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.