Regeneration of native trees in response to flood releases from the United States into the delta of the Colorado River, Mexico

Over the past 20 years, discharge of water from the United States to the delta of the Colorado River in Mexico has regenerated native trees that now account for 23% of vegetation in a 100-km, non-perennial, stretch of river below Morelos Dam at the United States–Mexico border. The discharges are associated with the filling of Lake Powell, the last large reservoir to be constructed on the river, and with ENSO cycles that bring extra winter and spring precipitation to the watershed. The discharges below Morelos Dam produce overbank floods that germinate new cohorts of Populus fremontii andSalix gooddingii trees. Relatively little flood water from the United States is required to support a pulse flood regime that can result in regrowth of native vegetation in the delta. Based on analysis of past flows and existing tree populations, we estimate that a February–April flow of 3×109m3at 80–120 m3s−1is sufficient to germinate and establish new cohorts of native trees. However, there was a positive correlation between frequency of flows and total vegetation cover over the years 1992–1999, showing that more frequent flows would further increase vegetation cover. The results support the importance of pulse floods in restoring the ecological integrity of arid-zone rivers