Long-term Outcomes of Natural-process Riparian Restoration on a Regulated River Site: The Rio Grande Albuquerque Overbank Project after 16 Years

Abstract

In 1998, a riparian restoration demonstration project was initiated with a target of efficiently establishing a dynamic patch mosaic of vegetation communities along a regulated river using available water and sediment and remaining natural hydrological processes. A point bar along the Middle Rio Grande, Albuquerque, New Mexico, dominated by the nonnative shrub Elaeagnus angustifolia (Russian olive), was mechanically treated by removing all vegetation and lowering a portion of the bar to allow overbank flooding during typical spring releases from an upstream dam (Cochiti Dam). Side channels and small islands were engineered in the lowered bar to slow flood waters, aid sediment deposition, and add site complexity. After treatment, a high-resolution monitoring grid was installed to track vegetation changes. Following an initial flood in the spring of 1998, over 10,000 cottonwoods per ha naturally established, but densities varied based on the fluvial landforms. Zones that were sufficiently wetted or naturally formed behind large woody debris were the most successful, while the artificial fill zone and the portion of the bar not lowered had the least native riparian tree recruitment. Over 15 years, cottonwood numbers declined through intraspecific competition and beaver browsing at all sites, but they continued to dominate. Natives also dominated a species-rich herbaceous layer, particularly on the lowered sites. The incursion of a new herbaceous invader, Saccharum ravennae (ravennagrass), was an unexpected outcome revealed by the long-term monitoring record. Yet, based on several criteria, the site reflects a successful application of a natural-process approach to restoration that can lead to increased ecosystem complexity and resilience.