Kenneth D. Adams

Shoreline processes and the age of the Lake Lahontan highstand in the Jessup embayment, Nevada

The well-developed shoreline record of pluvial Lake Lahontan in the Jessup embayment, Nevada, is used to refine the history of late Pleistocene lake-level fluctuations and to assess controls on shoreline development and distribution. Controls on the strength and type of shorelines developed include local slope, the amount and characteristics of sediment available for transport, the availability of accommodation space, and length of time the lake level resides at a particular shoreline elevation. At the Sehoo highstand and during the early part of the regression, strong storm winds and waves from the south-southeast set up a clockwise net shore-drift pattern near the head of the embayment. Although significant differences in local slope, geometry of the shoreline, and wave energy existed in the embayment, crestal heights of constructional shoreline features formed at the highstand vary <2.6 m in elevation and hence provide a relatively precise marker of the highstand elevation. Radiocarbon dating of a camel bone preserved in high shoreline deposits indicates that the lake reached its highest elevation of 1338.5 m in the embayment and receded from that elevation immediately prior to 13 070 ± 60 yr B.P. Similar and slightly older radiocarbon ages on gastropod shells preserved in barrier deposits at 1327 m (13 280 ± 110 yr B.P.) and 1331 m (13 110 ± 110 yr B.P.) suggest that the final rise to the highstand was very rapid and that the lake maintained its highest stand for a very brief period of time, perhaps only for years or decades. The brevity of the highstand is reasonable in light of the recent formation of similar barrier features in modern Pyramid Lake, which formed in less than seven months due to a rapid increase in lake level.

Isostatic rebound, active faulting, and potential geomorphic effects in the Lake Lahontan basin, Nevada and California

The high shoreline of the late Pleistocene (Sehoo) lake in the Lahontan basin is used as a passive strain marker to delineate the magnitude and character of regional deformation since 13 ka. The elevations of 170 high shoreline sites document that the once horizontal (equipotential) shoreline, which traverses almost 4° of latitude and 3° of longitude, is now deflected vertically about 22 m. Most of the deformation is attributed to isostatic rebound, but a small down-to-the-north regional tilting also appears to contribute to the overall deformation pattern. Active faults locally offset the high shoreline, but cannot explain the regional upwarping attributed to isostatic rebound since 13 ka. Preliminary models of the rebound yield an upper mantle viscosity of 10 18 Pa s that implies a Maxwell relaxation time of about 300 yr. The rapid Earth response, coupled with the rapid fall in lake level at the end of Pleistocene time, may have acted to divert some of the major rivers flowing into the basin from one terminal subbasin to another. The regional deformation caused by the rebound may also have acted to control the present location of Honey Lake. These shoreline data, therefore, support the potential for a link be

Viscosity structure of the crust and upper mantle in western Nevada from isostatic rebound patterns of the late Pleistocene Lake Lahontan high shoreline

Received 13 July 2005; revised 26 October 2006; accepted 31 January 2007; published 8 June 2007. [1] Large lakes can both produce and record significant crustal deformation. We present an analysis of the isostatic rebound pattern recorded in the shorelines of paleolake Lahontan, in western Nevada, using a layered Maxwell viscoelastic model. The inferred viscosity structure depends on loading history. We use three variants of a well-documented lake surface elevation model as input and recover corresponding estimates of viscosity and density structure. A simple two-layer model, with an elastic plate over an inviscid half-space, fits the observed elevation pattern quite well, with a residual variance of 32% of the data variance. Using multilayered, finite viscosity models, the residual variance is reduced to 20% of the data variance, which is very near to the noise level. In the higher-resolution models, the viscosity is below 10 18 Pa s over the depth range from 80 to 160 km. The minimum viscosity is very similar to the value that has been seen in the eastern Great Basin, from similar analyses of Lake Bonneville shorelines, but the lowviscosity zone is thinner beneath Bonneville. Making small adjustments to a seismically derived density structure allows an improved fit to the shoreline observations. Additionally, we find that small variations in proposed loading models can result in presumably spurious density inversions, and suggest that this modeling approach provides a test for loading histories.

The Lake Lahontan highstand: age, surficial characteristics, soil development, and regional shoreline correlation

Abstract The Lake Lahontan basin has been the site of numerous pluvial lakes during the Pleistocene. We address the question of whether or not the highest remnant shoreline features around the perimeter of the lake were produced during the most recent Sehoo highstand (∼13 ka), the penultimate Eetza highstand (∼140–280 ka), or both. To do so, we document surficial characteristics, morphologic preservation, and soil development on multiple Sehoo beach barriers in the Jessup embayment to define the range in characteristics displayed by latest Pleistocene beach features. Sehoo barriers generally exhibit original constructional morphology that has been little modified by erosion. Soils developed on Sehoo barriers are generally thin and weakly developed and are strongly influenced by the introduction of eolian fines into the predominately clast-supported coarse beach gravels. Similar observations from 13 other highstand barriers and from seven older-than-latest Pleistocene paleosols located around the basin form the basis for a regional comparison. Based on similar characteristics, including the degree of morphologic preservation and weak soil development, we conclude that the widespread and nearly continuous high shoreline around the perimeter of Lake Lahontan dates from the most recent major lake cycle in all areas except in the Walker Lake subbasin. In the Walker Lake subbasin, isolated early to middle Pleistocene lacustrine outcrops and landforms are elevated as much as 70 m above the late Pleistocene limit, but are differentiated by their degraded form and lack of continuity around the subbasin. Similar unambiguous landforms were not observed elsewhere and at similar elevations in the northern subbasins of Lake Lahontan.

Late Holocene sedimentary environments and lake-level fluctuations at Walker Lake, Nevada, USA

Walker Lake is a terminal lake located at the end of the Walker River in western Nevada. Lake-level lowering of ∼50 m during the past 100 yr has led to deep incision by the Walker River into the former lake bed. This incision has exposed a sedimentary record of lake-level fluctuations spanning at least the past 4000 yr. Fourteen new radiocarbon dates on plant material collected from diverse sedimentary environments, including fluvial, deltaic, beach, and offshore settings, have led to the refinement of the late Holocene lake-level curve for this basin. This new curve documents at least four highstands that have occurred since ca. 3500 cal yr B.P. between elevations of 1245 and 1255 m. The lake has also experienced several lowstands during this same period. Comparison of this outcrop-based record to existing lake-level curves and to an oxygen isotope curve for the past 1200 yr reveals many similarities between the records, but also some differences. Therefore, research should be conducted to integrate outcrop studies, where absolute water depth and lake volume can be estimated, with isotopic or other types of proxies from cored sediments that may have higher temporal resolution. The extensive and nearly continuous outcrops along the lower Walker River also provide valuable information on the relationships between various sedimentary features and their formative water depths in clastic-dominated lacustrine systems. The paleo–water depth relationships deciphered here can be applied in other basins, where outcrops are typically more limited, to arrive at better-constrained lake-level records.

Annually‐resolved late holocene paleohydrology of the southern Sierra Nevada and Tulare Lake, California

Here we present 2000 year long, annually resolved records of streamflow for the Kings, Kaweah, Tule, and Kern Rivers in the southwestern Sierra Nevada of California and consequent lake-level fluctuations at Tulare Lake in the southern San Joaquin Valley. The integrated approach of using moisture-sensitive tree ring records from the Living Blended Drought Atlas to reconstruct annual discharge and then routing this discharge to an annual Tulare Lake water balance model highlights the differences between these two types of paleoclimate records, even when subject to the same forcing factors. The reconstructed streamflow in the southern Sierra responded to yearly changes in precipitation and expressed a strong periodicity in the 2–8 year range over most of the reconstruction. The storage capacity of Tulare Lake caused it to fluctuate more slowly, masking the 2–8 year streamflow periodicity and instead expressing a strong periodicity in the 32–64 year range over much of the record. Although there have been longer droughts, the 2015 water year represents the driest in the last 2015 years and the 2012–2015 drought represents the driest 4 year period in the record. Under natural conditions, simulated Tulare Lake levels would now be at about 60 m, which is not as low as what occurred multiple times over the last 2000 years. This long-term perspective of fluctuations in climate and water supply suggests that different drought scenarios that vary in terms of severity and duration can produce similar lake-level responses in closed lake basins.

Response of the Truckee River to lowering base level at Pyramid Lake, Nevada, based on historical air photos and LiDAR data

The Truckee River in western Nevada has severely incised in response to a net 20 m of lake-level lowering of Pyramid Lake over the past 120 yr, leaving a suite of cut terraces that extend ~15 km upstream from the lake where incision was arrested by bedrock in the channel. Channel planform changes and terrace development were mapped over a 70 yr period using rectified aerial imagery and light detection and ranging (LiDAR) data in ArcGIS. Discrete point elevations were extracted from the LiDAR data for channel remnants on each of the different-aged terraces and combined with channel distance measurements derived from the appropriate aerial imagery to derive true channel gradients for each of the photo years. These same point elevation measurements were also resolved to a common valley distance (CVD), as is often done when studying prehistorical terrace sequences. Comparison of the true channel distance (TCD) profiles to the CVD profiles demonstrates that the TCD profiles are longer and less steep than the CVD profiles. These differences exist because resolving terrace elevations to a CVD does not account for decreasing sinuosities and straighter channel planforms through time. As base level fell to its historical low elevation, the channel became steeper, straighter, and smoother, which increased the rate of sediment transport and delivery to Pyramid Lake. Over the past 120 yr, ~60,000,000 m 3 of sediment have been removed from the bed and banks of the lower river and redeposited in the lake, temporarily increasing the sedimentation rate. Larger trenches through deltaic and lacustrine deposits occupied by rivers and streams flowing into Pyramid Lake and other closed basins may reflect similar rapid responses to lowering base levels at times in the past. Therefore, lowering lake levels at Pyramid Lake and elsewhere during relatively dry times may be at least partly responsible for transient increases in sedimentation rates in lacustrine archives.

Effects of Recreational use on Branchiopod Egg and Ephippia Density, Black Rock Desert-High Rock Canyon Emigrant Trails National Conservation Area, Nevada, USA

Numerous plays playas occur on valley floors of endorehic basins in arid regions of the western USA. Their openness makes them attractive for hiking, vehicle travel, military, and other uses when dry. Branchiopod crustacean dormant egg banks survive in these systems and are a rich food resource for migrating birds. Brachniecta gigas Lynch, 1937, B. mackini Dexter, 1956, and Lepiduras lemmoni Holmes, 1895, and Moina cf. macrocopa (Straus, 1820) occur in the Black Rock Desert playa, Nevada, USA. We collected playa egg bank samples to determine effects of human use in three studies. We compared intact egg and ephippia density in virgin playa areas with: 1) a heavily used vehicle track, and 2) recreational camping and vehicle activity mitigated by dust abatement in Black Rock City (site of the Burning Man Festival). We also attempted to quantify changes in intact egg and ephippia density through repeated vehicle travel over a track on virgin playa. We found no observed decrease in intact egg or ephippia density attributed to incrementally increased vehicle travel over virgin playa, which may be attributed to strength of the playa substrate matrix. Differences in intact egg and ephippia density were substantially lower in heavily used vehicle tracks than in adjacent playa, but differences were not statistically significant. Density of intact eggs and ephippia in Black Rock City were lower following the festival, but differences were statistically significant only for eggs in camping areas and not roads. Weak effects observed on these roads may be attributed to dust abatement that maintained substrate density.

A First Look at the Terminal Pleistocene/Early Holocene Record of Guano Valley, Oregon, USA

ABSTRACT Surface sites play a vital role in interpreting terminal Pleistocene/early Holocene (TP/EH) lifeways in the Great Basin. Two years of work in Oregon’s Guano Valley by crews from the University of Nevada, Reno have revealed a rich record of Western Stemmed Tradition occupations associated with an extensive delta system that brought freshwater into the valley from the adjacent tablelands. To date, we have recorded nearly 700 diagnostic TP/EH artifacts within the relatively small delta, making it one of the densest concentrations of Paleoindian artifacts in the region. This paper presents preliminary results from our archaeological and geomorphological investigations in Guano Valley as well as an extensive toolstone source provenance analysis.