Measurements of the oxygen isotopic composition (δ18O) of the total inorganic carbon (TIC) fraction from cored sediments of Walker Lake, Nevada, were conducted at an average resolution of ∼3 years per sample over the last 1200 years. On the basis of radiocarbon analysis on the total organic carbon (TOC) fraction, a δ18O time series was created to reconstruct changes in hydrologic conditions back to AD 800. The timings of variations in the TIC δ18O record are generally consistent with the tree ring-based Sacramento River flow record spanning AD 869 to 1977, indicating that Walker Lake δ18O contains information about past changes in at least regional hydrologic conditions. Comparison with the δ18O record from Pyramid Lake sediments indicates that both basins have recorded five century-scale oscillations in regional hydrologic conditions since AD 800. Several of these changes in hydrologic conditions appear synchronous with century-scale California Current water temperature changes derived from analysis of sediment cores from the Santa Barbara Basin also attesting to the regional extent of these climatic fluctuations. Nearly synchronous oscillations in the Sierra wetness and the California Current suggest that regional changes in atmospheric circulation may have played an important role in century-scale climate variability over the last millennium.