Cienega de Santa Clara, a remnant wetland in the Rio Colorado delta (Mexico): vegetation distribution and the effects of water flow reduction

The Cienega de Santa Clara is the largest remaining wetland in the Rio Colorado delta; it supports endangered bird and fish species. The Cienega is maintained by agricultural drainage water discharge from the USA which in the future may be diverted to the Yuma Desalting Plant. We examined the existing vegetation patterns and effects of flow disruption on vegetation using seasonal aerial and ground surveys. The Cienega was dominated by Typha domingensis and contained eight subdominant hydrophytes in addtition. The distribution of marsh plants was related to salinity and water depth within the Cienega. Disturbance in the form of burning of the Typha and grazing of cattle on the new growth had a marked effect on the status of the vegetation in accessible parts of the marsh. During 8 months of unplanned flow interruption due to the need for canal repairs, 60-70% of the marsh foliage died back. Green vegetation was confined to a low-lying geologic fault which retained water; however, Typha domingensis regenerated from dormant rhizomes following the return of canal flow. Though the vegetation proved resilient, prolonged flow reduction would unavoidably reduce the size of the wetland and its capacity to support associated wetland functions.