TIME, TIDES AND EARTHQUAKES: SATELLITE IMAGERY, STABLE ISOTOPES AND SMALL PLANES RESOLVE THE INFLUENCE OF TIDES ON TWO COLORADO DELTA WETLANDS

The Ciénega de Santa Clara is a 6,000 hectare wetland created in 1977 when agricultural return flows from the U.S were diverted to the Colorado delta in Mexico. The Ciénega is critical habitat for migratory birds and for resident marsh birds, including the largest population of the endangered yuma clapper rail. Operation of the Yuma Desalting Plant could affect the salinity of the inflow to the Ciénega. The Santa Clara Slough, an ephemeral hypersaline basin to the southeast, receives both tidal water from the Gulf of California and saline overflow from the Ciénega de Santa Clara. Although unvegetated, this is an important seasonal feeding station for migratory shorebirds.

We used satellite imagery from 1973 through July 2010, oxygen and deuterium isotopes from water samples, and observations from overflights to resolve the relative importance of freshwater, evapotranspiration and seawater in these adjacent wetlands.

Salinity controls the distribution of Typha domengensis (cattail), the plant that provides critical habitat in the Ciénega. The 6 ppt salinity limit is reached within the Ciénega and oxygen/deuterium isotopes confirm that the principal source of water in the Ciénega is the Colorado River. Analyses based on satellite images of this macrotidal delta show frequent tidal inundation of the Santa Clara Slough. The Ciénega’s margins are flooded a few times per year during high amplitude perigean spring tides. Observations from overflights during high tides confirm these conditions as captured by the satellite archive.

Extensive liquefaction occurred in the intertidal zone west of the Ciénega de Santa Clara as a result of strong shaking by the April 4th 2010 M7.2 El Mayor-Cucapah earthquake. Groundwater and sediments were pumped to the surface during liquefaction and localized subsidence followed. This subsidence altered pathways of tidal exchange and caused increased tidal flooding west of the Ciénega de Santa Clara. The Ciénega itself was unaffected. Preliminary observations show that the changes in tidal flows altered the pattern of shorebird usage of the area.

Water levels in the Ciénega de Santa Clara are controlled largely by the inflow of agricultural return flows; water levels in the adjacent Santa Clara Slough are controlled largely by tidal inundation and evaporation.