Neopluvial

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] The end of the neopluvial may align with a change of speckled dace populations.^[22]

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").^[23] Population around Lake Alvord increased during this time and lasted even after the Neopluvial had ended there.^[3] In Nevada, the largest indigenous houses were built during the neopluvial.^[24]

The beginning of the Neopluvial occurred about 6,000 years before present, but did not occur everywhere at the same time:^[12]

• The Neopluvial occurred between 4,000 and 2,000 years before present in the Carson Sink.^[16] The Neopluvial in the Lake Lahontan basin ended about 2,000 years ago.^[3]
• In Fallen Leaf Lake, the Neopluvial occurred 3,700 years before present in Fallen Leaf Lake. The end occurred 3,650 years before present;^[5] after that point precipitation became more irregular until the onset of the Little Ice Age about 3,000 years later.^[25]
• Its occurrence is dated between 5,100 and 2,650 years before present in the central-northern Great Basin,^[18]
• In the Great Salt Lake, the Neopluvial commenced 5,000 years before present and water levels reached their maximum between 3,000 and 2,000 years before present.^[4]
• It took place between 3,000 and 4,000 years before present in Lake Cochise.^[6]
• It occurred between 4,000 and 2,500 years before present in the Mojave Desert.^[7]
• In Pyramid Lake, the Neopluvial commenced starting from 5,000 years before present and reached a maximum between 4,100 and 3,800 years before present in Pyramid Lake.^[5]
• High elevation lakes in the Rocky Mountains with small watersheds, particularly sensitive to a changing water balance, showed synchronous increase in lake levels from 6,000 to 5,000 years before present, centered at 5,700 years ago.^[26]
• In the Summer Lake area, the Neopluvial is dated to have occurred between 4,000 and 1,900 years ago.^[13]
• Rising water levels in Lake Tahoe drowned trees between 4,800 and 5,700 years before present.^[12]
• In Tulare Lake, the Neopluvial lasted between 4,500 and 2,800 years before present; after that a severe drought occurred.^[9]

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.^[27] 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.^[26] Strengthening ENSO variability, a cooling of the North Pacific and a southward shift of the Pacific jet stream also coincided with the Neopluvial.^[28] The neopluvial resembles the Pluvial period that occurred in western North America during the late Last Glacial Maximum,^[29] 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.^[30]