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Responses of Aquatic Macroinvertebrates to Stream Channel Reconstruction in a Degraded Rangeland Creek in the Sierra Nevada

Streams on western rangelands are sometimes degraded to the point that reestablishing lost ecological values requires rebuilding their physical structure, shape, and form. We evaluated the biological response to reconstruction of a small rangeland stream channel through comparisons of aquatic invertebrate communities before and after engineering activities and in relation to local and regional reference stream conditions. We measured geomorphic and riparian habitat features along with benthic macroinvertebrates for two years prior to restoration actions and for two years afterward. Stream restoration activities included the construction of a new channel to replace an incised meadow stream reach (including addition of coarse rock substrate, erosion control fabric, and willow planting) and the rehabilitation of gullies and roads in the meadow and its watershed. In postproject monitoring, we found statistically significant changes in the macroinvertebrate community and trophic structure at the restored site. These were exhibited as increases in EPT taxa (mayflies, stoneflies, caddisflies) and the proportion and diversity of sensitive taxa, decreased tolerant taxa, and an increase in consumers of riparian organic matter (shredders) and decrease in fine organic particle filter-feeders. A site monitored downstream of the restoration activities exhibited increased deposition of fines and sand one year after project construction, but was similar to preproject conditions in the second year, and the macroinvertebrate community was unchanged. Improved biological integrity at the restored site showed that rapid recovery can occur in rebuilt streams within rehabilitated watersheds, but neither degraded nor enhanced conditions were transferred to downstream habitat, at least over the initial postproject period.

Keywords: livestock grazing impacts, macroinvertebrate bioassessment, rangeland improvement, Sierra Nevada, stream restoration