Abstract
Large state or regional environmental flow programs, such as the one based on the California Environmental Flows Framework, rely on broadly applicable relationships between flow and ecology to inform management decisions. California, despite having high flow and bioassessment data density, has not established relationships between specific elements of the annual hydrograph and biological stream condition. To address this, we spatially and temporally linked USGS gage stations and biological assessment sites in California to identify suitable paired sites for comparisons of streamflow alteration with biological condition at a statewide scale. Flows were assessed using a set of functional flow metrics that provide a comprehensive way to compare alteration and seasonal variation in streamflow across different locations. Biological response was evaluated using the California Stream Condition Index (CSCI) and Algal Stream Condition Index (ASCI), which quantify biological conditions by translating benthic invertebrate or algal resources and watershed-scale environmental data into an overall measure of stream health. These indices provide a consistent statewide standard for interpreting bioassessment data, and thus, a means of quantitatively comparing stream conditions throughout the state. The results indicate that indices of biological stream condition were most closely associated with flow alteration in seasonality and timing metrics, such as fall pulse timing, dry-season timing, and wet season timing. Magnitude metrics such as dry-season baseflow, wet season baseflow, and the fall pulse magnitude were also important in influencing biological stream conditions. Development of ecological flow needs in large-scale environmental programs should consider that alteration to any of the seasonal flow components (e.g., dry-season baseflow, fall pulse flow, wet-season baseflow, spring recession flow) may be important in restructuring biological communities.
Highlights
Flow alteration is a pervasive and significant issue globally and in California, where over 95% of California’s gaged streams have altered flow (Poff et al, 2007; Grantham et al, 2014; Zimmerman et al, 2017)
Despite a relatively large pool of sites to work with, after filtering and pairing, we identified 233 Algal Stream Condition Index (ASCI) and 231 California Stream Condition Index (CSCI) sites associated with 222 USGS gages across the state
Three of the top metrics for CSCI related to timing, while the remaining significant metrics were associated with flow magnitudes (Table 1, Figure 5)
Summary
Flow alteration is a pervasive and significant issue globally and in California, where over 95% of California’s gaged streams have altered flow (Poff et al, 2007; Grantham et al, 2014; Zimmerman et al, 2017). While the causes and related impacts of flow alteration are well documented (Poff et al, 2007), significant gaps remain in linking flow management with ecological responses to track current stream conditions, evaluate restoration efficacy, and provide future flow recommendations (Poff and Zimmerman, 2010). A critical component of developing ecological flow needs for management is to identify relationships between specific flow metrics that represent distinct characteristics of the annual hydrograph and measures of biological stream conditions at broad spatial scales (Poff and Zimmerman, 2010). Evaluation of direct relationships between individual flow metrics and biologic response across broad spatial scales and assessment of whether such relationships provide a means of quantitatively comparing stream conditions across large regions remains limited
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