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Mitigation and Modification Alternatives and steel, are subjects influencing the design process. Access routes and construction area limits also help define the temporary and permanent ecological impacts associated with the construction activity and the related ecological mitigation which may be necessary. With this basic information available, preliminary engineering, which provides the basis for an improved construction cost estimate, can proceed simultaneously with initial ecological evaluations and preliminary permitting activity. Should engineering design options exist they would be re-examined at this stage prior to proceeding to final design. Public and landowner involvement and resolution of related concerns is a critical element at this stage in order to proceed with final preconstruction activities. Remaining investigations would be conducted, followed by final engineering design and completion of related ecological activities. Final permitting activity is followed by construction. Special construction issues at the sites of these structures will include: • Mitigation of instream and riparian ecological disturbance during construction; • Timing of construction to permit uninterrupted diversion of waters for off-stream use; • Timing of construction to minimize ecological disturbance; • Timing of construction to minimize water handling requirements. Fish Passage Criteria Knowledge of fish behavioral characteristics is needed to determine the low-flow fish passage design criteria. Fortunately, design and monitoring of the City of Craig diversion structure have laid valuable groundwork here. Most of the structure types have small enough head differentials to permit instream passage by swimming. Design information required includes maximum swimmable velocity for the full range of flows, minimum depth, maximum burst speed and duration (to determine slope), and fish movement characteristics so that approach channels can be configured to direct fish movement to passage routes. Rock size is also a significant consideration as rock voids must either be large enough to swim through or small enough to be grouted closed (to force water into navigable sections). Fish passage provisions at each site are uniquely different due to site physical constraints and the physical and behavioral characteristics of fish. In general, passage criteria address the following key characteristics: • Attraction water to signal the passage route. • Adequate quantity, depth and velocity of flow to permit passage. • Pools for resting areas, if the passage exceeds the swim-through capability of the fish. 4-10