Neopluvial is a term referring to a phase of wetter and colder climate that occurred during the late Holocene in the Western United States. During the Neopluvial, water levels in a number of now-dry lakes and closed lakes such as the Great Salt Lake rose and vegetation changed in response to increased precipitation. The event was not exactly synchronous everywhere, with neopluvial lake-level rises occurring between 6,000 and 2,000 years ago. It is correlative to the Neoglacial period.
The neopluvial took place in the western United States during the late Holocene,[1] causing the levels of lakes in the Great Basin to increase[2] and previously dry lakes and springs to refill.[3] It has been observed in Great Salt Lake,[4] Fallen Leaf Lake,[5] Lake Cochise,[6] the Mojave Desert,[7] Mono Lake, Owens Lake, Pyramid Lake,[5] San Luis Lake,[6] Silver Lake,[7] Summer Lake,[8] Tulare Lake,[9] Walker Lake[5] and Winnemucca Lake.[10]
During the Neopluvial, the Great Salt Lake became fresher,[4] and Pyramid Lake reached a water level of 1,186 metres (3,891 ft) above sea level.[5] Walker Lake, Owens Lake and Mono Lake experienced their highest Holocene water levels,[5] with the volumes of the latter two lakes more than doubling.[11] Likewise, water levels in Lake Tahoe rose to the point of overflowing into the Truckee River.[12] Silver Lake in the Mojave Desert formed a perennial lake and vegetation was more widespread in the Little Granite Mountains.[7] Summer Lake rose above its present-day level to an elevation of c. 1,278 metres (4,193 ft),[13] although it was not as high as during the mid-Holocene.[8] Water levels rose in Tulare Lake as well.[9]
In the White Mountains, meadows formed during the Neopluvial.[14] Ice patches in the Beartooth Mountains[15] and glaciers grew in the Sierra Nevada,[16] sagebrush steppe, green Mormon tea and other vegetation expanded in the Great Salt Lake region,[17] marshes expanded in the central and northern Great Basin,[18] mammal communities in the Lake Bonneville basin changed with the return of the long-tailed pocket mouse, the Great Basin pocket mouse and the Western harvest mouse to sites where they were not present before and increased abundances of even-toed ungulates,[19] and tree lines dropped, with the lower limit of wooden vegetation penetrating into deserts.[20] Counterintuitively, higher tree line elevations in the Lake Bonneville area occurred during the Neopluvial, which may indicate warmer summers.[21]
In the Owens Valley region, during the Neopluvial the human population became more sedentary and trans-Sierra Nevada trade became established ("Newberry"/"Middle Archaic Period").[22] Population around Lake Alvord increased during this time and lasted even after the Neopluvial had ended there.[3]
The beginning of the Neopluvial occurred about 6,000 years before present, but did not occur everywhere at the same time:[12]
The Neopluvial is in part correlative to the Neoglacial,[18] and might have been caused by a change in winter conditions over the North Pacific.[25] This cooling is primarily explained by steadily declining summer insolation, though synchronous patterns in hydrological responses at sub-millennial scales may be linked to atmospheric circulation shifts driven by factors such as internal variability in ocean-atmosphere teleconnections.[24] Strengthening ENSO variability, a cooling of the North Pacific and a southward shift of the Pacific jet stream also coincided with the Neopluvial.[26] The neopluvial resembles the Pluvial period that occurred in western North America during the late Last Glacial Maximum,[27] but was much weaker than the LGM wet period.[4]
The term "neopluvial" was coined in 1982 and originally referred to high lake levels in Summer Lake.[10] The term has also been used for a mid-to-late Holocene phase of increased moisture noted in the form of increased wetness in eastern Texas, potentially linked to a stronger monsoon or to the neopluvial of the western US.[28]