Over the past 30 years, I have experienced a variety of water levels on the Bighorn River. During the decade starting in 1990, the flows in the river were above the recommended minimum of 2,500 cfs, and in the decade of 2000 to 2010, most of the flows were below the recommended minimum flow.
Each time there was an extended period of above-normal or below-normal flows, changes occurred to the aquatic ecosystem.
When there are above-normal flows in the Bighorn River during the spring and summer, the water temperature rises and by the time mid- to late- July arrives, the water temperature has creeped up into the mid- to upper-50-degree range. By the time August ends, the water temperature has reached 60 to 65 degrees.
During low water years, the water temperature remains quite cold. By the end of June, the water temperature may only be 42 to 45 degrees. By the end of July the temperature may have risen to only 48 to 50 degrees. The maximum water temperature by the end of September may only be 53 to 55 degrees.
While you might think that a stream that only reaches 55 degrees is beneficial to trout, it isn’t quite so. You see, trout are cold-blooded, so they can only metabolize at a rate corresponding to the water temperature. Hence, when the water temperature is in the low 50s, they grow less than when the temperature reaches into the low- to mid-60s. One fact I remember from a fisheries biology course is the optimum temperature for rainbow trout is 62 degrees. At that temperature, rainbows are their healthiest and have great growth rates.
Ken Frazer, fisheries supervisor for the Billings region, once told me, “During warm-water years the Bighorn River trout have growth rates of 4 to 8 inches; during cold-water years they grow 2 to 4 inches.”
The water temperature also affects aquatic insects. During warm-water years, blue-winged olive mayflies will start emerging in mid-April and peak sometime around May 20 and then taper off and end in mid-June. During the peak of the hatch in mid-May, the blue-winged olives come off in such numbers that the predators cannot possibly eat them all and the survival of the species is insured by the flood of insects and their subsequent ability to lay fertilized eggs.
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During low-water years, such as occurred in 2013, the water stayed cold throughout the summer. The blue-winged olive hatch started in late April and seemed to stay at a low level throughout May and June. I witnessed blue-winged olives in late August. It seemed as though there never was a flood hatch of blue-winged olives in 2013.
In 2013, the cold water temperatures depressed the yellow Sally hatch in the upper 20 miles of the Bighorn River. Though the Sallies did hatch in the Mallards to Two Leggins stretch, there were few to be seen upstream from there. It appears to me that yellow Sallies are highly dependent on a water temperature in the mid- to upper-50s in order to reach a flood hatch level.
Another manifestation of the flow levels is the presence or absence of water in the side channels. During high-water years the side channels are full and there is more water for the trout to inhabit and more water for the aquatic invertebrates to inhabit as well. Trout fry and fingerlings can find refuge in the side channels, too. Full side channels also help to spread fishing pressure out and causes less conflict on the river.
It only stands to reason with more water in the river there will be more fish per mile of river. The carrying capacity of an aquatic ecosystem is expressed as pounds per surface area. If there are more surface acres per mile of water, there will be more trout per mile. This was demonstrated graphically during the stretch of 1995 through 1999 (a high-water period) when the trout numbers reached as high 9,500 per mile.
During low water years, most of the side channels are dry; there is less water for the trout to inhabit and there are fewer aquatic invertebrates as a consequence. Without the side channels, the young of the year trout have to stay in the main channel and end up being forage for the older trout.
Another aspect to consider is that high water flushes out the sediments that have accumulated in the backwaters and pools. The high water also dilutes the irrigation returns that enter the river at various points from above Bighorn access to the Two Leggins access. During high-water years the returns are diluted and diffused so that fishing is possible all the way to Two Leggins. In low water years, the waters become too murky by the time the river flows past Mallard Landing.
By the looks of things, we are going to have high flows in the Bighorn River this spring and summer. The flow is already at 6,000 cfs and will probably climb as the mountain snowpack melts. Who knows? Perhaps we’ll see some monster trout at the end of the summer. At least we’ll be able to fish some of those lovely little side channels again. Stay tuned, this could be a great year.