Surficial deposits in Bear Lake Valley, Utah and Idaho, demonstrate that Bear Lake and its surrounding wetlands expanded and contracted several times during the Quaternary Period. Deposits in 40 outcrops and the geomorphology of the valley were studied to reconstruct the water-level history. Temporal control was provided by amino acid geochronology on 29 samples of aquatic mollusks, calibrated by 14C dating and tephrochronology. Samples of three molluscan genera were heated and analyzed to determine intergeneric differences in aspartic (Asp) and glutamic (Glu) acid racemization. The covariance of Asp and Glu racemization in these genera was used to identify aberrant amino acid analyses. The rate of Glu racemization was calibrated by using a parabolic model that was used to estimate sample ages.
Although the paucity of exposures, uncertainty of the depositional setting, and postdepositional uplift of deposits hamper an accurate reconstruction of water level in Bear Lake Valley, several conclusions can be drawn. During the middle Pleistocene, lacustrine or wetland environments in northern Bear Lake Valley may have extended up to 1830 masl (meters above sea level; ∼6000 ft) twice between ca. 1000–100 ka (Bear Hollow phase). During the late Pleistocene, the shoreline of Bear Lake transgressed at least three times: (1) to 1817 masl (∼5960 ft) at 47–39 cal. (calendar-year-corrected) ka (Jensen Spring phase); (2) to 1814 masl (5950 ft) at ca. 16–15 cal. ka (Raspberry Square phase); and (3) to 1814 masl at ca. 9 cal. ka (Willis Ranch phase).
Bear River currently bypasses Bear Lake, but flowed into the lake during highstands and allowed the water level to rise to the valley threshold at Nounan narrows. Water levels have lowered to 1806 masl (5923 ft) since the middle Pleistocene, likely because of downcutting by Bear River at Nounan narrows. Because downcutting at Nounan narrows controls the maximum attainable water level in Bear Lake Valley, effects of climate change on lake level are limited, and Bear Lake may actually have been smaller than today when Lake Bonneville transgressed to its maximum shoreline during the Last Glacial Maximum.