Continuous, high-resolution d 18 O records from cored sediments of Pyramid Lake, Nevada, indicate that oscillations in the hydrologic balance occurred, on average, about every 150 years (yr) during the past 7630 calendar years (cal yr). The records are not stationary; during the past 2740 yr, drought durations ranged from 20 to 100 yr and intervals between droughts ranged from 80 to 230 yr. Comparison of tree-ring-based reconstructions of climate change for the past 1200 yr from the Sierra Nevada and the El Malpais region of northwest New Mexico indicates that severe droughts associated with Anasazi withdrawal from Chaco Canyon at 820 cal yr BP (calendar years before present) and final abandonment of Chaco Canyon, Mesa Verde, and the Kayenta area at 650 cal yr BP may have impacted much of the western United States.During the middle Holocene (informally defined in this paper as extending from 8000 to 3000 cal yr BP), magnetic susceptibility values of sediments deposited in Pyramid Lake's deep basin were much larger than late-Holocene (3000-0 cal yr BP) values, indicating the presence of a shallow lake. In addition, the mean d 18 O value of CaCO 3 precipitated between 6500 and 3430 cal yr BP was 1.6m less than the mean value of CaCO 3 precipitated after 2740 cal yr BP. Numerical calculations indicate that the shift in the d 18 O baseline probably resulted from a transition to a wetter (>30%) and cooler (3-51C) climate. The existence of a relatively dry and warm middle-Holocene climate in the Truckee RiverPyramid Lake system is generally consistent with archeological, sedimentological, chemical, physical, and biological records from various sites within the Great Basin of the western United States. Two high-resolution Holocene-climate records are now available from the Pyramid and Owens lake basins which suggest that the Holocene was characterized by five climatic intervals. TIC and d 18 O records from Owens Lake indicate that the first interval in the early Holocene (11,600-10,000 cal yr BP) was characterized by a drying trend that was interrupted by a brief (200 yr) wet oscillation centered at 10,300 cal yr BP. This was followed by a second earlyHolocene interval (10,000-8000 cal yr BP) during which relatively wet conditions prevailed. During the early part of the middle Holocene (8000-6500 cal yr BP), high-amplitude oscillations in TIC in Owens Lake and d 18 O in Pyramid Lake indicate the presence of shallow lakes in both basins. During the latter part of the middle Holocene (6500-3800 cal yr BP), drought conditions dominated, Owens Lake desiccated, and Lake Tahoe ceased spilling to the Truckee River, causing Pyramid Lake to decline. At the beginning of the late Holocene (B3000 cal yr BP), Lake Tahoe rose to its sill level and Pyramid Lake increased in volume. r

The utility of a high-resolution snow-hydrologic model to derive climatological indices that describe the variability in radial growth of four conifer species in two Sierra Nevada sites is presented herein. Nine annual indices associated with radial growth were developed to represent the winter dormancy, characteristics of the snowpack and soil water content, and the duration of the seasons. Site chronologies of earlywood (EW) and latewood (LW) ring widths were developed for mountain hemlock (Tsuga mertensiana), red fir (Abies magnifica), white fir (Abies concolor), and ponderosa pine (Pinus ponderosa) at two sites on leeward and windward slopes. The signal strength for annual climatological indices derived from model output was tested with correlation and regression, in combination with principal components analysis. Results show significant snow-related climate signal in the tree-ring data, with substantial differences between species and between EW and LW. Dependence on previous year's snow and soil moisture (a lagged response) were found for EW of hemlock and red fir. The primary EW-LW signal contrast for those species is a shift toward dependence on current-year moisture conditions for LW, especially for red fir. Lagged climate response was less evident for white fir and ponderosa pine. Regression of tree-ring series on principal components of climatological indices showed a stronger average signal in EW (R 2 = 0.48) than in LW (R 2 = 0.35). Differences in tree-ring hydrologic signal at the two sites are attributed to microclimate and contrasts in snow regime. Results attest to the hydrologic model usefulness for investigating temporal relationships between tree rings and local climate.