HIGH-RESOLUTION PAGE

DROUGHT AND FLOOD FREQUENCY IN THE NORTHERN GREAT BASIN

The western sector of the United States is particularly vulnerable to extended, multi-year climatic fluctuations because of naturally marginal water resources. This region is experiencing rapid increases in population which put additional stress on scarce water resources. For example, several multi-year climatic anomalies have severely impacted the western United States during the last two decades. Among the most important anomalies were the record increases in sizes of Great Basin lakes in the mid 1980s followed by an abnormally dry period between 1987 and 1994. During this the winter of 1996/1997, the Sierra Nevada is experiencing possibly the wettest winter on record.

In May of 1997 a research team composed of U.S. Geological Survey, University of Nevada, and University of Southern California scientists obtained a series of sediment cores from Pyramid Lake, Nevada (40oN). These cores are being used to quantitatively reconstruct variation in the Truckee River-Pyramid Lake surface-water system at less-than-decadal scales for the past 3500 cal yr. Pyramid Lake is by far the deepest closed-basin lake in the Great Basin and, by virtue of its great depth, may have persisted throughout the Holocene. The depth of Pyramid Lake (105 m) relative to other Great Basin lakes (e.g., Mono Lake (43 m), Walker Lake (30 m), Great Salt Lake (25 m), Goose Lake (8 m), Lake Abert (6 m) also tends to minimize the effects of sediment reworking in its deepest areas; i.e., the deepest part of Pyramid Lake lies much below wave base and may have remained so during the past 3500 years. Arguably, Pyramid Lake is the most intensively studied closed-basin lake in North America. The Truckee River-Pyramid Lake system has served, for some time, as a natural laboratory for the study of biological, chemical, thermal, and isotopic processes that occur in temperate closed-basin systems.

The principal approach will be to employ high-resolution total inorganic carbon and oxygen-isotope measurements on well-dated carbonate sediments to determine changes in the water balance of Pyramid Lake. Numerical simulations of the hydrologic-balance records will be used to estimate the magnitudes of climate change that led to variability in lake volume. We anticipate that the results of our lake-size reconstructions will allow us to evaluate the impact of Little Ice Age (A.D.1450-1850) and Medieval Warm Period (A.D.1100-1250) and ENSO climatic anomalies on the climate of the study area. A coarser resolution study of the period 7000 to 2000 cal yr B.P. will be performed to set the timing and intensity of the mid Holocene dry period.

The data records and hydrologic-balance reconstructions resulting from this study will provide estimates of the return periods and intensities of dry and wet regimes for major population centers on either side of the central Sierra Nevada. In addition these data will serve as essential inputs to model calculations of future climate variability.

Pyramid Lake and the lands surrounding it belong to the Pyramid Lake Paiute tribe. The cui-ui, a fish belonging to the sucker family, is endemic to Pyramid Lake. Natural reproduction of this endangered fish is minimal in dry years because of consumptive use of Truckee River water by the cities of Sparks and Reno and by upstream diversion of Truckee River water to the Newlands irrigation project. For example, natural reproduction of cui-ui did not occur between 1988 and 1992. During this dry period, fish hatcheries located on Paiute lands were able to produce only (106 fry as compared to (109) fry naturally produced in relatively wet years. Our proposed work on variability of the late Holocene hydrologic balance of Pyramid Lake would help to determine the intensities and return periods of low and high-discharge periods of the Truckee River, information that is highly important to those responsible for allocation of Truckee River water and management of its several reservoirs.