Impacts of Regulated Flows on Riparian Cottonwood Seedling Recruitment in The South Fork Boise River, Usa

Pioneering riparian vegetation, like cottonwood trees and willows, are highly productive and important species for riparian and aquatic ecosystems. Large riparian vegetation provides benefits for aquatic and riparian species that include shading and maintaining favorable water temperature for fish, introducing seasonal sources of carbon and nutrients, providing nesting and foraging habitat, and introducing physical complexity from large woody debris in the channel. To investigate long-term sustainability of riparian forests below water management infrastructure like dams and diversion, we developed a coupled two-dimensional (2D) hydraulic and riparian vegetation recruitment model to predict favorable areas for cottonwood and willow seedling recruitment in the South Fork Boise River, Idaho, USA. We analyzed the influence of reservoir flow releases for average (WY 2016) and wet climatic conditions (WY 2017) on successful riparian vegetation seedling recruitment. The validated model simulated favorable areas with field surveyed vegetation seedling recruitment patches. Our results suggest high reservoir releases during a wet climatic year (WY 2017) can be used to create natural recruitment that may not be possible during an average year (WY 2016). This study highlights the importance of adaptive management and of leveraging inter-annual climatic variability to develop flow releases that benefit downstream riparian ecosystems and minimize impacts of dam regulation.