Flood Recurrence Interval and Egg-to-Fry Survival

“Effects of peak flows on egg to migrant fry survival have been documented in Chinook salmon in the Skagit River basin (Kinsel et al. 2007), and are presumed to reflect scour of eggs in the gravel due to the overlap in timing of floods and the incubation period. A similar effect is presumed to occur in other river basins. The data from the Skagit River basin are shown in Figure I.9, along with predicted survivals using an exponential regression relationship. Note that the recurrence interval on the x-axis is based on the Skagit River near Mount Vernon, which is low on the mainstem Skagit.
In the Chehalis basin, we rescale this relationship to create a multiplier that ranges from 1 at RI = 0 to near zero at RI = 100 (Figure I.10). The equation for the relationship is:

where MultiplierPi is the incubation productivity multiplier and RI is the flood recurrence interval. In each year of the model run, the multiplier is then calculated using this function, based on the flood recurrence interval for that year. We used peak flow at the USGS gage on the Chehalis River at Porter as the index flow. Peak flows, probabilities of occurrence, and recurrence intervals were calculated using the PeakFQ USGS software (https://water.usgs.gov/software/PeakFQ/). Results for that gage are shown in Table I.3.” (Beechie et al. 2021, p. 230)