I have not fished those streams specifically except to say that the WI Driftless area contains limestone-rich groundwater sourced streams. This says a LOT: Fertility (means productivity in terms of food production), seasonal stability in temperature (means growth), stability in water levels can offer fingerlings more consistent survival. More surface water sourced streams (most streams in the US) tend to be less fertile and less stable in temperature and flow, resulting in overall less productivity.
There are likely to be some large trout in these "lesser" (more average) waters, but they cannot grow as large on insect fodder as they might in more productive and stable environments like spring creeks and tailwaters. The best way to up the odds in average waters is to target "gators" -large browns that have switched from insect fodder to fish. Since there are rarely many of these, this takes some effort. Some streams have more than others. Your fisheries people should be able to help you ID these.
You can break down the season from the fish’s perspective into four periods: -Summer feeding, migration, spawn, wintering. Each have their own habitat requirements, and the migration periods occur in between.
These large fish are probably most vulnerable during the migratory periods (fall into mid-spring) when they are most apt to be "exposed". You’ll need to start thinking about streams in terms of the entire watershed, as if it’s a lake, bc that’s the scope these big trout have. Think about seasonal habitat requirements, water levels for fish to move with, and barriers that might hold these fish for long enough to have a crack at them.
The spawn is the first impetus to move (behaviors run from late August through November). Wintering turf usually means large deep slow pools, often with wood cover. Early spring, associated with runoff/spates, is the time / water volume when these fish return to their summer home ranges. You may find a big fish almost anywhere at this time and it’s when most anglers “get lucky” and find a big one.
In spring and summer, these fish are in stretches that offer two things: lotsa large food (young trout, chubs, crayfish), the most common location on most watersheds being below “classic” trout water where the stream gets larger and warmer –down into “chub water”. Use a thermometer bc the water must still have periods in the day when the water is in the mid 60s or less, often at night and early AM. Too warm? Start working back upstream. These fish are often nocturnal, esp during the summer.
Effective presentation possibilities:
First, in general, drop the standard dry flies and nymphs and fish big meaty stuff that may look more like bass flies than typical trout stuff. This also means you may want to tackle up -leave the 3 and 4 wt at home and use a 5 or 6wt.
-The exception to tackling up is during emergences of larger flies (if sufficient in number) that can bring up a few of the bigger fish, esp the larger end of the “insect eaters” (usually 14-16” fish). Know your hatches and be there, although hitting it right is not clockwork.
-Big deer hair mouse, large wet fly, or streamer, fished down and/or across at night in large cover strewn pools in summering ranges.
-During daylight hours, “Flippin’”, a technique used by bass fisherman to target specific cover spots with a heavy jig is also very effective for large cover oriented trout –which most big browns are. Use a stout tippet and a nose-weighted wooly bugger and probe deep dark undercuts, wood, and boulder crevices.
-Hanging (or fishing across) large wets/streamers into large pools and under deep undercuts during spate periods anytime of year. Summer rain spates, or just deep overcast, can bring larger carnivores out of the woodwork during daylight hours.
-During the spawn, swing streamers along gravel-filled stretches, esp tailouts of pools. Also check undercuts and wood or boulder cover above and below these good spawning sites. Pre-spawn and post-spawn fish are most vulnerable/willing. Post-spawn fish are especially ravenous to feed and dead-drifted egg patterns work really well.
-Nymph/egg fishing eddies in deep, slow wintering pools. These wintering fish are often hungry, and good habitat can collect up more than one.
Others may have more to add to this list.
You have your work cut out for you. But pleasant work, if you can find the time to REALLY get to know your watersheds.