ABSTRACT While drought represents a serious threat to the Pacific southwestern United States, floods represent an equally formidable threat. This risk is so significant that the U.S. Geological Survey created the ARkStorm Project. This project aims to prepare California for a future storm(s) on the scale of the disastrous A.D. 1861–1862 events. Unfortunately, our knowledge of premeasurement floods in the Pacific southwestern United States is sparse. To date, the best paleoflood record consists of flood layers in the Santa Barbara Basin, spanning the past 9000 calendar yr B.P. (cal yr B.P.). As an alternative to marine archives, the lakes of the Pacific southwestern United States represent untapped resources for possible premeasurement flood reconstructions. Here, we present evidence for a flood between ca. 4860 and 4820 cal yr B.P. using sediment from Lake Elsinore core LEGC03-4. Core LEGC03-4 is predominantly clayey silt with occasional sandy silt units of variable centimeter-scale thickness. Here, we focus on a specific core section between 350 and 315 cm, where an ~11-cm-thick “unusual” sediment unit (330–319 cm) is well preserved and complete. The core section was analyzed for a variety of physical and chemical properties, including magnetic susceptibility, loss-on-ignition (LOI) at 550 °C and 950 °C, grain size, C org :N total ratios, and δ 13 C (bulk organic matter) . The unit is characterized by an erosional basal contact and microflame structures. It is normally graded, with laminae occurring in the upper section of the unit. It contains predominantly terrestrial organic matter, and the upper boundary is gradational. It is coeval with the fourth highest sand peak in a previously dated central basin core. Consequently, it is our conclusion that the unusual sediment unit represents a turbidite associated with a large flood-producing precipitation event with a maximum limiting age between 4860 and 4820 cal yr B.P.

ABSTRACT This paper summarizes the hydrological variability in eastern California (central Sierra Nevada) for the past 3000 yr based on three distinct paleoclimate proxies, δ 18 O, total inorganic carbon (TIC), and magnetic susceptibility (chi). These proxies, which are recorded in lake sediments of Pyramid Lake and Walker Lake, Nevada, and Mono Lake and Owens Lake, California, indicate lake-level changes that are mostly due to variations in Sierra Nevada snowpack and rainfall. We evaluated lake-level changes in the four Great Basin lake systems with regard to sediment-core locations and lake-basin morphologies, to the extent that these two factors influence the paleoclimate proxy records. We documented the strengths and weaknesses of each proxy and argue that a systematic study of all three proxies together significantly enhances our ability to characterize the regional pattern, chronology, and resolution of hydrological variability. We used paleomagnetic secular variation (PSV) to develop paleomagnetic chronostratigraphies for all four lakes. We previously published PSV records for three of the lakes (Mono, Owens, Pyramid) and developed a new PSV record herein for Walker Lake. We show that our PSV chronostratigraphies are almost identical to previously established radiocarbon-based chronologies, but that there are differences of 20–200 yr in individual age records. In addition, we used eight of the PSV inclination features to provide isochrons that permit exacting correlations between lake records. We also evaluated the temporal resolution of our proxies. Most can document decadal-scale variability over the past 1000 yr, multidecadal-scale variability for the past 2000 yr, and centennial-scale variability between 2000 and 3000 yr ago. Comparisons among our proxies show a strong coherence in the pattern of lake-level variability for all four lakes. Pyramid Lake and Walker Lake have the longest and highest-resolution records. The δ 18 O and TIC records yield the same pattern of lake-level variability; however, TIC may allow a somewhat higher-frequency resolution. It is not clear, however, which proxy best estimates the absolute amplitude of lake-level variability. Chi is the only available proxy that records lake-level variability in all four lakes prior to 2000 yr ago, and it shows consistent evidence of a large multicentennial period of drought. TIC, chi, and δ 18 O are integrative proxies in that they display the cumulative record of hydrologic variability in each lake basin. Tree-ring estimations of hydrological variability, by contrast, are incremental proxies that estimate annual variability. We compared our integrated proxies with tree-ring incremental proxies and found a strong correspondence among the two groups of proxies if the tree-ring proxies are smoothed to decadal or multidecadal averages. Together, these results indicate a common pattern of wet/dry variability in California (Sierra Nevada snowpack/rainfall) extending from a few years (notable only in the tree-ring data) to perhaps 1000 yr. Notable hydrologic variability has occurred at all time scales and should continue into the future.

ABSTRACT Shorelines formed by terminal lakes record past changes in regional moisture budgets. In the western Great Basin of North America, winter precipitation accounts for nearly half of the annual total and is well correlated with northeast Pacific storm track activity and moisture transport. We evaluated these relationships and found that historical precipitation between 1910 and 2012 was better correlated to moisture transport (0.78, p < 0.01) than to storm track activity (0.54, p < 0.01) because moisture transport better captures dynamics associated with the Sierra Nevada rain shadow. We derived modern analogs of enhanced and reduced storm track activity and moisture transport from reanalysis products and used associated winter precipitation anomalies with these analogs as inputs to a coupled water balance and lake evaporation model of the Walker Lake basin. Simulated lake-level responses were compared with a radiocarbon-dated lakeshore chronology spanning the past 3700 yr. Wet analogs developed from winters in the 90th and 75th percentiles for storminess and moisture transport produced lake levels that exceeded estimated late Holocene highstands by 50 m. Dry analogs (10th and 25th percentiles) produced lake levels corresponding to Medieval megadrought lowstands. The twentieth century is shown to be as wet as any century in the past 3700 yr. Our results demonstrate the sensitivity of terminal lakes to winter season circulations and highlight the value of using moisture transport as a predictor of cool season precipitation and to evaluate how past or future changes in regional circulations will influence the water balance of dryland regions.

ABSTRACT We extend a published 9000 yr fire history record from Little Lake, in the Oregon Coast Range, to 35,000 yr and compare it with the established pollen record from the site. The fire history is based on a high-resolution analysis of charcoal preserved in lake sediments, providing a fire history record that spans the Last Glacial Maximum in North America. The data enabled us to address questions regarding the interactions between large-scale climate changes associated with the shift from glacial to interglacial conditions and the accompanying changes in forest vegetation and fire regimes. The vegetation history indicates a change from open subalpine forests to closed western hemlock and Douglas fir forests as climate moved from cold and dry full glacial to warm and wet Holocene conditions. The fire history indicates that although there was more biomass burned in the Holocene, the frequency of fires between glacial and interglacial conditions was not significantly different, and the fire frequency did not change in concert with regional shifts in vegetation. This suggests that fire is a product of seasonal or multiyear variations in climate that may not cause significant shifts in vegetation. Also, as this short-term climate variability becomes more common in the near future, conditions for fires in these mesic forests may become more common as well.

Abstract High-elevation tropical glaciers provide records of past climate from which current changes can be assessed. Comparisons among three ice-core records from tropical mountains on opposite sides of the Pacific Ocean reveal how climatic events are linked through large-scale processes such as El Niño–Southern Oscillation. Two distinctive trans-Pacific events in the mid-fourteenth and late-eighteenth centuries are distinguished by elevated aerosol concentrations in cores from the Peruvian Andes and the Tibetan Himalaya. Today aerosol sources for these areas are enhanced by droughts accompanying El Niños. In both locations, large-scale atmospheric circulation supports aerosol transport from likely source regions. Oxygen isotopic ratios from the ice cores are significantly linked with tropical Pacific sea-surface temperatures, especially in the NIÑO3.4 region. The arid periods in the fourteenth and eighteenth centuries reflect droughts that were possibly connected to strong and/or persistent El Niño conditions and Intertropical Convergence Zone migration. These ‘black swans’ are contemporaneous with climate-related population disruptions. Recent warming, particularly at high elevations, is posing a threat to tropical glaciers, many of which have been retreating at unprecedented rates over the last several thousand years. The diminishing ice in these alpine regions endangers water resources for populations in South Asia and South America.

Abstract Afghanistan is a mountainous, arid country with limited surface water supplies. The complex geology in this country includes active tectonics and mountain ranges. Afghanistan is subdivided into three distinct hydrogeological areas: the Central Highlands, the Northern Plain, and the Great Southern Plain. Most groundwater is located in the Central Highlands, where water of sufficient quantity to meet the needs of the population is available primarily by digging wells into unconsolidated alluvial aquifers located in mountain valleys. A lack of sustainable, high-quality water supplies can have a negative impact on the ability to conduct military operations. An understanding of hydrogeological conditions is required in order to minimize exposures to natural and anthropogenic sources of contamination that may pose either acute or chronic health risks to military forces. This same scarcity of potable water can have a negative impact on the local population. Projects that improve the quantity and quality of water available to both military forces and the local population are important to improve the overall stability of Afghanistan.