Post-dam sediment dynamics and processes in the Colorado River estuary: Implications for habitat restoration

Abstract

River-sea connectivity is essential for restoring ecosystem services in the Colorado River delta. The mixing of river water and seawater sustains biodiversity and provides brackish-water nursery grounds for both commercially important and endangered marine species. The Colorado River no longer reaches the sea except during particularly high tides and anomalously wet years. The river's relict channel is now obstructed by an accumulation of sediments deposited during flood tides; ebb flows are not strong enough to keep the channel open. Landsat 5-TM and Landsat-7 scenes from the Colorado River delta and tide prediction tables were used to reconstruct river-sea connectivity and geomorphic processes after 50 years of extensive human manipulation of the Colorado River. Historical documentation, previous topographic surveys and sediment cores were used to estimate sedimentation rates in the lower river channel. Satellite images and tide charts show that currently the river reaches the sea or the sea reaches the river about 12 days per year, unlike 10 years ago when a year-round connection existed. Reduction in connectivity results from the evolution of a tidal sandbar located within the bedload convergence zone, about 35km upstream from the river's mouth. Historical documentation and sediment core analyses suggest sedimentation rates in the range of 10–21cm per year. With the current conditions prevailing, active management – dredging – is required and needs to occur once every 5–10 years to reconnect the remaining riparian wetlands in the Colorado River to the Gulf of California.