Re-restoration on the South Platte repairs flawed attempt at restoration

摘要

A flawed attempt at fishery enhancement caused great disappointment for this private property owner on the Middle Fork of the South Platte near Fairplay, Colo. A consultant had attempted to enhance the fishery by creating deep pools and complex habitat features. More than 200 large boulders were added to the stream in various configurations, and unnaturally deep holes were dredged in the channel. Many of the side channels, which historically served as flood relief channels and backwater spawning habitat, also were filled with the dredged material. At the property owner's request, Trout Headwaters, Inc. assessed the site and documented what the property owner suspected: the flawed design and installation had resulted in a poorly functioning stream system.The placement of boulders and dredge and fill activities had restricted the floodplain; and decreased water velocity caused fine sediments from upstream to accumulate in deepened pools. The altered hydrologic regime triggered increased erosion and reduced the width of the riparian corridor. In addition, the majority of backwater habitat had been removed by filling braided channels, and large boulders placed across the entire stream channel altered the natural riffle/run ratio. This project is what THI refers to as "re-restoration", where a prior attempt at stream enhancement and restoration has actually caused harm to the system. To achieve a stable and self-maintaining channel morphology, with riffle and pool elevations and ratios that can be naturally maintained by spring runoff, more than 90 instream boulders imported by the prior consultant were adjusted in location and elevation. To minimize site disturbance, a modified excavator with large rubber tires and fully rotating arm, nicknamed "the spider," was used to carefully reach and place materials without causing the problematic soil compaction of heavier, track excavating equipment. The new design and installation allows fine sediments to be transported through the channel without active maintenance, and prevent pools and spawning gravels from acting as sediment traps. Point bars were re-graded and side channels reopened to allow floodplain access, promote flood relief, and restore backwater spawning habitat. To maintain these desirable morphologies, THI installed bank stabilizing erosion control fabrics, toe stabilization materials, and willow plantings to provide structural root support along suitable reaches. Revegetation of these banks will increase biodiversity, add shade and cover, and provide ideal edge habitats for fish.