 121 185 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
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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
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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
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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
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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: