Mountain Air

Glacier Institute's Blog

Fly Fishing in the Shadow of Glacier National Park

People often wonder how our rivers in northwest Montana compare to some of the more “famous” trout
rivers of southwest Montana when it comes to a fly fishing vacation. Let’s provide some very concise
observations on the generalized differences between our little corner of heaven and how it stacks up
against those celebrity rivers in other portions of our state.

Southwest Montana rivers have a much higher nutrient load in their waters. This in turn provides for a
much larger population of invertebrates in the food chain Fish grow bigger and faster with the
improved invertebrate presence in the food chain. There is also a greater diversity of trout waters in the
area. Spring creeks (O’Dells, Depuys, Armstrong) and tail waters (Missouri, Beaverhead, Bighorn)
provide for more consistent stream flows and ideal temperatures that are optimal for maximizing
invertebrate population which in turn enhances the growth of trout. Other trout rivers in the area have a
more diverse complex of pools, riffles, and runs compared to northwest MT that provide an ideal mix
for supporting healthy fish populations.

Southwest Montana has good populations of Brown, Rainbow, and Yellowstone Cutthroat trout.
Although Browns and Rainbows are introduced species, they have established a naturalized existence
in the southwest part of our state due to ideal and fecund river conditions. Although all of these trout
can reach very large sizes (5+ lbs), Brown trout are optimally suited to growing to these large sizes as
they tend to become more piscatorial and nocturnal as they reach larger sizes. Both of these attributes
allow for significant numbers of large Brown trout to exist in several rivers. They can also tolerate
warmer water temperatures than other kinds of trout which only enhances their survivability.

Challenges for fly fishing in this area include dewatering for irrigation (raises stream temperatures,
lowers the dissolved oxygen level in the river which the fish need to breathe) and fishing pressure
(which can be off the charts especially in the summer). With a significant invertebrate population, the
fish that can also be very selective because of the availability of natural food sources in the water
column. This is where the term “match the hatch” really holds true.

Our area in northwest Montana has mostly freestone streams (exceptions…Kootenai and the lower
South Fork Flathead). Runoff typically comes from forested areas which contribute fewer nutrients to
the watershed.. These relatively low nutrient waters support reduced levels of macro invertebrate life.
Bull trout are the apex species in our waters. Bull trout can grow to significant sizes, but tend to be
much less prolific (don’t start breeding until 5 years old) … and are much less tolerant of man induced
changes to their ecosystem. Bull trout tend to be migratory (adfluvial) which also requires more
diverse, high quality habitat and subjects the fish population to more environmental stressors as part of
their behavioral movements. With all these factors, bull trout can grow to significant sizes in our
watersheds, but their numbers are very low. Bull trout fishing in our area is severely restricted as a

Westslope cutthroat trout are the primary quarry for fly fishers in our area. These natives evolved with
our landscape and are very opportunistic when it comes to feeding. If it looks edible, they are likely to
take a swipe at it. Growing up in these nutrient poor environments tends to reward the fish that are
most aggressive when pursuing a meal opportunity. While good for fly fishers, it does make cutthroat
populations vulnerable to fishing over-harvest.

Westslope cutthroats rarely achieve sizes beyond 20 inches (3+ lbs). Westslopes also tend to be the
least piscatorial feeders amongst all cutthroat trout. Lack of a small fish based diet for mature trout
excludes a significant food source for mature trout attaining large sizes. Theses adaptation seems to be
the result of co-evolution with Bull trout that ensures a minimum of overlap on food source
competition (1).

Much less de-watering (agriculture, ranching, municipality) of our northwest rivers improve stream
flows and stream temperatures, especially in the summer (haven’t needed “hoot owl” fishing
restrictions so far). Fortunately, this provides a safe haven for bull trout populations since they require
colder water temperatures compared to other trout species.

Scenery on our northwest rivers…second to none !!.

Want to learn more about fly fishing and our fishing waters up here in the Crown of the Continent ?
There is still time to sign up for Glacier Institute’s fly fishing introductory or women’s fly fishing
courses in July and August. Don’t miss this opportunity to get connected with our
piscatorial friends in the shadow of Glacier National Park.

Summary at a glance…

Southwest MontanaNorthwest Montana
River types (predominant)Freestone, Tail water, SpringFreestone
Nutrient levelMedium-HighLow
Primary speciesRainbow, Brown, Yellowstone
Cutthroat trout
Westslope Cutthroat trout
Fishing pressureHigh to Extremely HighLow to Medium
Scenery (predominant)Ranch lands, National ForestsNational Parks, National Forests

Mike Carpenter, Fly Fishers International Certified Casting Instructor, Glacier Institute fly fishing instructor

(1) Tomelleri, Joseph R., 2002, Trout and Salmon of North America, New York, NY, The Free Press