Geomorphology of a Recurring Tidal Sandbar in the estuary of the Colorado River, Mexico: Implications for restoration

Abstract

An obstructive tidal sandbar forms in the estuary of the Colorado River, Mexico, during periods of low fluvial discharge. The sandbar typically develops at a bedload convergence zone centered approximately 30km landward from Montague Island near the river's mouth. The estuary provides important spawning habitat for the endangered totoaba (Totoaba macdonaldi) and commercially exploited Gulf corvina (Cynoscion othonopterus), as well as habitat for larval shrimp, all of which are impacted by reduced freshwater/sea water mixing when the sandbar is present.Sequential satellite images, aerial photographs, overflights, and ground observations were used to document the geomorphology of the tidal sandbar and its formation, removal, and subsequent reappearance in response to long-term variations in fluvial flow following completion of Glen Canyon Dam in 1964. Evidence for the formation of the sandbar during low-discharge periods prior to anthropogenic manipulation of the river's flow was also examined.Water data sensors were installed upstream from the current sandbar to monitor the frequency of tidal overflow and document the effect of tides on salinity. Topographic surveys of the sandbar were completed both before and after the 2010 Mw 7.2 El Mayor-Cucapah Earthquake which caused subsidence of lands immediately east of the sandbar's crest.Utilizing topographic data and knowledge of the sandbar's geomorphologic and sedimentary history, we designed and installed a system of dredged pilot channels to improve connectivity between the upper and lower estuary by facilitating tidal and fluvial flow across the barrier. The effectiveness of the pilot channels is currently under evaluation. If the pilot channels fail to establish the desired level of connectivity, installation and maintenance of a more substantial dredged channel may be necessary to meet restoration objectives.