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butions for four different levels of permeability. Median particle size was plotted against permeability, and fitted by least squares to approximate particle sizes for intermediate permeabilities. Having determined the particle size giving the desired permeability and apparent velocity, a tractive force was required which would prevent smaller particles from depositing. It should be noted that the surface hydraulic conditions associated with embryo survival is not independent of slope nor of sediment concen- tration. For small slopes the tractive force for a given depth is small compared to those for large slopes. Water with a small suspended sedi- ment concentration requires a lower tractive force to prevent deposition of fines, but also has an excess of energy with which to attack the streambed. Conversely, water carrying a load of suspended sediment requires a higher tractive force to prevent deposition on the gravel. Therefore, two different sources for tractive force were used. Incip- ient motion tractive forces were obtained from Leopold, et al. (1964). These values were used for clear water situations where the tractive force required is only great enough to prevent deposition of fines on the gravel. In those situations where the sediment concentration ex- ceeds 200 mg/L or is composed largely of silt and fine sand, the trac- tive force must be higher. These higher values were obtained from Chow (1959) and reflect tractive forces which will maintain channel stability. Having determined the appropriate tractive force for a given condi- tion of slope and sediment concentration, equation (5) was then used to determine the critical depth (R) for that situation. This value of R was then substituted into equation (4) to determine the surface velocity associated with each value of R and Va. These values of R and Us were then correlated to survival rates of eggs for small values of Rand V. For large values of Rand V, movement of the gravel becomes limiting. It was found that for particle sizes larger than about 6 mm median diameter, the permeabilities became so high that the apparent velocity could be eliminated as a potential limiting factor. Survival of eggs in large gravels was estimated as a function of the size of gravel capable of being moved by spawning fish. For brown trout (Salmo trutta), for example, a cumulative frequency distribution of redds located in various gravel sizes was constructed from data provided by Reiser and Wesche (1977)(Figure 5). All of the redds were located in gravels smaller than 80 mm median diameter, and 38% were located in gravels finer than 25 mm median diameter. Therefore, it was assumed that movement of the 25 mm gravels would result in the loss of 16