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INDIRECT PARAMETER ANALYSIS The determination of flow criteria for salmonid egg incubation is an example of indirect analysis, relating surface hydraulic conditions to subsurface hydraulics in the gravels. Essentially, a redd is located in an unconfined, submerged aquifer. Therefore, hydraulic conditions of the surface waters play an important role in determining hydraulic conditions through the gravels. Unfortunately, most studies examining the survival of eggs in redds were exclusively concerned with sub- surface flow conditions, without discussion of surface flow factors. Given an initial dissolved oxygen content high enough to provide good embryo survival, two hydraulic factors remain which determine the survival of eggs. The first is the velocity of water through the gravel, the apparent velocity, which is important in the delivery of oxygen to the embryos and in the removal of metabolic wastes. The second hydraulic factor is the tractive force, or shear stress, of the water. Certain shear stresses are desirable because they prevent the deposition of fines in the interstices of gravel.- However, higher shear stresses can cause movement of gravel, resulting in redd destruc- tion and egg mortality. Therefore, the objective of the calculation of incubation criteria was to determine water column hydraulic conditions which would result in the desired apparent velocities, without scouring or filling the sub- stratum. The apparent velocity is a function of surface slope) and the permeability of the defines apparent velocity as: Va = KS where, Va = Apparent velocity in cm/hr K = Permeability in cm/hr S = Hydraulic gradient (dimensionless) (1) Apparent velocity may be related to surface hydraulics through the use of Manning's equation, the hydraulic slope (water gravels. Terhune (1958) 14