Yup, what Riptide said.
The nymphs of early black and early brown stoneflies are active this tiem of year. They're dark and a lot skinnier slender than most other stonefly nymphs.
A "Pheasant Tail Nymph" tied with dark brown turkey tail fibers is a good imitation. A 3xl size 14 hook weighted with nontoxic wire and a black bead head helps to to get it down in fast water that the nymphs prefer.
Other dark nymphs about that size tied with peacock herl like Prince Nymphs, Zug Bugs, wire bodied stuff like Black Copper Johns etc or dubbed dark black or brown patterns are a good choice.
Here's a better imitation tied up by member Smalliesrule for our Nymph Fly Swap that's in progress:
Personnally I haven't had as much luck with the dries for this hatch but there's a Black Caddis (Chimera aterima) around size 16 that hatches about the same time, so some dark caddis patterns are worth trying too.
A better imitation for the adults is a size 16 black or dark brown Dancing Caddis on a 2xl or 3xl light wire hook (like a Tiemco 200R). If you tie, here's a link to it in the pattern library: Dancing Caddis