Restoring riverine resilience: Using historical ecology to reconstruct eco-geomorphic complexity on an intermittent river, lower Santa Clara River, Ventura County, CA

Background/Question/Methods

The maintenance and creation of resilience has been suggested as one of the most effective ways to manage ecosystems in the context of climate change. This is particularly true for extremely dynamic ecosystems such as Mediterranean-climate rivers, which were naturally resilient to highly variable climatic and hydrologic conditions in the recent past. Research on riverine systems of coastal California has demonstrated that historical ecology can be a powerful tool for understanding the attributes that made ecosystems resilient to climatic variability in the recent past, suggesting potential future landscape trajectories.

The lower Santa Clara River (Ventura County, California) is a flashy alluvial river that is the focus of ongoing conservation and restoration efforts. We designed a historical ecology study to provide context for these efforts by answering fundamental questions about the river’s history, such as the magnitude of floodplain reclamation, the extent of riparian forest, and the presence/absence of dry season flow. We integrated hundreds of historical cartographic, textual, and visual accounts to reconstruct ecological and hydrogeomorphic characteristics of the river prior to major modification (early 1800s). Data were collected from over 30 archives, assessed for accuracy and reliability, and synthesized into a GIS database of early conditions.

Results/Conclusions

Our reconstruction documented an extremely broad former river corridor that supported a diverse mosaic of riparian communities, from extensive willow-cottonwood forests to xeric scrublands and including several large (>100 ha), discontinuous nodes of forested wetlands. This reach-scale ecological heterogeneity was linked to local variations in dry season water availability, with approximately 70% of the river sustaining summer surface water and the remainder going seasonally dry. Comparison to contemporary conditions indicates a loss of nearly half (49%) of the 19^th century river corridor extent, including a dramatic reduction in riparian habitat and floodplain area, though historical flow patterns have remained somewhat intact. These findings enrich our understanding of how the lower Santa Clara River provided reliable refuge and habitat for target species that persisted through major flood and drought events. This functional, spatially explicit approach helps identify opportunities for reestablishing similar attributes to enhance system resilience in the future. These findings suggest that different restoration strategies may be appropriate for different reaches, rather than a “one size fits all” approach. Far from being irrelevant, history is a valuable tool that can suggest new strategies and opportunities for designing resilient, climate-adaptive systems in a time of global change.