Hydrologic monitoring at Hot Creek Fish Hatchery is done by the U.S. Geological Survey in cooperation with Mono County. Hot Creek Fish Hatchery, operated by the California Department of Fish and Game, is one of the largest fish hatcheries in the Eastern Sierra.

In the early 1980's, renewed interest in the geothermal energy potential in Long Valley Caldera highlighted the need to balance the benefits of energy development with the established recreational activities of the area. In 1986, the Mono County Board of Supervisors passed a resolution establishing the Long Valley Hydrologic Advisory Committee (LVHAC). A mission of the committee is to implement a monitoring program for tracking changes in hydrologic features that could be impacted by geothermal or water resource developments. Hot Creek Fish Hatchery was identified by LVHAC as one of the key hydrologic sites to be protected.

Hydrologic conditions, including water flows, temperatures, and quality, at Hot Creek Fish Hatchery are uniquely suited for raising trout. Four groups of springs discharge about 25 cfs (cubic feet per second) of water from fractured basalt flows that have been eroded to form a low cliff along the southern side of the hatchery. Water temperatures of the springs range from 10.8 to about 18°C (local nonthermal ground-water temperatures range from 6 to 12°C). Water discharged from the springs is a mixture nonthermal and thermal ground water. Most of the water is cold shallow ground water recently recharged from snowmelt. This water is very low in dissolved solids, similar to water in the upper reaches of Mammoth Creek. The thermal water component makes up 5 percent or less of the total discharge. The thermal water contains higher concentrations of dissolved solids including several elements common to geothermal systems, for example, arsenic, boron, chloride, and lithium. Boron and chloride are conservative elements with respect to water/rock interaction and therefore are well suited for tracing water types and calculating mixing ratios.

Each of the spring groups includes several vents that contribute water to pools or channels within the hatchery. The principal spring groups have been given alphnumeric identifiers by hatchery personnel. Three (AB, CD, and H-2,3)of the four principal spring groups are monitored for continuous discharge and water temperature. Periodic measurements of water quality constituents are also made at these springs.

Variations in discharge, water temperature, water chemistry, and isotopic composition among hatchery spring groups demonstrate that the hatchery vicinity is the discharge area for a complex hydrologic system wherein thermal water mixes in variable proportions with nonthermal water from more than one source area.

Spring group AB has the highest recorded discharge of any of the spring groups and also has greater variability in discharge than springs to the east. AB has a strong seasonal variation in discharge, with the highest flows in June or July and the lowest in December or January. AB also has substantial inter-annual variations in discharge related to variations in snow pack in Mammoth Basin. Spring group CD has the second highest recorded discharge and has a smaller range in discharge than AB. Spring group H-2,3 has the lowest discharge and a smaller range than AB.

The warmest springs (group AB) are located in the western part of the hatchery, water temperatures become progressively cooler in springs to the east. Chemical composition of the water is strongly correlated with temperature. Spring group AB has the highest concentration of dissolved constituents (> 220 mg/L) and the highest temperature (17-18° C); spring group H-2,3 has the lowest concentration of dissolved solids (<140 mg/L) and the lowest temperature (10.8° C). Distribution of water temperatures and chemical compositions indicates that the percentage contribution of thermal water varies among spring groups, with the greatest percentage of thermal water in the westernmost spring group (AB) and the least in the easternmost spring group (H-2,3).

The stable isotopes ratios, ¹⁸O/¹⁶O and ²H/¹H, of water from springs also vary from west to east and provide evidence of differences in ground-water source areas. The heaviest ²H/¹H are in the west, at AB, which has an average dD= -115 and the lightest are in the east (H-2,3) with dD = -120. The isotope ratios are primarily dependent on the location of recharge areas for the nonthermal components of flow. Fractionation of oxygen and hydrogen isotopes in precipitation due to rainout and topographic effects in Long Valley Caldera results in significant differences (>20 per mil dD) between areas in the western part to areas in the eastern part. The isotopically heaviest precipitation falls in the western part of the area. The waters in nonthermal springs and streams in the western part are also isotopically heavier than nonthermal springs in the eastern and southern parts.

FH Springs on Maps

10 meter resolution digital elevation model

• with all monitoring sites on the Hydrologic Studies Homepage (FH sites not labeled)

• with all spring sites on the Springs Homepage

• detailed map of the Hot Creek Fish Hatchery