TWO YEARS AFTER THE FLOW: HYDROLOGIC AND ECOLOGICAL EFFECTS OF THE 2014 ENVIRONMENTAL FLOW TO THE COLORADO RIVER DELTA

Thanks to a five-year, binational agreement between the US and Mexico (Minute 319 to the US-Mexico Water Treaty of 1944), 130 million cubic meters of Colorado River water was released to the mostly dry riparian zone of the Colorado delta during eight weeks in the spring of 2014. The environmental flow was only one part of a complex agreement. Conservation NGOs are implementing restoration activities and a binational team of agency, university and conservation NGO scientists is monitoring the flow’s effects. The environmental flow inundated 1,600 ha of the floodplain and terraces and connected the river from the international boundary to the sea for the first time since 1998. Peak flows attenuated downstream and 94% of the flow infiltrated and recharged the underlying aquifers. The water table rose as much as 9 m and mostly returned to pre-flow levels within 6 months. The flow did not widen the channel nor scour or bury much of the existing vegetation. Native, woody riparian species established with highest densities in areas where water was delivered through the canal system and that had been prepared by clearing non-native vegetation and contouring channel banks in advance of the flow. Surface and subsurface flows prompted a 16% increase in overall riparian “greenness” (NDVI) in 2014 compared to 2013, following a 10-year decline. 2015 levels were similar to pre-flow NDVI values. Bird diversity and abundance increased in 2014 and 2015 over pre-flow levels. Bird diversity and abundance were highest in the restoration sites. The flow that reached the estuary had minimal effects. The social response to the flow was positive, as demonstrated by local celebrations, media coverage, and fund-raising. Ecological restoration was maximized by site preparation, the use of irrigation canals to deliver environmental flows, and careful application of supplemental water. The current agreement expires in late 2017. Future environmental flows depend on the outcome of ongoing binational negotiations over several aspects of transboundary water management.