Benthic macroinvertebrates as indicators of biological condition below hydropower dams on west slope Sierra Nevada streams, California, USA

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AbstractOver 50 hydropower dams in California will undergo relicensing by the Federal Energy Regulatory Commission (FERC) in the next 15 years. An interpretive framework for biological data collected by relicensing studies is lacking. This study developed a multi‐metric index of biotic integrity (IBI) to assess biological condition below hydropower diversion dams on west slope Sierra Nevada streams based on benthic macroinvertebrates (BMIs). Ten streams were sampled above the upstream influence of peak reservoir storage and at five downstream sites sequentially spaced 500 m apart. Reference conditions were defined by screening upstream study sites and 77 other regional streams using quantitative GIS land use analysis, reach‐scale physical habitat (PHAB) data and water chemistry data. Eighty‐two metrics were evaluated for inclusion in the IBI based on three criteria: (1) good discrimination between reference and first downstream sites with some indication of recovery over the distance sampled; (2) sufficient range for scoring; (3) minimal correlation with other discriminating metrics. The IBI showed good discrimination between reference and downstream sites with partial recovery as distance downstream increased, and was validated with an independent dataset. Individual metrics, IBI scores and multivariate ordination axes were poorly correlated with PHAB variables across sites. When only reference and first downstream sites were evaluated, decreased IBI scores were related to lower habitat variability and substrate coarsening below dams. Lower IBI scores below dams were most strongly associated with altered hydrologic regime, especially non‐fluctuating flows as defined by the flow constancy/predictability index. Flow restoration experiments would be valuable in developing management actions that achieve a sustainable balance between conflicting human and ecological needs for freshwater. Copyright © 2008 John Wiley & Sons, Ltd.