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The paleoenvironmental record of sedimentation and development of an in-channel wetland that spans the border of southeast Arizona, USA, and northeast Sonora, Mexico, suggests a complex history between climate change and regional groundwater levels. From the late-Pleistocene to early Holocene, the channel occupied by the San Bernardino Cienega was largely aggradational with cobbles to sands being deposited in the oldest and deepest sediments. Changes in grain size indicate lowered stream power through the early to middle Holocene. Around 7200 years ago, sediments changed from sand and cobble dominance to significant proportions of silts and clays suggesting standing or still waters. Along with changes in sediment texture, preservation of organic material initiated, providing a record of regional and local vegetation change. The upland transitioned from woody/grassland co-dominance between 7000 and 5400 cal BP, to grassland dominated conditions after 5400 cal BP. Locally, wetland vegetation established within the stream channel after 7000 cal BP and persisted until ca. 4100 cal BP. From 4100 to similar to 2400 cal BP, the cienega remained stabilized, but was covered with grasses and forbs. Wetland vegetation returned after 2400 cal BP. The change in sediment characteristics and inferred establishment of wetland vegetation is interpreted as increased importance of regional groundwater hydrology from the middle Holocene to present. The timing of this change is coincident with increasing El Nino variability and suggests a causal mechanism for increasing regional groundwater, namely increase in winter precipitation in the American Southwest. (C) 2010 Elsevier Ltd and INQUA. All rights reserved.