Application of Automated Riparian Vegetation Recruitment Potential Model for Habitat Enhancement Pilot Study

The alteration of flow regimes and landscapes has led to a decline in riparian forests across western North America. River restoration designs often include plantings to address the loss of riparian vegetation. Plant survival may be assessed based on estimated groundwater and scour dynamics. Plantings are labor intensive, expensive to implement, and have high risk of failure. A sustainable outcome of habitat enhancement projects requires the consideration of natural stresses and mortality going beyond plantings alone to include a process for natural riparian recruitment. This study involved development and application of a new software tool integrated into the River Architect analysis software to evaluate a river projects riparian recruitment potential. In the code, two-dimensional (2D) hydraulic numerical model results are paired with historical flow records to identify optimal site locations and hydrologic patterns that promote Fremont cottonwood (Populus fremontii) recruitment. Seasonally sensitive physical parameters are accounted for to determine if all requirements of cottonwood seeds necessary for germination and subsequent seedling survival are met. The algorithm considers the timing of peak flows required for bed preparation in relation to seed dispersal, stress and mortality due to stage recession rates after germination, the elevation of recruited seedlings above the base flow stage, and the scouring or uprooting and the inundation seedlings experience in their first year. The algorithm was applied to assess Fremont cottonwood recruitment potential for 2017 baseline conditions and three design alternatives at the Long Bar site on the lower Yuba River in northern California.