Last month, my son Clint and I gave a presentation to the Little Bighorn Chapter of Trout Unlimited in Sheridan on our combined 40 years of guiding on the Bighorn River, including observations about various aspects of the river, its fishery and anglers.
One thing we noted was the increase in anglers over the years and the increased number of anglers who prefer to fish without guides, in part due to the influx of inflatable one-person pontoon boats.
Clint made the observation that fishing success plummets when there is a strong southwest wind. He felt that it was probably due to a high-pressure system setting up.
We both noted that in years with low flows, the water temperature remained relatively cold and rarely went above 55 degrees. In high water years, the water temperature warmed up and often exceeded 60 degrees and, at times, approached 70 degrees. In the low flow years, the aquatic vegetation seldom became a problem but during the high water years the warm water temperatures helped to cause a luxuriant growth of widgeon grass, water buttercup and filamentous algae.
I pointed out that plants and cold-blooded animals could only grow at a rate that the water temperature allows.
I later pointed out that the flows during the '90s were largely above the minimum flows of 2,500 cubic feet per second; in fact they were often above 7,500 cfs and even reached 14,600 in 1995. Conversely, in the decade starting in 2000 the flows were seldom above 2,500 and were as low as 1,300.
Sharing some of the information I had recorded in my daily log, I found that: In 1995, my clients caught 53 trout on yellow sallies, a small yellow stonefly dry-fly pattern, and 103 trout on trico patterns. (Tricos are a small (size 18) mayfly that normally hatch in September on the Bighorn). Amazingly, in 2005 my clients caught zero trout on either yellow Sallies or Tricos.
This was rather startling to me to see such a marked difference in the two years' results. At the time I attributed the lack of fish caught on the patterns to the lack of hatches of the insects. I had not seen enough yellow sallies and tricos in the decade to even mention.
What caused the diminution of the insects? I recently finished reading a book, "The Mulligan" by Nathan Jorgenson. In it the author states that tricos need the water temperature to reach 63 degrees before they can complete their life cycle.
From some other source, I knew that large stoneflies need the water temperature to reach into the mid-50s before they can emerge.
It has become fairly apparent to me that low flows on the Bighorn River not only reduce the number of trout per mile but the low water temperature adversely affects some species of aquatic insects.
In short, much of fly fishing on the Bighorn is driven by flows and their effect on water temperature.