A late Holocene carbonate spring mound and associated wetland deposits at Cienega Amarilla, New Mexico, contain a 4000-yr record of geomorphic, paleoenvironmental, and hydroclimatic change on the southern Colorado Plateau. Forty-four 14C dates support a century-scale chronostratigraphic framework. Pollen, plant macrofossil, mollusk, ostracode, and soil analyses indicate rapid spring mound growth and wetland expansion beginning ~2300 cal yr BP, followed by a pronounced decline in groundwater discharge (GWD) between ~1500 and 1000 cal yr BP. The isotopic composition of Cienega Amarilla springwater suggests GWD is driven primarily by winter precipitation. Historical climate data indicate that El Niño and warm Pacific Decadal Oscillation (PDO) conditions foster wetter-than-average winters in the Cienega Amarilla area, whereas dry winters are associated with La Niña conditions regardless of PDO phase. The ~2300–1500 cal yr BP Cienega Amarilla pluvial appears to represent an interval of peak, late Holocene cool-season precipitation that implies unusually strong or persistent El Niño–like and warm PDO–like conditions in the Pacific. Other southwestern paleoenvironmental records corroborate atypically wet conditions during this interval, and pluvial conditions related to increased winter precipitation likely fostered significant prehistoric cultural changes throughout the region, including increased sedentism, population, and dependence on agriculture.

Stratigraphy and dating provide an 18,000 yr record of erosion and deposition by the middle Gila River. Prior to 18,000 cal yr B.P., the Gila River cut into its floodplain and created a deep, wide channel. Sand and gravel accumulated within this channel until 4250–4400 cal yr B.P. By this time, the channel of the Gila River was narrow and deep. Around 5000 cal yr B.P., fine-grained sediments began to accumulate on the floodplain. At 800–950 cal yr B.P., there was a major period of channel widening. After this erosional episode, the channel again narrowed as it filled with sand and gravel until around 200 cal yr B.P. On the floodplain, deposition continued from about 5000 cal yr B.P. to 500 cal yr B.P. A period of stability and soil formation occurred on the floodplain between 500 and 200 cal yr B.P., after which overbank deposition resumed and buried the soil. Channel widening again occurred in the late 19th century. The changes observed in the sedimentologic history of the Gila River show that during the late Pleistocene, the Gila River was a competent stream capable of carrying and depositing coarse sediment loads. Later deposition of sand and gravel during the first half of the Holocene implies an increase in sediment yield from upstream watersheds. Changes during the last 4000 years reflect the response of the river to climatic perturbations, the timing of large floods, internal landscape thresholds, and human impacts.