SquatchHunter-

All aquatic insects require some particular thermal regimen in order to mature. This is customarily measured in degree days. You can probably search the archives of this site for an article by the Stream Doctor, which will provide a more complete explanation of this principle. In any event, a Salmonfly nymph requires 2-4 years to acquire the thermal regimen necessary for maturity. Once that maturity has been attained, a specific water temperature does seem to trigger emergence, and that is likely the mid-50's to which you refer. Hope this helps.