Introduction

Surrounding land use heavily influences river systems (Hawley et al., 2013; Navratil et al., 2013; Vietz et al., 2014). Specifically, pressures from urban development contribute to changes in water and sediment delivery. This effectively alters the flow mechanics and sediment dynamics of river systems. To maintain fluvial geomorphological connectivity and aquatic ecological function (i.e., fish passage) in ecosystems impacted by urban development, river restoration is required (Ward and Standford, 1995; Clarke et al., 2003; and Jasson et al., 2007). River restoration practices, including in-stream vortex rock weir (VRW) construction, are implemented as a design component to maintain overall channel form and function (Gilvear, 1999; Clarke et al., 2003; Klingeman et al., 2004). Rock weirs are increasingly preferred because they provide physical channel stability, habitat enhancement, and the hydromechanics to allow upstream fish passage for local fish species (Corwin et al., 2007). Rock weirs have conflicting goals, where they are installed as a high flow feature to maintain channel stability but provide conditions for low flow processes such as fish passage (Thomas et al., 2000).
Rock weirs vary in gradient, structure, and geometry, and are uniquely tailored to site-specific conditions depending on the local channel morphology, hydrologic regime, and target fish communities. Many studies evaluate fish passage effectiveness through conventional fishways (Bice et al., 2017; Romao et al., 2018; Zhang and Chanson, 2018), however, few experiments consider the irregularity of natural materials and the unique design of rock weirs for fish passage effectiveness (Dodd et al., 2017). Further, many conventional fishways are designed for large-bodied fish species on large-scale river systems, and as such, few studies evaluate fish passage effectiveness for small-bodied fish species on small-scale systems. According to Hatry et al. (2013), only 9% of all documented fishways in Canada (211) are evaluated using appropriate methods to understand geomorphological and ecological effectiveness. It is important to note that of the 211 documented fishways in Canada, most are applied on large-scale systems and target large-bodied fish species.
The purpose of this research was to evaluate the effectiveness of rock weirs for small-bodied fish passage in Weslie Creek. Field assessment was completed under varying water level conditions to observe rock weir structural geometry and flow. Rock weir structural geometry and flow characteristics were applied to identify fish passage effectiveness based on providing appropriate velocities through gap and over-weir flow pathways. The structural dimensions of the rock weirs were assessed to understand how design and construction influences fish passability and opportunities for fish refuge. Determining the critical design and construction components to facilitate fish passage through rock weirs, while also ensuring channel stability, will inform future river restoration works. This work will also provide recommendations for balancing opposing goals, such as fish passage and channel stability on a small-scale, in natural channel design.