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Mitigation and Modification Alternatives • Sufficient pool volume and pool residency time for the number of fish expected. • Operation over the proper flow range during the appropriate time period (migration periods). • Unobstructed navigation path of concentrated flow (for aesthetic; low flow and attraction flow purposes). Design criteria which evolve from these characteristics include: -" • Fishway consisting of a sloped drop followed by a slower velocity run (versus a pool). • Fishway with one or more stages (12 max) not exceeding one foot drop per stage. • Watercraft navigation capability and structural stability at all normal flow rates. • Fishway located as close as possible to the most upstream point of the diversion structure. • Minimum run dimension of 4' deep by 10' long and wide; with a velocity not exceeding 8 ft/second at flow rates between 10 and 150 cubic feet per second. • Fishway contains the majority of instream flow passing the structure at average annual low flows. • A staging area at the base of the fishway with a minimum depth of 4 feet and an attractor velocity of 4 to 8 feet per second. • Minimum flow depth through fishway of 1 foot. These design parameters are based upon limited knowledge of the Colorado Squawfish and upon fishways developed for other similar fish species about which more information is available. In particular the following Colorado Squawfish characteristics are important to note: • Upstream migration occurs in a mid-July through August time frame and consists primarily of fish from 500-700 mm in length. • The potential number of fish to be provided for in passage is unknown due to their low population density. • Their swimming characteristics are believed to be much the same as the Northern Pike, White Sucker, Flannelmouth Sucker and the Roundtail Chub, all of which successfully navigate the City of Craig diversion structure. CONSTRUCTION COSTS For the purpose of establishing the general magnitude of costs associated with diversion structure modifications, conceptual-level cost estimates (January, 1995) were prepared for 4-11