As many anglers know, the Big Horn River has
experienced lower than average flows for the
past three years. For instance, over the past
two years the average flow has been 1,500 cubic
feet per second (cfs) while the normal average
low flow would be 2,500 cfs. Along with the low
flows have come some substantial changes in the
trout fishery and the aquatic ecosystem.
Perhaps the most notable change has been the dramatic
decrease in the number of trout per mile. Back in
the good old days of the mid-90s the total number
of brown and rainbow trout per mile in the upper
river was estimated to be 7,200. Nowadays the
figure is pinned at 1,600.
What has happened? Well, one obvious difference
is the decrease in the amount of surface acres
per mile of river. Many of the side channels and
shallow riffles are high and dry. Consequently,
there are fewer places for trout to live.
Another aspect of the loss of the side channels
has been that the young of the year trout have to
inhabit the main channel where the big browns can
feed on them. For the first couple of years of the
diminished flows, the browns decimated the young
fish and all but eliminated the year classes.
What isn't reflected in the population per mile
estimates is the average size of the trout. Unlike
terrestrial animal populations, population ecologists
don't express the carry capacity of a stream in numbers
per surface acre but in pounds per surface acre. While
the number of fish per surface acre is down markedly,
the average size of the trout has increased dramatically.
Back in the good old days, the average brown was about
14 inches while average rainbow was about 16 inches.
Nowadays the browns and rainbows are between 18 and
The brown trout have had the opportunity to become what
I call, "Real brown trout." What I mean is that in
free-stone streams, brown trout normally switch from
an aquatic invertebrate to a predominantly fish diet
when the browns reach about 16 inches in length.
Subsequently, the free-stone browns would become
lunker-sized and hard to catch. The Big Horn has
few baitfish species, particularly in the upper
stretches. Previously the browns would have to
continue to feed on the invertebrates. They would
also have to feed continuously since they couldn't
obtain enough food to be full. The habit of
continuously feeding made the browns more susceptible
to being caught by anglers.
When there were large numbers of smaller 12 to 16-inch
brown trout in the river, many of the larger browns
would become gaunt and starve to death because they
couldn't eat enough of the small invertebrates to
During the past couple of seasons the browns have
become harder to catch and also have sported girths
that we had up until then only seen on rainbows.
In short, the small trout diet has enabled the browns
to be more like their free-stone stream cousins.
During the heydays of the Big Horn, the brown trout
outnumbered the rainbows about six to one; presently
the ratio is close to even with maybe the rainbows
holding a slight edge. According to Montana Fish,
Wildlife & Parks fisheries biologist, Ken Frazer,
"It is typical for rainbows to be the predominant
species in a tailwater fishery; the change that is
occurring is not exceptional."
A slight digression is necessary at this point. Last
year there were erroneous reports that there were
only 800 trout per mile. The persons who posted this
information had not read the population estimate
correctly. The statistical model used for estimating
the population is called the Lincoln Method or Mark
and Recapture. The method entails capturing the trout
by electro-fishing and marking them. In the case of
the Big Horn trout, a hole is punched in the tail of
A few days later, the same stretch of river is
electro-fished again. The biologists note how many
of the fish are marked in the total of all the fish
captured. A simple equation then can be used to make
the population estimate: the number of marked fish
in the second sample is to the total number of fish
in the sample as to the total number of marked fish
is to the total population. For instance, if there
were 100 marked fish in the second sample of 500 and
there were 500 total marked fish then the total
population would be 2,500 fish.
What happened was that the biologists marked very
adequate numbers of browns and rainbows on the first
pass. On the second pass, the biologists recovered
a statistically valid number of browns but not rainbows.
They were able to estimate that there were 800 browns
per mile in the upper river but could not make a
statistically valid estimate for the rainbows. Ken
Frazer told me that if he had to make an estimate,
it would be at least as many rainbows as browns.
Hence, 1600 trout per mile is a more realistic
estimate not the 800 that so many have posted
on the Internet.
As I stated earlier, there is a lot less trout
habitat in the Big Horn River due to the decreased
flows. The decreased flows also mean that the water
temperatures will average colder than when there is
a high flow year. In 2003, I never recorded a water
temperature in the upper river that exceeded 58
degrees. It wasn't until August that the temperature
warmed into the mid-50s.
Colder temperatures mean slower growth rates, not only
for fish, but for aquatic invertebrates as well. Hatches
that normally occur in early July don't begin until late
July or early August. Consequently, anglers who are
accustomed to fishing specific July hatches as yellow
Sallies or pale morning duns are out of luck if they
plan on fishing early in the month.
Another aspect of the low flows is that the habitat
for specific aquatic invertebrates has been diminished.
Species that require heavy riffles and large gravel or
cobblestone have declined in numbers. Over the past two
years, the yellow Sally and pale morning dun hatches
have been minimal to say the least.
Both black caddis and tricos have had sparse hatches
through the low flow years.
Mahogany duns, who were rare prior to the low flow
years, have burgeoned in numbers. The mahogany duns
are silt dwellers. Without heavy flows, silt deposits
have increased, particularly in the lower stretches
of the river where irrigation return waters are laden
with silt. Also, the plains streams flowing into the
river carry heavy loads of silt.
During the high flow years of the mid to late 90s,
scuds were relatively rare and sowbugs were numerous.
As the flows decreased the sowbugs followed suit while
scud numbers climbed.
Anglers had become accustomed to landing lots of trout
each time they fished the Big Horn. With fewer, but
bigger trout, anglers have had to settle for
considerably fewer trout landed per day's fishing.
However, anglers hook a fairly high number of trout
but the landing percentage is abysmal—the large trout
are too big and powerful to allow a high landing
percentage. An expert fly fisher can expect to land
only about one half the trout he or she hooks; novices
probably will land less than one quarter.
The tried and true method of consistently catching trout
on the Big Horn is nymph fishing. While rainbows continue
to feed heavily on invertebrates throughout their lives,
browns switch to a fish diet. Anglers wishing to catch
more browns will have to change tactics in order land
the slab-sided browns. Of course, I am told "Streamers
don't work on the Big Horn except during the brown trout
spawning time." Maybe it is time to rethink this credo.
In the previous decade any pattern imitating sowbugs
would get the job done, but now scuds seem to have become
the go to fly. While the tan ostrich herl nymph has done
a great job as a sowbug imitation, it is also doing a
passable job of imitating scuds—the tan herl with the
fire orange thread under wrapping gives a hint of orange
just as the natural does.
There are higher numbers of midges in the river
nowadays thanks to the silt deposits so midge
patterns are effective from early on, say March 1,
to the end of June. Some days the midge clusters
can be awesome to behold and to fish. The ideal
midge day is a cloudy, calm one—a phenomenon that
occurs four to six times a month. When such a day
occurs, it is not unusual to see in the late afternoon
midge clusters the size of dimes floating down the
Another aspect of the Big Horn River that I would
like to address is the crowds or lack thereof. In 2003,
the Big Horn had large numbers of anglers, particularly
on the weekends in April and May. Many of the anglers
came from points south, in other words, Colorado. As
the season wore on there were weeks where the crowds
were sparse and a person could fish the river without
seeing but a handful of boats. June and July were low
numbers months. August and September had a few more
anglers but at no time were the conditions crowded.
In October and November the river was close to deserted.
While the snow pack conditions aren't encouraging, it
is safe to say that the flow won't go lower than 1,500
cfs. If we have a wet late winter and early spring, it
might be such that the flow could be bumped up a bit
but I wouldn't hold my breath!
The Big Horn has settled down and should provide good
fishing for fairly large trout for several seasons to
come. This year there will be an influx of 12 to 14
inch browns and rainbows to compliment the large trout
in the river. However, the big fish will probably most
all die off this fall. Next year they will be in the 16
to 20 inch range and a good number of trout in the 10
to 14 inch range. Basically, the population will adjust
to the flows and will come into equilibrium with it.
When we finally have high flows, the first year will
be tough fishing because the lower number of trout
will be spread out over a large area. (Then, too,
the fish will have so much more feed that they will
grow at a geometric rate!)
Well, I hope I have helped to dispel some of the rumors
about the Big Horn River and I hope, too, that you will
try fishing it again. All you will have to do is to
relax and go with the flow. ~ Bob
Bob Krumm is a first-class guide who specializes on fishing the Big Horn River in Montana,
(and if there terrific fishing somewhere else he'll know about that too.) Bob has
written several other fine articles for the Eye Of The Guides series. He is also
a commericial fly tier who owns the Blue Quill Fly
Company which will even do your custom tying! Bob is a Sponsor here! You can reach him at:
1-307-673-1505 or by email at: