[This story is excerpted from the Tight Lines Rocky Mountain Fly Fishing Forecast, which is available at flyshops in Montana, Idaho, and Wyoming, or you can download a digital copy of the 135-page book for $4.95 at the Fly Fisherman on-line store. The Editor.]

If you fish the northern Rocky Mountain's rivers and streams you'll catch a plethora of trout, some small, some medium-size, and, if you are either lucky or very good with a fly rod, you may place a trout in the net topping 20 inches. If you fish eastern Idaho's Henry's Lake, one of the most productive trout waters on the planet, you will catch fish exceeding 20 inches--healthy, firm cutthroat, rainbow/cutthroat hybrids, and brook trout often measured in pounds. And this year, some of those fish are running larger than ever.

"This will be the first year that we will see that extra year of growth so there will be some giant fish in the lake. Probably quite a few hybrids in the 8- to 12-pound class."

Henry's Lake, which sits at an elevation of 6,500 feet, bordered on three sides by mountains forming the Continental Divide, is the true headwaters of the Henry's Fork River and it holds the title of eastern Idaho's most respected stillwater fishery. Located just 20 miles from the west entrance to Yellowstone National Park the lake sees lots of angling pressure. But that isn't a problem--biologists estimate that nearly one million trout swim in Henry's Lake and the hybrids are growing to incredible size due to management decisions made several years ago.

"In the past we saw great growth and survival of our hybrid trout to age three and four," said Dan Garren, a biologist for Idaho Department of Fish and Game in Idaho Falls. "Then the fish disappeared due to stresses associated with spawning -- they died without achieving their desired lifespan.

"In 1998 we began sterilizing our hybrids, which eliminated their desire to spawn," Garren added. "By stocking sterilized hybrids we eliminated the threat of them breeding with our Yellowstone cutthroats and we anticipate that they will live an extra year because they aren't dealing with the stresses of spawning. This will be the first year that we will see that extra year of growth so there will be some giant fish in the lake. Probably quite a few hybrids in the 8- to 12-pound class.

"The fish are phenomenal," Garren added. "It's just freakin' incredible how fast they are growing. At age three our hybrids are 17 inches long and the cutthroats are 16 inches. Our brook trout are 20 inches by age 5. You can catch a 5- to 7-pound hybrid in this lake without much trouble. And if you get lucky you may pull out an 8- or 10-pound hybrid. And there are still a few big brook trout around--last year I saw one brook trout that was 21 inches long and I heard of some in the 22-inch range. At that size, those are three and four-pound fish."

In 1988, IDF&G ceased stocking brook trout due to concerns about those fish competing with Yellowstone cutthroat. Unfortunately, the brook trout failed to sustain their population so the department will restore its brook trout stocking program this year.

"We will put in 100,000 sterile brook trout this season," Garren said. "So we should see the brook trout fishery improve in the next year or two."

If you choose to fish Henry's Lake, which opens May 24 and closes October 31, bring or rent a float tube, pontoon boat or an aluminum skiff. Wadefishing opportunity is limited, especially in July, August, and September. Wade fishermen catch a few fish early and late in the season but not in the numbers found by anglers who stray from shore.

Water temperature and aquatic plant growth play a big part in the success of a Henry's Lake angler. During the closed season the lake is covered in ice. During late May, June and July the water temperature is moderate and weed growth is bearable. By August the surface temperatures may range to 70 degrees and aquatic plant growth is heavy. During the hot days of summer, trout find underwater springs and congregate near stream mouths. They are not spread out and anglers must do their research to find willing fish at that time. The early season through mid-to-late July and again in September and October are prime times to fish Henry's Lake. However, large fish are taken throughout the season.

Last year Henry's Lake was lower than normal and warmer than normal and aquatic plant life thrived. Fishermen recorded the worst catch rate in the history of the lake, according to IDF&G. However, the water level is expected to be adequate this summer, meaning the water temperature will likely be lower and the catch rate higher than last season.

Despite the presence of mayflies and caddis, Henry's Lake is mostly a sub-surface fishing affair, with leeches, scuds, Krystal Buggers, and Utah chub imitations, drawing the most takes. Those patterns are fished off sink-tip and full-sinking lines, which allow anglers to reach fish that hug the bottom. Overall Henry's Lake is shallow, averaging 18 feet in depth, but it has some holes that range to 25 feet deep.

One of the best times to fish the lake is in late June and early July when damselflies are present. These 2- to 3-inch long insects demand the attention of trout and the fish feed on them heavily. Use a floating line or a medium-sinking line and retrieve damsel patterns in short, 3- or 4-inch strips. Size 6-8 Marabou Damsel Nymphs and Nyergess Nymphs work well during the hatch, which usually gets going between 10 A.M. and 11 A.M. each morning. The hatch extends into the afternoon hours. It's not uncommon to see huge trout boiling just under the surface, sucking down an unfortunate damsel nymph as it swims from the weed beds toward the relative safety of shore or your float tube. The action offers even the most dedicated anglers a case of the big-fish shakes.

If looking specifically for brook trout, anglers are wise to arrive in late September and throughout October when brookies begin movement toward the spawning streams. The mouths of many creeks, including Duck and Kelly host big brookies. The actual streams are closed to fishing.

Access to Henry's Lake is limited due to private development around much of its shore. However, public put-ins are located at Henry's Lake State Park; Henry's Lake Lodge; the state fish hatchery; Wild Rose Ranch; Staley Springs; Frome County Dock; and Hope Creek. Some of those areas require payment of a reasonable launch fee. Camping is available at several sites including Henry's Lake State Park. If arriving early in the year--around the opener--or during the damselfly emergence, reserve your site early.

Whenever and wherever you fish Henry's Lake consider the weather. Wild storms can kick up even during the middle of summer and wind is a huge danger if fishing the lake from a boat or float tube. In fact, anglers should expect the wind to kick up in the afternoon every summer day on Henry's Lake. Don't venture too far away from shore. Another note: the lake closes to fishing at 9 P.M. each night.

If you are looking specifically for large trout Henry's Lake may be the top spot in the northern Rocky Mountain region to find that fish. The lake is loaded with hogs and they willingly strike a variety of patterns. Anglers who spend a fair amount of time on the lake will catch trout ranging between 5 and ten pounds. You can't say that about many public waters in the West.