This Skwala nymph still has a couple months left to go before hatching, but it's still a good representative of its species, which was extremely abundant in my sample for a stonefly of this size. It's obvious why the Yakima is known for its Skwala hatch.
I recently came across a book which does exactly that, The Fly Hatches by David Richey. It was published in 1980, and is no longer in print. However, it is available on the internet used book market for at a real bargain.
The book lists the hatches by state, specifying scientific name (common name), start date for hatch, duration of hatch, hours of peak activity, and best imitating fly pattern and size. For New York, it lists (21) separate insect hatches, (15) mayflies, (3) caddisflies, and (3) stoneflies.
I was in the process of developing an updated taxonomy for the book, but unfortunately, got temporarily distracted by some other higher priority projects.
The Hatch Guide for New England Streams by Thomas Ames (Frank Amato Publications, 2000) is a good little reference for Eastern hatches. It covers most of the important mayflies, caddisflies, and stoneflies, as well as a few incidentals (dragonflies, damselflies, true flies, and terrestrials)--lots of nice photos and useful information.
PS--The website at www.flyfishingconnection.com gives a number of stream-specific hatch charts for popular NYS waters. Unfortunately, they resort to the usual practice of using many vague common names (tan caddis, brown stonefly, BWO), which somewhat limits the usefulness of these charts.