Patterns of hydroclimatic change in the Rocky Mountains and surrounding regions since the last glacial maximum

The paleohydrologic record of western North America since the last glacial maximum reveals a wide range of hydroclimatic variability in time and space. To improve the understanding of abrupt hydroclimatic shifts and millennial-scale hydrologic changes in the central Rocky Mountains, we reconstruct the lake-level histories of two small lakes in the Beartooth and Bighorn Mountains in northern Wyoming over the past 17 ka. To do so, we use ground-penetrating radar (GPR) and sediment cores to track the elevations of shoreline sediments within the lakes through time. We compare the stratigraphies with those from four other lakes in Wyoming and Colorado, and find widespread evidence for a Terminal Pleistocene Drought from 15 to 11 ka, an early Holocene humid period from 11 to 8 ka, and mid-Holocene aridity from 8 to 5.5 ka. The northern Wyoming lakes also provide evidence of high levels in the Pleistocene, possibly before ca. 15 ka, and rapid hydroclimatic changes that may have correlated with Heinrich Event 1 (ca. 16.8 ka). We place the changes in a broad context by summarizing and mapping water-level changes from 107 additional, previously studied lakes. Important patterns include 1) extensive drying across the western U.S. after 15 ka; 2) sub-regional differences during the Pleistocene-Holocene transition; 3) a north-south contrast from 9 to 6 ka consistent with a northward shift in storm tracks as the influence of the Laurentide Ice Sheet diminished; and 4) rapid increases in effective moisture across much of western North America from 6 to 4 ka.