Red Rock Pass Research Articles

Transgressive-phase barriers as indicators of basin-wide lake-level changes in late Pleistocene Lake Bonneville, Utah, USA

Lake Bonneville was the largest of the late Pleistocene pluvial lakes that once filled part of the Great Basin of the interior western United States. As the lake reached its highest level at the Bonneville shoreline and overflowed, it eroded through landslide deposits and alluvium near Red Rock Pass, Idaho, and quickly dropped over 100 m to the Provo shoreline. This unique flooding history and resulting rapid lake-level fall allows us to assume that all shorelines with elevations between the Bonneville and Provo levels (the intermediate shorelines) formed during the lake's transgressive phase. Various types of depositional features characterize the transgressive-phase shorelines, including barrier spits and bayhead barriers. This study uses 5 m auto-correlated digital elevation models (DEMs), airborne light detection and ranging (lidar), and ground penetrating radar (GPR) surveys to measure the internal and external structure of intermediate barriers. Analysis of 102 barriers at 26 different locations further documents the lake's constructional shorelines while employing a statistical approach to show patterns in basin-wide lake levels. These methods identify only one confidently correlatable intermediate shoreline at an adjusted elevation of 1530 m (±4 m), providing evidence of a still-water level during the late-transgressive phase of Lake Bonneville. The variability of elevations along barrier crests indicates that the formation of depositional features is highly dependent on local wave climate, in addition to basin-scale climatic controls.

Computational Fluid Dynamics simulations of the Late Pleistocene Lake Bonneville Flood

At approximately 18.0 ka, pluvial Lake Bonneville reached its maximum level. At its northeastern extent it was impounded by alluvium of the Marsh Creek Fan, which breached at some point north of Red Rock Pass (Idaho), leading to one of the largest floods on Earth. About 5320 km3 of water was discharged into the Snake River drainage and ultimately into the Columbia River. We use a 0D model and a 2D non-linear depth-averaged hydrodynamic model to aid understanding of outflow dynamics, specifically evaluating controls on the amount of water exiting the Lake Bonneville basin exerted by the Red Rock Pass outlet lithology and geometry as well as those imposed by the internal lake geometry of the Bonneville basin. These models are based on field evidence of prominent lake levels, hypsometry and terrain elevations corrected for post-flood isostatic deformation of the lake basin, as well as reconstructions of the topography at the outlet for both the initial and final stages of the flood. Internal flow dynamics in the northern Lake Bonneville basin during the flood were affected by the narrow passages separating the Cache Valley from the main body of Lake Bonneville. This constriction imposed a water-level drop of up to 2.7 m at the time of peak-flow conditions and likely reduced the peak discharge at the lake outlet by about 6%. The modeled peak outlet flow is 0.85·106 m3 s−1. Energy balance calculations give an estimate for the erodibility coefficient for the alluvial Marsh Creek divide of ∼0.005 m y−1 Pa−1.5, at least two orders of magnitude greater than for the underlying bedrock at the outlet. Computing quasi steady-state water flows, water elevations, water currents and shear stresses as a function of the water-level drop in the lake and for the sequential stages of erosion in the outlet gives estimates of the incision rates and an estimate of the outflow hydrograph during the Bonneville Flood: About 18 days would have been required for the outflow to grow from 10% to 100% of its peak value. At the time of peak flow, about 10% of the lake volume would have already exited; eroding about 1 km3 of alluvium from the outlet, and the lake level would have dropped by about 10.6 m.

New insights into the outlet conditions of late Pleistocene Lake Bonneville, southeastern Idaho, USA

Geologic, geomorphic, and geophysical analyses of landforms, sediments, and structures in northern Cache Valley, USA, document a revised history of flooding and recession of Lake Bonneville, the world9s premier pluvial lake. Crosscutting relationships suggest that the Riverdale fault produced a surface-rupturing earthquake ∼25 km southeast of Zenda shortly before, during, or after the Bonneville flood, as well as possible younger surface ruptures. Thus fluctuating stresses and pore pressure induced by changing lake levels may have triggered a large earthquake that, in turn, triggered the Bonneville flood. The flood scoured ∼25 km of Cache and Marsh Valleys and activated landslides during its ∼100 m incision to a new outlet near Swan Lake, Idaho. One to two thousand years of steady outflow produced the main ∼4775 ± 10 ft (1455 ± 3 m) Provo shoreline, ∼10 m above the commonly accepted altitude. Later Lake Bonneville oscillated below the main Provo shoreline, incised the Swan Lake outlet, rose and paused briefly at a new lower Provo sill (4745 ± 10 ft [1446 ± 3 m], P9?) 23 km south of Zenda, before reverting to a closed-basin condition. Correlation to the Blue Lake chronology of Benson et al. suggests that aridity during the Heinrich 1 event activated the lower Provo sill ∼15.9 ka. An abandoned, meandering riverbed, north of the lower Provo sill, records a large northward flowing river. The Great Basin9s modern divide at Red Rock Pass formed in the Holocene when a small alluvial fan filled the dry bed of this paleoriver.

Paleodischarge of the late Pleistocene Bonneville Flood, Snake River, Idaho, computed from new evidence

The Bonneville Flood resulted from catastrophic outflow from Pleistocene Lake Bonneville about 15,000 yr ago, when the lake overtopped its rim at Red Rock Pass in southeastern Idaho and discharged a vast volume of water down the Snake River. This paper provides revised estimates of the paleodischarge, volume, and duration of the Bonneville Flood, based on new evidence of its height and on current understanding of the amount of lowering of Lake Bonneville. Evidence for the revised height of the flood is derived from the altitude of erosional features and flood deposits at the head of a constricted reach of the Snake River Canyon at the mouth of Sinker Creek and from the altitudes of flood deposits at several places about 53 km upstream. Using the step-backwater method, we estimate that peak discharge for the Bonneville Flood through the constricted reach was from 793,000 to 1,020,000 m 3 /s and most likely was 935,000 m 3 /s. This discharge is 2.2 times the discharge previously reported and is the second largest flood known to have occurred in the world. At this rate of discharge, the shear stress for the flood would have been 2,500 N/m 2 , and the unit stream power would have been 75,000 N/m/s, as compared with values of 6 to 10 N/m 2 and 12 N/m/s for recent floods on the Mississippi and the Amazon. Other recent studies of the history of Lake Bonneville show that the volume of water released was 4,700 km 3 , or about 3 times greater than the volume previously inferred. Although this volume indicates a flood duration of 8 weeks at constant peak discharge, an accurate estimate of the duration would require dam-break modeling at Red Rock Pass. From a dam-break model, flood hydrographs at Red Rock Pass and the hydraulics of the flood wave along the Snake River could be computed.

Radiocarbon Chronology of Lake Lahontan and Lake Bonneville II, Great Basin

The climate chronology proposed by Broecker and Orr (1958) for the two major Great Basin pluvial lakes has been confirmed and expanded by coupling 80 new radiocarbon measurements with uranium-series isotope measurements. The validity of the ages is evaluated by comparing different materials, chemical fractions of the same material, radiocarbon ages with Th 230 -U 234 ages, and chronology based on geomorphic features with that based on stratigraphic units. The lake-level chronology has been extended to the postpluvial period ( 30,000 years ago) by Th 230 -U 234 measurements. The results suggest that both lakes were relatively high four times during the late Wisconsin: 17,000 years ago; 14,500 years ago; 12,000 years ago; and 9500 years ago. The downcutting of Red Rock Pass probably occurred during the high-water period about 12,000 years ago. An attempt is made to correlate this chronology with those worked out for the pluviation of the Searles Lake basin, the pollen sequence of Northern Europe, and the glacial oscillations in the North American mid-continent.

EVIDENCE OF LAKE BONNEVILLE FLOOD ALONG SNAKE RIVER BELOW KING HILL, IDAHO

A catastrophic flood, probably exceeding 10 million cfs, was released down the Portneuf River at Red Rock Pass, Idaho, when ancient Lake Bonneville broke its natural dam in Wisconsin time. The flood entered the Snake River and overflowed banks at bends and constrictions, scoured deep channels (many of which are now abandoned), and left extensive high bars of sand, gravel, and boulders. The flood was at least 410 feet deep at Brownlee damsite near Homestead, Oregon.