I'm willing to bet you are witnessing natural population dynamics and maybe even stream dynamics. I have seen changes in streams and lakes; natural shifts in entire stretches, reaches, and even one basin. A flash flood knocks out one hole and created another. Sediment loaded reaches from a slide 12 years earlier turned gravel to silt bottom and slowed the water for hundreds of yards. With these changes so changes the entomology.
Populations go up and down and a cycle can be years or decades. The are are predators, grazers, shredders and filter feeders. These populations are all interdependent and, at the same time, some independent. Populations can also vary up and down the river, laterally, and a sample can be inadvertently skewed by the time the sample was taken (day, night, spring, summer, fall, pre/post hatch). Other impacts to these populations are water temp/dissolved gases, flow, detritus composition/ nutrient load, light penetration, etc.. So these cycles can be very complex.
I saw the giant stone fly hatch here peak almost two weeks early this year, early hatch. This was the result of a very mild winter and low flows. Last year we had record flows during the same time period, weak hatch. The year before the flows were normal but a wet rainy cool spring, delayed hatch. All three samples, if taken on the same day of month over three years, would have yielded three vastly different results. In other words, three years is nothing in the scheme of insect populations and can't be used to infer population trends. Want trends then you better start the baseline studies now so they can be assessed in +10 years. Still laughable IMO.
Number of risers? Some days rises are everywhere while other days none; it can vary as much as one evening to the next. Watching PMDs float by and no takers? Clouds of caddis swarming the shoreline and no splashes? Really? For every one fish feeding on the surface there are 10 feeding below. Where is the easy food source? It's drifting by underwater. Transported directly to the mouth of a waiting fish the emerging insect is an EASY meal requiring little to no exertion of energy; potato chip eating sofa rider watching the game. And, countering what is drilled into the angler's head, the worst fishing can be during an intense hatch.
I've noticed less risers over the years....I've noticed more people as well. Some of the best dry fly fishing I've seen is in the depths of winter today when only the insane have walked in to fish. Think about conditioning. Why would a fish stop eating during the best time of the year? It wouldn't and won't. It just changes its habit. The reward for this change is more food, easier food, and with less disruption in the process. I think numbers of people have conditioned feeding habits on many stretches. Taylor River. (I use this river a lot as an example because it's full of conditioned fish and just about everyone has fished or read about it. lol). Sure you can catch fish on dries. There are days when dries can be very productive. Overall though, if you want to catch the big fish or numbers of fish you will nymph, even during hatches. Why would a fish expose itself if it doesn't have to? It won't. Why focus on a dry only to stop when the next angler walks up, next splash of a fly line, or the way wrong bug?
Diminishing hatches? I have yet to witness this anywhere I have fished in over my 40 years of angling. Intensity depends on year; dynamic populations. Yeah, I hear stories of how thick hatches used to be and I take them with a grain of salt. These are from the same guys that sat around the shop and call giant stones 'Hellgramites'. Sorry, but many of the 'old timers' never left 50 miles from where they lived/died and knowing the difference between species, casings aside, is beyond the knowledge of the vast majority of the fly anglers today despite the availability of picture ready information. I get texted all the time from guys asking "What's this bug...'shell'....nymph?" Their knowledge of aquatic entomology begins and ends with newest and hottest fly pattern and are amazed when they find out the real bugs are not purple, don't sparkle in the light or glow in the dark.
So, before determining the storm direction by testing the wind with a licked finger, a look at doppler would be in order. Surface winds don't mean a damn thing and could possibly result in one being caught in flashes of negatively charged conclusions.
My grain of salt
" ..... 'I bet three (or four) would be better!"
