PALEOHYDROLOGY OF THE LOWER GILA RIVER PRESERVED BY LATE CENOZOIC LAVA FLOWS IN SOUTHWESTERN ARIZONA

The Gila River is a westerly-flowing, ~1000 km tributary of the Colorado River with a ~212,000 km² drainage basin that joins the Colorado River near Yuma, AZ. The lower Gila River is in the Basin and Range province of southwestern Arizona. The lower Gila-Colorado River system probably connected to the Sea of Cortez ~4 Ma (Spencer and Patchett, 1997). Late Pliocene to early Pleistocene basaltic lava flows of the Sentinel-Arlington Volcanic Field dammed the river multiple times, forming small ephemeral lakes, possibly diverting the river, and preserving ancient river terraces and channels. The river has subsequently incised ~30-40 m. The SAVF is relatively unaltered by tectonism or epeirogenic uplift or depression. Interactions between the river and the SAVF provide insight into the hydrology of the ancient Gila River, including channel morphology and sediment transport, which can be used to estimate longitudinal discharge. Preliminary calculations suggest a maximum discharge of ~450,000 cfs for the paleo Gila River. The largest historical flood on the modern lower Gila River occurred in 1891 with an estimated discharge of ~300,000+ cfs. High-albedo, lobate remnant alluvial and colluvial fans in the Gila Bend Mountains, Arizona, were investigated through remote sensing and field study. The data support the hypothesis that these features are deltaic fans whose termination corresponds to the shoreline of an ephemeral paleolake initiated by lava dams formed by SAVF lava flows near Sentinel, AZ. Also, river gravels underneath a spatter and cinder cone south of the Painted Rock Mountains share similar major and trace elemental compositions to SAVF shield volcanoes and suggest that the Gila River transverse drainage near Painted Rock Dam could have been initiated by the early stages of SAVF emplacement, diverting the river north of the Painted Rock Mountains.