HYDROLOGIC MODIFICATION FROM HYDROELECTRIC POWER OPERATIONS IN A MOUNTAIN BASIN

The natural flow paradigm suggests that components of the natural streamflow regime and variability should be managed to maintain important ecosystem functions and services. Mountain rivers can exhibit extreme flow variability and provide critical aquatic habitat and ecosystem services but can be severely impacted by hydroelectric power (HEP) development and operations that will likely increase in the future. The hydrologic modification from HEP operations in the Upper San Joaquin River Basin, California, was evaluated across 15 river and stream locations throughout the basin. Flow modifications in Bear Creek, an otherwise unimpacted high‐elevation subbasin, were evaluated in detail using a number of hydrologic metrics, including Indicators of Hydrologic Alteration (IHA), environmental flow components of IHA, flow duration curves, ecodeficit and comparisons using equivalence testing. The uncertainty of the metrics based on confidence intervals was also evaluated for unimpaired (upstream) and existing (impaired or downstream) conditions. Results showed that metrics for median values changed considerably for most locations under impaired conditions, but the direction and extent of change varied depending on the location and flow metric. Metrics for variability (coefficients of dispersion) changed even more. Most metrics showed that flow modifications in Bear Creek were substantial, including decreases in high flows and increases in most low‐flow metrics. However, some flow variability metrics increased because of large flood flows during several years overwhelming and bypassing the dam/diversion structure. Uncertainty in metrics varied considerably throughout the basin but generally increased for impaired conditions. Uncertainty should be explicitly considered when evaluating hydrologic modification from HEP in mountain watersheds. A number of metrics should be used depending on objectives and spatial scale, including a subset of key IHA metrics across multiple sites and other methods to provide detailed information on flow modification in conjunction with other environmental flow assessment techniques at key locations. Copyright © 2011 John Wiley & Sons, Ltd.