The unique Mediterranean climate and geography of California contributes to predictable flow patterns in its rivers, enabling researchers to discern key hydrologic metrics indicative of stream health for diverse species. The California Environmental Flows Working Group (CEFWG) introduced the California Environmental Flows Framework (CEFF) in 2021 to provide a holistic approach to analyze environmental flows throughout the diverse state. CEFF identifies five functional flow components described by 23 functional flow metrics that capture historical environmental functionality. However, challenges arise when applying these metrics to less predictable flashy streams or highly altered river systems, such as those impacted by dams or diversions, resulting in inaccurate assessments. The alterations to California's hydrology through dam construction, water transfers, and reservoir management have significantly modified natural flow regimes, affecting downstream ecosystems and native species like salmon. Improved management of streamflow is crucial for sustainable water use and the preservation of native ecosystems.

To address these challenges, the Functional Flow Calculator - Flashy (FFC-F) was developed to accurately calculate functional flow metrics in naturally flashy streams and highly altered systems. Unlike the original reference-based Functional Flow Calculator (FFC-R), which performs well in streams with natural predictable seasonal flows but struggles with accurate assessments in highly altered systems, the FFC-F minimizes data smoothing and relies on abrupt changes in flow to identify seasonal transitions. Through an alternate functional flow identification algorithm, the FFC-F targets rapid rates of change or flat-lined flows, common in altered basins or less predictable rain-driven streams. By eliminating extensive data filtering, the FFC-F captures the nuanced flow dynamics characteristic of these systems, offering a more accurate representation of functional flow components. The FFC-F thus provides a valuable complementary tool within CEFF for regulators, planners, and dam operators to navigate the complexities of managing environmental flows in a rapidly evolving hydrological landscape.