Borax Lake, a thermal lake confined within a biomorphic mound, Pueblo Valley, Oregon, USA

Relations among geothermal system dynamics, evolution of an 8-m thick biomorphic mound, and requirements to protect critical habitat of Gila boraxobius, a fish protected under the Endangered Species Act of 1973, were explored at Borax Lake, a shallow thermal lake in the tectonically active northwestern Basin-and-Range Province. Neutral pH, thermal water at ~ 105 °C discharges within a carrot-shaped vent at a depth of 29 m and rapidly mixes within the vent and lake which is mostly < 1 m deep. The 14 hectare thermal lake lies at the crest of a biomorphic mound composed of diatoms, silt-size opal-A silica particles, silica encrusted and permineralized vascular plant fragments, sinter fragments, and discontinuous carbon-rich plant and charcoal layers. Siliceous sinter lines the vent, part of the lake bed, and forms a discharge apron that caps the mound and gently dips (< 3°) away from the shore. The hydrothermal deposit unconformably overlies lacustrine deposits of Pleistocene Lake Alvord and began to accumulate prior to BC 3775 to 3655, the age of a charcoal layer near the mound crest. Slope degradation occurs primarily from rilling, gullying, and solifluction as the result of seasonal seepage, overland flow and freeze/thaw cycles. Winter seepage on the flanks of the mound ceases as the water table drops (~ 1.5 m) during the summer dry season. Piping from an upward seepage gradient also degrades slopes. Stability analysis indicates that the mound is stable under current static stress conditions, however, high water table and cohesionless diatomaceous silt with shear wave velocities less than 164 m s− 1 and proximity of seismic sources indicates the mound may be susceptible to liquefaction and slope failure during dynamic earthquake loading. Borax Lake is the unique product of the geothermal system and the biological communities it supports. Portions of the mound may have failed in the past during seismic loading, eroded, and been rebuilt by processes similar to those active today.