Stream Habitat Characterized by Stage -Specific Flows and Three-Dimensional Geomorphological Complexity: Development of Ecological Criteria for Stream Restoration Design

Abstract

In a low-gradient, third-order stream in East-Central Illinois, a new classification scheme for mesohabitat units was developed based on three-dimensional geomorphological complexity, and associated flow structures at low- and high-flow stages. In this two-tier classification scheme, the term fluvial habitat unit was used because fluvial refers to both form and flow. At low-flow stages nine habitat units were identified; they included pool-front, pool-mid, pool-rear, scour pool, glide, riffle with and without a raceway, point bar platform, and a lateral area in the lee of an obstruction. At high-flow stages where flows are contained in the active channel, four habitat units were identified as the channel thalweg, deflection eddy, expansion eddy, and local hydraulic dead zone at margins. Near bankfull flow, floodwaters inundate geomorphic features in a channel-floodplain transition area, forming three additional high-flow units, which included the concave-bank bench, vegetated point bar platform, and floodplain slough. Ecologically, fluvial habitat units represented important habitat patches that provide for some biological need of fish, such as feeding, spawning, cover, temperature refuge, and flow refuge. Organized by season and flow stage, relationships between these biological needs and fish use of fluvial habitat units were quantified using prepositioned areal electrofishing devices (PAEDs). A new PAED design was developed using rod electrodes so that a predictable electrical field immobilizes fish for capture. PAEDs were successfully used to capture fish in specific habitat unit types, varying in shape and size. Patterns of fish habitat use were summarized from three main studies including (1) monthly samples at low-flow stages for three years, (2) extensive sampling during summer low-flow, and (3) a five-day sample during a single high-flow event. In summary, ecological relationships developed in my research illustrated the importance of the pool-riffle sequence in maintaining diversity of the fish community, and the importance of bed and bank morphology during high-flow stages to form hydrodynamic conditions necessary for fish refugia. Patterns of fish habitat use associated with specific geomorphic features of the channel support the development of ecological criteria for habitat-based stream restoration design.