This project was initiated at the direction of Eric Kuhn from the Colorado River Water Conservation District, which has supported the project with District funds and also motivated the Colorado Water Institute’s support of the project. Datasets from 66 tree-ring sites were compiled, and then several different statistical methods were used to reconstruct annual naturalized flow for the Gila at its mouth and the Lower Colorado mainstem between Lees Ferry and Imperial Dam, back to at least 1612. The tree-ring reconstructions indicate that annual flow were generally lower before 1900 than during the modern gaged period. The tree-ring reconstructed flows for the Lower Basin were then used to inform a basin water-balance model similar to Rajagopalan et al. (2009), but modified to represent the flow inputs from the Gila River. System-response modeling under several different scenarios of water demand, shortage-sharing policy, and climate-change flow reductions showed that the periodic inflows from the Gila under current management substantively improve system outcomes.