Restoration Treatments to Improve Native Riparian Tree Establishment Following Environmental Flow Releases in the Colorado River Delta, Mexico

Schlatter, Karen Jean

Graduate Thesis Or Dissertation

Restoration Treatments to Improve Native Riparian Tree Establishment Following Environmental Flow Releases in the Colorado River Delta, Mexico Public Deposited

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Citeable URL: https://scholar.colorado.edu/concern/graduate_thesis_or_dissertations/c534fp27c

Abstract

In the mid-1900s, the Colorado River Delta in Northwest Mexico stopped regularly receiving freshwater flows due to the diversion of Colorado River water for human consumptive use. Despite the highly altered hydrologic regime and resulting desiccation of 90% of Delta habitat, sporadic flood flows in the 80s and 90s regenerated riparian habitat along the river corridor in Mexico, suggesting ecosystem resilience. Since 2001 however, ongoing drought has led to a steady decline in ecosystem conditions in the Delta. In 2014, an experimental pulse flow of water was delivered to the Colorado River in Mexico as part of the U.S.-Mexico binational agreement, Minute 319, to determine if a designed flow could improve biologic and hydrologic conditions with a relatively small amount of water. Laguna Grande, a target restoration site in the central Delta, was not expected to receive high-magnitude flows needed to create conditions for riparian tree recruitment. To improve recruitment, areas were cleared of tamarisk (Tamarix spp.) and arrowweed (Pluchea sericea) prior to the pulse flow, and some areas were seeded with Goodding’s willow (Salix gooddingii) and Fremont cottonwood (Populus fremontii) during flow recession to promote riparian tree establishment. A controlled, replicated experiment was implemented to assess effects of four restoration treatments on riparian tree species recruitment: 1) control – no inundation from pulse flow, 2) inundated, 3) inundated and cleared, and 4) inundated, cleared, and seeded. Vegetation was monitored in March, May, and October of 2014 (before, immediately after, and one growing season following the pulse flow). Only cleared (3) and seeded (4) treatments had successful cottonwood-willow establishment, and establishment was affected by site conditions and flow delivery hydrology. Mixed tamarisk and native species established in cleared areas, suggesting the persistence of novel plant communities following restoration treatments and flow deliveries in Laguna Grande. In this highly altered riparian system, components of resilience such as cross-scale interactions and connectivity greatly depend on water management decisions and restoration actions. Studies such as this one provide a quantitative assessment of restoration outcomes that can inform decision makers and feed into adaptive management of restoration sites to achieve riparian resilience.

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