Well; the SOHO sulphur hatches often last into December some years and the hatch is pretty well reliable day in and day out regardless of flow... one can pretty well expect dry fly action more often than not...

The Clinch by comparison is a nymphing river more often than not.... when it's on for sulhurs it's awesome no doubt but I show up expecting to fish wet, not dry.

To date I have never experienced a caddis hatch on the Holston that could be described as "strong" compared to so many other rivers I have fished...

As to "substrate" I would say the SOHO has more snot and this might acoount for more bugs.