ENVIRONMENTAL AND GEOMORPHIC EFFECTS ON THE LOWER COLORADO RIVER DELTA FROM THE APRIL 4, 2010 EARTHQUAKE, BAJA CALIFORNIA, MEXICO

The M7.2 El Mayor-Cucapah earthquake in Baja California caused widespread flooding, localized subsidence/lateral spreading, and social and economic disruption in the lower Colorado River Delta. We assessed the earthquake’s effects with sequential satellite images, overflights, water-level sensors and ground observations. Most of the flooding in the lower delta was caused by liquefaction, with water coming to the surface through sand boils and fissures. Some flooding was caused by ruptures of irrigation canals and artificial levees. Liquefaction continued for several days following the earthquake and resulted in ephemeral ponds and new drainages. Subsidence resulting from liquefaction has increased the area affected by the northern Gulf of California’s high-amplitude tides. Extensive fields of sand boils at the head of the estuary may cause local realignment of the upper estuary/remnant river channel and may affect the connectivity of wetlands with the Gulf. Local flooding from liquefaction, subsidence and dike ruptures caused evacuation of some farming communities, disruption of agricultural activity, damage to tourist facilities, and restricted access to commercial fishing sites. The Ciénega de Santa Clara, a 6,000 ha wetland supported by agricultural drain water from Arizona, was largely unaffected by the earthquake. Water levels in the Ciénega increased because of diversion of river water away from the delta’s ruptured irrigation canals toward the Ciénega. Water diversion occurred through active management of the lower river, not because of direct effects of the earthquake.