Record of Holocene Lake Levels in Bear Lake Utah/Idaho

Sediment grain-size data and other observations indicate that Holocene Bear Lake experienced large lake-level changes, and that on average the lake was about 20 m below the modern full-lake level. Bear Lake, in northeastern Utah and southern Idaho, lies in an active half graben at an elevation of about 1800 m, covers an area of 280 km², and is 63 m deep. During most of the Holocene the Bear River bypassed Bear Lake, but river water was diverted into the lake circa 1912. In 1909 lake elevation was about 1805.5 m, and modern full-lake level is close to this datum. Holocene sediments are comprised of about 70% endogenic carbonate, 25% detrital siliciclastic material (largely quartz), and small amounts of other material. Except for one calcite interval, the endogenic carbonate is aragonite. After removal of carbonate, biogenic silica, and organic material, grain-size data were acquired using a laser particle-size analyzer. Analyses of 30 samples of the uppermost 1.5 cm of sediment, taken along 4 depth profiles, demonstrate progressive decrease in grain size with water depth. Median grain sizes range from >200 μ m in 1 m of water to <6 μ m in depths greater than 30 m. Differences among the depth/grain-size relations defined by these profiles are probably related mostly to differences in lake-bottom morphology and in position with respect to fetch and prevailing winds. Paleo-depths were calculated for Holocene sediments from two ¹⁴C dated cores using the depth/grain-size relation defined by the profile closest to the coring sites. These modeled depths indicate that the lake was commonly about 20 m below modern levels and that there were three periods when lake levels were significantly lower. Two of these low-water periods occurred in the early Holocene (both occurring between 10.2 and 9.2 cal. ka) and one in the late Holocene (ending at about 2.8 cal. ka). A bench and scarp on the west side of the lake, observed in reflection seismic profiles, are consistent with a prolonged period with water 20 m to 25 m below modern levels. Low-water levels are further corroborated by shell gravels in one of the cores that coincide with the coarsest grained siliciclastic material and by a horizon containing roots dated at about 9.9 to 10.1 cal. ka from a third core taken from a depth of about 40 m. At face value, the grain-size data suggest that only once during the Holocene did water depth reach or exceed the modern level, although thin intervals of deep-water deposition may have been missed. Shortly after 9.2 cal. ka, water depth increased to a maximum greater than the modern full level. The period of deep water lasted about 1000 years and coincides with an interval in which aragonite is replaced by calcite, indicating that the water was not only deeper but also fresher. Isotopic data (e.g., ⁸⁷Sr/⁸⁶Sr) indicate that the Bear River was flowing into the lake during this period. The high-water level coincides with formation of an elevated shoreline 8 m above modern lake level (dated by Laabs and Kaufman at 9.2 cal. ka, GSA Bull., 2003). During much of the Holocene Bear Lake was topographically closed and water levels probably reflected climatic conditions. Because the lake had attained its modern level prior to diversion of Bear River water, effective precipitation in the late 19^th and early 20^th centuries must have been greater than for the average Holocene climate. The Holocene high stand may also be climatically controlled, but the possible roles of tectonics and surficial processes in diverting the river into the lake cannot be discounted.