COMPARISON OF FLOW-RELATED HABITAT EVALUATIONS DOWNSTREAM OF LOW-HEADWEIRS ON SMALL AND LARGE FLUVIAL ECOSYSTEMS

The focus of within-channel restoration is the placement and construct ion of instream habitat structures to enhance the capture of organic d etritus and aufwuchs, as well as, colonization by macroinvertebrate an d fish species. Structural design is based upon the assumption that th ese habitat requirements can be controlled through the design of struc tures that produce preferred physical and chemical conditions in relat ion to flow conditions. Restoration scientists are assuming that hydra ulic conditions are one primary template that govern the distribution of lotic organisms. For benthic macroinvertebrates, substrate composit ion is the most easily manipulated habitat characteristic. The most co mmon structures for fish habitat enhancement have been current deflect ors, overpour structures (dams and weirs) and instream cover, especial ly for juveniles. These instream structures also modify local hydrauli c conditions to present preferred habitat to benthic invertebrates. Th e physical habitat simulation (PHABSIM), a software package used in th e instream flow incremental methodology, was used to evaluate stream e nhancement activities on a low-order stream, with the placement of a s eries of three-log weirs on Brushy Branch, a second-order stream in Te nnessee, and compared with published results of a hydraulically simila r concrete structure on a large-order system used to re-regulate flows downstream of peaking hydropower facility on the Cumberland River, Te nnessee. On Brushy Branch, the simulation demonstrated that benthic ma croinvertebrate habitat can be dramatically increased at low flows (up to five times higher) after placement of structures that improve hydr aulic conditions to sustain maximum diversity of the benthic community . In this case, then, the structures acted to augment habitat under lo w flow conditions. Reregulation dams, on large rivers, modify the wate r surface elevation and dampen high velocities, which enhances habitat for juvenile and adult salmonids when subject to the high discharge s urges of peaking generation. Thus, these low-head structures augment h abitat under high flow conditions. Hydraulic habitat models, then, can be a useful tool to evaluate the benefit of certain restoration activ ities and, in the case of weir-like structures, indicate that similar structures impart similar benefit, regardless of scale of application.