Cooperative Fish and Wildlife Research Units Program: all
Education, Research and Technical Assistance for Managing Our Natural Resources

Flow regulation, water abstraction, and riparian vegetation encroachment are causing habitat simplification with severe consequences for native fishes. We experimentally added wood to simplified habitat in a large, arid-land river to assess the effectiveness of enhancing simplified habitat. We added wood at 19 paired treatment and reference (no wood added) subreaches (50 - 100m) within the main channel of the San Juan River (SJR). Using a before-after-control-impact design, we sampled fishes and macroinvertebrates, and quantified habitat complexity. After wood addition, native fish densities were 2.2x higher in treatments compared to references, whereas nonnative fish densities exhibited no response. Macroinvertebrate densities were 6.8x higher, and habitat variation increased in treatments. The number of geomorphic features in treatments increased from 1 to a maximum of 11 following wood addition, whereas the number of features in references remained unchanged. The results suggest wood addition as a viable management strategy, enhancing native fish habitat by increasing food sources and facilitating habitat complexity at fine scales. As flow management faces challenges due to water overallocation and increased aridity, managers may consider integrating non-flow alternatives like abundant, invasive wood addition to improve habitat conditions.