COMPARISON OF FLOW‐RELATED HABITAT EVALUATIONS DOWNSTREAM OF LOW‐HEAD WEIRS ON SMALL AND LARGE FLUVIAL ECOSYSTEMS

The focus of within-channel restoration is the placement and construction of instream habitat structures to enhance the capture of organic detritus and aufwuchs, as well as, colonization by macroinvertebrate and fish species. Structural design is based upon the assumption that these habitat requirements can be controlled through the design of structures that produce preferred physical and chemical conditions in relation to flow conditions. Restoration scientists are assuming that hydraulic conditions are one primary template that govern the distribution of lotic organisms. For benthic macroinvertebrates, substrate composition is the most easily manipulated habitat characteristic. The most common structures for fish habitat enhancement have been current deflectors, overpour structures (dams and weirs) and instream cover, especially for juveniles. These instream structures also modify local hydraulic conditions to present preferred habitat to benthic invertebrates. The physical habitat simulation (PHABSIM), a software package used in the instream flow incremental methodology, was used to evaluate stream enhancement activities on a low-order stream, with the placement of a series of three-log weirs on Brushy Branch, a second-order stream in Tennessee, and compared with published results of a hydraulically similar concrete structure on a large-order system used to re-regulate flows downstream of peaking hydropower facility on the Cumberland River, Tennessee. On Brushy Branch, the simulation demonstrated that benthic macroinvertebrate habitat can be dramatically increased at low flows (up to five times higher) after placement of structures that improve hydraulic conditions to sustain maximum diversity of the benthic community. In this case, then, the structures acted to augment habitat under low flow conditions. Reregulation dams, on large rivers, modify the water surface elevation and dampen high velocities, which enhances habitat for juvenile and adult salmonids when subject to the high discharge surges of peaking generation. Thus, these low-head structures augment habitat under high flow conditions. Hydraulic habitat models, then, can be a useful tool to evaluate the benefit of certain restoration activities and, in the case of weir-like structures, indicate that similar structures impart similar benefit, regardless of scale of application.