RESTORING THE CONNECTION BETWEEN THE COLORADO RIVER AND THE SEA: IMPLICATIONS OF SEDIMENTATION RATES IN A LOWER CHANNEL

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 sediments deposited during flood tides; ebb flows are not strong enough to keep the channel open. We used Landsat 5-TM and Landsat-7 scenes from the Colorado River Delta and tide prediction tables to reconstruct river-sea connectivity and geomorphic processes. We used historical documentation, previous topographic surveys and sediment cores 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 35 kilometers upstream from the river’s mouth. Historical documentation and sediment core analyses suggest sedimentation rates in the range of 10-21 cm per year.

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. Active management -dredging - is required to reconnect the remaining riparian wetlands in the lower Colorado River to the Gulf of California. Based on our estimates of sedimentation rates, dredging would need to occur once every 5-10 years to maintain this connection. Pulse flows from controlled releases could simulate pre-dam spring floods and would scour and clear away sediment accumulated in the lower river channel. This would reduce the need and cost of frequent dredging.