Over 26 days in March and April 2014, 130 hm3 of water were released into the dry riverbed of the Lower Colorado River. Though once a mighty river with perennial flows to the Delta, the final reaches of the Lower Colorado have largely been dry for several decades. One goal of the 2014 environmental flows program was to understand the ecological response to intermittent flows in the river, necessary for current stream restoration activities. To achieve this goal, it was necessary to develop a water balance characterising the fate of the water released into the dry stream reach. Transmission losses during the flow included infiltration into an unsaturated streambed (with a variable depth of between 3 and 16 m to underlying groundwater), evaporation, and lateral flow to the shallow aquifer. It was hoped that these losses could be accurately described using an integrated, numerical surface water-groundwater model.
The challenge of building such a model was not insignificant. These included complex river morphology and connectivity, with a so called “pilot channel” intersecting older channel meanders, side channels and wetlands for the water to pool in, and additional tributary flows entering from irrigation canals to supplement the main release. This reach of the river flows along the international boundary between the United States and Mexico, which limits the access and safety of scientists quantifying instream flows and groundwater responses. Heterogeneity in the surrounding aquifer is not well characterised. Finally, the transition from dry to wet (and often still unsaturated) model cells are numerically difficult to model. All these challenges explain why there are few examples of such modelling attempts in the literature. Nonetheless, four years later the results of the model aid greatly in understanding observed physical changes along the river corridor, and provide a useful tool for planning future environmental flows.