The Snake River along with many other western tailwaters is primarily used for irrigation. This means that flows tend to be high and steady from memorial day into early September. This is great for aquatic insects and fish alike as steady flows of cool water is essential for growth. The high flows also grant baitfish and minnows a relatively safe place to live and grow in the shallows.
When irrigated crops are ready to be harvested, farmers typically call for less water from rivers and flows can drop dramatically. Shallows along the sides of these big rivers dry up and force baitfish and minnows to leave their safe summer haunts and venture out into the main stream. This can make for some exiting angling for fall fishers throwing streamers. The "Minnow Hatch" can be some of the best action for larger fish until cold water temps slow fish down in November.
Matching the hatch can be a bit harder than matching insect hatches. During insect emergence and mating its fairly easy to identify what the fish are eating by just being observant. The "Minnow Hatch" on the other hand may require some additional investigating to know what baitfish pattern and size to fish. Small kick nets or minnow nets can help to get samples of small fishes present in a given fishery and lend some insight on what to throw. However, a phone call or visit to your local fly shop should provide some good tips on what and where to fish.
We typically try to match color and size of minnows available on the waters we are fishing. Much of this comes from many years of trial and error on these tailwaters. Generally, the baitfish tend to be smaller early in the fall then they will be in October. We will fish larger imitations later as they grow.
For the better part of our streamer fishing during September and October we will throw drylines with 9 foot 1X or 2X leaders in faster water with plenty of bank structure. These set ups allow for precise placement and enhanced action in ambush areas that large fish tend to wait for a stray minnow or baitfish to swim by. The strikes can be sudden and vicious as the large predator does not want to let a good meal get away.
As temps begin to cool and water flows slow, We may switch to a sink tip line to allow for deeper and slower presentation. We prefer 6 and 7wt rods for this and typically run short, stout leaders. 20lb Maxima around 36" seems to be My favorite. We will allow streamers to get deep and swing through likely holding water and wait for the sudden stop or tug.
Some of our favorite patterns for the fall minnow hatch are Whitlocks Near Nuff Sculpin, The Double Bunny and other Zonker type patterns, SRF Buggers and Krinklezon Clousers, Orange and Olive Blossems Specials as well as any Galloup Streamers will fit the bill.