PALEO-LAKE LEVELS OF BEAR LAKE (UTAH/IDAHO): A RECORD OF HOLOCENE ARIDITY

Bear Lake (UT/ID) lies within an active half graben in the northeastern Great Basin, a short distance west of the Green River drainage, a part of the Upper Colorado River Basin. It is reasonable to expect that variations in water availability at Bear Lake will be similar to variations in the northern part of the Upper Colorado River Basin because the lake lies on the track of winter storms that deliver snow to the mountainous headwaters of the Green and Colorado Rivers. This supposition is borne out by similarity of the historic Bear Lake hydrograph and flows on the Green River. During the most prolonged and severe droughts of the 20^th century lake level dropped ~6 m below the modern full level. Numerous radiocarbon dated sediment cores were used to construct a lake-level record by comparing characteristics of cored sediments (e.g., mineralogy, grain size, and sedimentary structures) to the depth distribution of analogous facies in the modern lake. The sedimentological interpretations are supplemented and strengthened by mapping of elevated shorelines and by seismic-reflection evidence for lower lake levels. The lake-level record exhibits changes of tens of meters on millennial and shorter time scales and indicates that throughout much of the Holocene, lake levels were significantly lower than during the period of European settlement. Features of the lake level record include (1) an early Holocene lowstand characterized by levels largely between 10 and 25 m below the modern full level, (2) a high-stand (~8.8 cal ka to ~7.8 cal ka) with levels at or above the modern full level, (3) a mid-Holocene lowstand with levels similar to those of the early Holocene, and (4) a rise in level at ~4.3 cal ka with fluctuations between the modern full level and >15 m below full (averaging about 8 m below full) for the remainder of the Holocene. The Bear Lake record strongly supports the conclusion of other lake and tree-ring based studies that the 20^th century was anomalously wet in the western U.S. and may in fact be among the wettest intervals of the entire Holocene.