The Effects of Restored Aquatic Large Woody Debris Structures on Invertebrate Populations in the Napa River

Agricultural encroachment and habitat destruction within the riparian zone of many California ecosystems have created a need for restoration of stream hydrology in order to enhance and support native flora and fauna. On the Napa River, adjacent vineyards have caused the channel to become deeply incised, so that the stream now lacks geomorphic variability and biodiversity. In order to restore channel complexity, large woody debris (LWD) structures were installed in the river in the summer of 2010. In this study I evaluated the effects of installing LWD structures within the Napa River on benthic invertebrates in the first year after installation. Six 150 m study sites were sampled monthly from June 2011 to September 2011 using the kick sampling method. Areas that received LWD treatment were compared to control sites of the same habitat type. Although in-stream invertebrate diversity and abundance varied with stream geomorphology along the length of the river, in no instance did invertebrate abundance increase in the first year after installation of large woody debris. In fact, in several months LWD structures were associated with lower invertebrate abundance and diversity as well as lower dissolved oxygen. Overall, added LWD did not function as planned at base flow during the first year after installation