Hi Steely.
I just got in from the coast and online. It's been a while since I fished the Koot'. But I do recall that the 'tailrace' waters right below Libby Dam down river to about D. Thompson Bridge had the best drift fishing type water; similar to much of our steelhead water. We caught larger rainbows, 'plenty over twenty' inches, drift fishing small red or pink corkies with a little white yarn as you would for steelhead. But there is also very good water from Libby down to the mouth of the Yaak River (tributary that has good fishing) below Troy. Beware the big Thompson Falls in that stretch! We also did very well drifting half a live nightcrawler with a little pink yarn tied on above the hook. And using a small corky with half a worm or one or two cured red Pautzke's salmon eggs worked great. We used about 28" leaders in 6 lb. strength - I think with size 4 or 6 hooks. ...
If you have a driftboat, backtrolling size 50 crawfish pattern Hot Shots work great at times. One thing we didn't get to trying that I'm sure would work great is Hot N Tot divers with 50+" leaders with the tinyest SpinNGlo and a nightcrawler. And I'm also quite sure that 1/8th oz. jigs in brown/orange or black/red would work there too. In fact, give the rubber pink worm a try on these big 'bows and I bet you score big time! Either on a 1/8oz. jighead with smaller hook (size 2 or 4) and about 2 1/2" to 3" of the back end of the Berkley Power pink worm. Let me know how this works if you try it.
I know the water gets very cold there earlier than here, so your results may not be great until just before and after spring runoff, which is usually mid-May thru mid-June or so; but you should catch a few good ones in cold colored water. Don't forget that sinktip flyline and nymph patterns work too; and floating flylines and topwater flies work real well from mid-summer thru early fall. >
BTW, just to get ya pumped up with some facts (don't tell anyone
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) in a Western Outdoors Magazine issue a few years ago, in the 'Dateline West' section, they reported in back to back years on the Kootenai River in NW Montana a 29 1/2 pound rainbow was caught and then a 30 1/2 pound rainbow was caught!!! These Koot' bows are the huge Gerrard strain from Kootenay Lake up in B.C. These same strain of trout were taken from there several decades ago and planted in northern Idaho's Lake Pend de Orielle (pronounced Pond da Ray) and became the famous giant Kamloops Trout. Lots of them between 20 to 30 pounds were caught for many years in that lake until they were mostly fished out. There are a few of those monsters swimming around in the Kootenai River! Good luck.
RT