Abstract Geologic, geomorphic, and geophysical analyses of landforms, sediments, and geologic structures document the complex history of pluvial Lake Bonneville in northern Cache Valley, NE Great Basin, and shows that the outlet of Lake Bonneville shifted ~20 km south after the Bonneville flood. The Riverdale normal fault offsets Bonneville deposits, but not younger Provo deposits ~25 km southeast of Zenda, Idaho. Rapid changes in water level may have induced slip on the Riverdale fault shortly before, during, or after the Bonneville flood. Although other processes may have played a role, seismicity might have been the main cause of the Bonneville flood. The outlet of Lake Bonneville shifted south from Zenda first 11, then another 12 km, during the Provo occupation. The subsequent Holocene establishment of the drainage divide at Red Rock Pass, south of Zenda, resulted from an alluvial fan damming the north-sloping valley. Weak Neogene sediments formed sills for the three overflowing stages of the lake, including the pre-flood highstand. Field trip stops on flood-modified landslide deposits overlook two outflow channels, examine and discuss the conglomerate-bearing sedimentary deposits that formed the dam of Lake Bonne ville, sapping-related landforms, and the Holocene alluvial fan that produced the modern drainage divide at Red Rock Pass. The flood scoured ~25 km of Cache and Marsh Valleys, initiated modest-sized landslides, and cut a channel north of a new sill near Swan Lake. Lake Bonneville dropped ~100 m and stablilized south of this sill at the main, higher ~4775 ± 10 ft (1456 ± 3 m) Provo shoreline. Later Lake Bonneville briefly stabilized at a lower ~4745 ± 10 ft (1447 ± 3 m) Provo sill, near Clifton, Idaho, 12 km farther south. An abandoned meandering riverbed in Round Valley, Idaho, shows major flow of the large Bonneville River northward from the Clifton sill. Field trip stops at both sills and overlooking the meander belt examine some of the field evidence for these shorelines and sills. The Bear River, which enters Cache Valley at the mouth of Oneida Narrows, 17 km ENE of the Clifton sill, was the main source of water in Lake Bonneville. It produced 3 sets of deltas in Cache Valley—a major delta during the Bonneville highstand, a larger composite delta during occupation of two Provo shorelines, and at least one smaller delta during recession from the Provo shoreline. The Bonneville delta and most of the Provo delta of the Bear River were subaqueous in Cache Valley, based on their topsets being lower than the coeval shorelines. The Bonneville delta is deeply dissected by closely spaced gullies that formed immediately after the Bonneville flood. The delta morphologies change sequentially from river-dominated to wave-dominated, then back to river-dominated. These unique shapes and the brief, intense erosion of the Bonneville delta record temporal changes in wave energy, erosion, vegetation, and/or storminess, at the end of the Pleistocene. Stops on a delta near Weston, Idaho, reveal many of the distinguishing features of the much larger deltas of the Bear River in a smaller, more concentrated form. We will see and discuss the ubiquitous gully erosion in Bonneville landforms, the nearly undissected Provo delta, the subaqueous topset of the Provo delta, and the wave-cut and wave-built benches and notches at the upper and lower Provo shorelines.

Major-ion chemistry, strontium isotope ratios (87 Sr/ 86 Sr), stable isotope ratios (δ 18 O, δ 2 H), and tritium were analyzed for water samples from the southern Bear Lake Valley, Utah and Idaho, to characterize the types and distribution of groundwater sources and their relation to Bear Lake’s pre-diversion chemistry. Four ground-water types were identified: (1) Ca-Mg-HCO 3 water with 87 Sr/ 86 Sr values of ~0.71050 and modern tritium concentrations was found in the mountainous carbonate terrain of the Bear River Range. Magnesium (Mg) and bicarbonate (HCO 3) concentrations at Swan Creek Spring are discharge dependent and result from differential carbonate bedrock dissolution within the Bear River Range. (2) Cl-rich groundwater with elevated barium and strontium concentrations and 87 Sr/ 86 Sr values between 0.71021 and 0.71322 was found in the southwestern part of the valley. This groundwater discharges at several small, fault-controlled springs along the margin of the lake and contains solutes derived from the Wasatch Formation. (3) SO 4 -rich groundwater with 87 Sr/ 86 Sr values of ~0.70865, and lacking detectable tritium, discharges from two springs in the northeast quadrant of the study area and along the East Bear Lake fault. (4) Ca-Mg-HCO 3 -SO 4 -Cl water with 87 Sr/ 86 Sr values of ~0.71060 and sub-modern tritium concentrations discharges from several small springs emanating from the Wasatch Formation on the Bear Lake Plateau. The δ 18 O and δ 2 H values from springs and streams discharging in the Bear River Range fall along the Global Meteoric Water Line (GMWL), but are more negative at the southern end of the valley and at lower elevations. The δ 18 O and δ 2 H values from springs discharging on the Bear Lake Plateau plot on an evaporation line slightly below the GMWL. Stable isotope data suggest that precipitation falling in Bear Lake Valley is affected by orographic effects as storms pass over the Bear River Range, and by evaporation prior to recharging the Bear Lake Plateau aquifers. Approximately 99% of the solutes constituting Bear Lake’s pre-diversion chemistry were derived from stream discharge and shallow groundwater sources located within the Bear River Range. Lake-marginal springs exposed during the recent low lake levels and springs and streams draining the Bear Lake Plateau did not contribute significantly to the pre-diversion chemistry of Bear Lake.

Pollen analysis of sediments from core BL96-2 at Bear Lake (42°N, 111°20′W), located on the Utah-Idaho border in America’s western cordillera, provides a record of regional vegetation changes from full glacial to the late Holocene. The reconstructed vegetation records are mostly independent of Bear Lake’s hydrologic state and are therefore useful for identifying times when climate forcing contributed to lake changes. The Bear Lake pollen results indicate that significant changes in the Bear Lake vegetation occurred during the intervals 15,300–13,900, 12,000–10,000, 7500–6700, 6700–5300, 3800–3600, and 2200–1300 cal yr B.P. These intervals coincide with regional shifts in vegetation and climate, documented in pollen, isotope and biogeographic records in the Basin and Range region, suggesting that large-scale climate was the primary forcing factor for these intervals of change. Maximum aridity and warmth is indicated from 12,000 to 7500 cal yr B.P., followed by intervals of generally more mesic and cool conditions, especially after 7500 cal yr B.P.