1 | INTRODUCTION
Instream structures have been employed to restore streams and improve fish habitat for more than century, yet the design of many structures has changed very little over that time (Thompson, 2005). Stream restoration and habitat enhancement projects have utilized a variety of structures designed to reduce erosion and improve aquatic habitat (Miller & Kochel, 2010). These structures can generally be classified as dams, deflectors, or bank protection/cover, and are typically constructed from boulders, logs, and sediment (Thompson, 2005). Projects have used structures to address a variety of objectives, including bank erosion, riparian vegetation, grade control, recreational boating, flow diversions, fish habitat, reintroduction of large wood, infrastructure protection, flooding, sediment transport, energy dissipation, and other stream functions (Rosgen, 2011; Wohl, Lane, & Wilcox, 2015). Some structures are designed to enhance aquatic habitat by providing velocity refuge, overhead cover, improved over-winter habitat, more profitable feeding positions, and increased habitat complexity (Roni, Hanson, & Beechie, 2008; Richer, Gates, Kondratieff, & Herdrich, 2019). Although stream restoration has grown into a multibillion dollar business (Cockerill & Anderson, 2014), the effectiveness of treatments is rarely evaluated for most projects (Bash & Ryan, 2002; Roni, Aberg, & Weber, 2018). Understanding the effectiveness of stream restoration structures will inform the design, evaluation, and expectations for future projects.
This case study evaluates a stream restoration project in the headwaters of the Arkansas River near Leadville, Colorado. The project was intended to rehabilitate and enhance aquatic habitat for a 17.7-km reach of the Arkansas River and Lake Fork to restore natural resources equivalent to those injured by the release of hazardous substances from the California Gulch Superfund Site (Stratus Consulting Inc., 2010a). The primary goal of restoration was to improve instream aquatic habitat and increase Brown Trout Salmo trutta populations. Restoration treatments were designed to stabilize streambanks, reduce the risk of erosion at fluvial tailings deposits, create diverse stream morphology including deep-water pools for overwinter habitat, provide overhead cover for trout, increase spawning habitat, and provide refuge for juvenile trout. Monitoring targets included instream structures, fish populations (Richer et al., 2022), riparian vegetation (Cubley et al., 2022), benthic macroinvertebrates (Clements, Vieira, & Church, 2010; Wolff, Duggan, & Clements, 2019), and habitat quality (Richer, Gates, Kondratieff, & Herdrich, 2019). The goal for instream structures was that at least 90% of all habitat improvement structures be stable and functional three years after implementation (Stratus Consulting Inc., 2010b). The objectives of this study were to evaluate structure performance by evaluating (1) the integrity and function of instream structures and (2) the change in pool depths following completion of instream construction.
2 | METHODS