Hydraulic Features of Engineering Log Jams and Their Influence on Salmonid Behavior

In an effort to recreate channel complexity and habitat, construction of Engineered Log Jams (ELJs) is increasing.  However, while research indicates that fish do use the structures, a lack of direct observations of flow hydraulics and fish use around the structures results in questions regarding how, why, and when fish use ELJs.

We surveyed three forms of ELJs in the cascades of western Oregon, with the objective of identifying hydraulic regions downstream of ELJs and of relating fish (cutthroat trout) behavior to the fluid behavior in those regions. The structures and near-structure stream environments downstream of the structures were surveyed for bathymetry, were snorkeled every four hours for a 24-hour period during the summer to observe the behavior of salmonids, and 3D velocities were measured using an Acoustic Doppler Stream Profiler (ADCP). Based on these observations, velocities were plotted as vector and tensor fields to visualize circulation and flow intensity in and around the ELJ structures. We then mapped regions defined by hydraulic features (e.g. flow convergence, flow divergence, deep plunge pools, high or low turbulence, and cover-hydraulic combinations) and regions defined by fish behaviors (e.g. feeding, resting, aggression).  Fish species, size, location and depth, orientations, time of day, and behaviors were mapped onto channel bathymetry. 

We found that fish avoided areas of intense velocity, high turbulence, and complex circulation. Variation in the size of cutthroat trout was present in areas with greater variation in the velocity, circulation, depth, and cover.  The greatest variety in sizes of trout were observed in pools that offer both deep/fast-flowing and shallow/slow-flowing areas. Areas with cover and low velocities were used for feeding during the day and resting at night, particularly near the bed of the channel.  Areas of flow divergence and convergence were used for feeding, with fish oriented into the flow in areas of higher velocities. Boulders appeared to generate important hydraulic diversity; Fish were consistently observed feeding and resting upstream, adjacent to, and downstream of boulders, during both the day and night, because of the combination of overhead cover, fast and slow velocities, and vertical mixing.  Protruding logs appear to offer similar habitat benefits as boulders at our study sites.  In addition, slowly circulating flows were utilized extensively by all sizes of fish at night.