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<br />Introduction <br /> <br /> <br />This report summarizes the final results of fluvial geomorphic research conducted <br /> <br /> <br />along the North Fork Cache Ia Poudre River (herein referred to as the North Fork) below <br /> <br />Halligan Reservoir. Since September 1996, when approximately 7,000 m' of clay- to silt- <br /> <br /> <br />sized sediment were released from Halligan Reservoir into the North Fork, two research <br /> <br /> <br />projects have been conducted to investigate the effects of a massive sediment release on <br /> <br /> <br />the downstream channel. One project focused on channel sedimentation and subsequent <br /> <br /> <br />sediment movement out of the North Fork over time (see report by E. Wohl, November <br /> <br /> <br />1997). The focus of the second project was to assess the ability of one-dimensional <br /> <br /> <br />sediment transport models to simulate observed patterns of scour and deposition within <br /> <br /> <br />pools (see report by S. Rathburn. February 1997). <br /> <br /> <br />Two goals of this tinal report are to I) address sediment management issues <br /> <br /> <br />within Halligan Reservoir and the North Fork below the dam to help mitigate existing <br /> <br /> <br />sedimentation hazards, and 2) to prevent future sedimentation hazards. Although <br /> <br /> <br />sediment accumulation in Halligan Reservoir will continue to atkct the storage capacity <br /> <br /> <br />within the reservoir. indiscriminate releases of sediment have been shown to be <br /> <br /> <br />detrimental to the downstream ecosystem, One-dimensional sediment transport modeling <br /> <br /> <br />conducted during the second project facilitated the evaluation of two flo",; scenarios that <br /> <br /> <br />resulted in substantial tlushing of sediments out of pools within the North Fork. <br /> <br />Effects of Sediment Release on Channel Morphology <br /> <br /> <br />Pools were filled to varying degrees for 12 km below Halligan Reservoir by clay- <br /> <br /> <br />to gravel-sized sediment released in September 1996, The degree of pool intilling <br /> <br /> <br />decreased downstream. as did the grain size of material, in accordance with diminished <br /> <br /> <br />transport capacity of the tlow, At a distance of 500 m below the dam, pools were filled <br /> <br /> <br />completely, up to 3.5 m deep, with coarser sediment (graveI- to clay-sizes), At 3 km and <br /> <br /> <br />8 km downstream pool volume loss was 50% and 30%, respectively, with sediments <br /> <br /> <br />primarily silt and clay in size, <br /> <br />.\ <br />