Water Temperatures and Bug Hatches :: Cutthroat Anglers


Yeah, we all did it when we wanted to ditch school. Just put that old mercury next to a light bulb for a second or two and, yep, you were sick enough to watch TV all day and feed that fever (or is it starve a fever?) until you really were sick. But we are now older and more mature (well, some of us). We don’t need to play with thermometers anymore. Or do we?

This past summer, we recommended our customers carry thermometers to determine when the water reached a temperature that caused too much stress on a hooked/caught fish; that temperature is somewhere around 65 degrees and definitely 68 degrees and higher. But there are other reasons to pack a thermo. One of which is the role water temperatures plays in determining when a specific hatch might occur. So here goes with a basic, and admittedly “local,” tutorial on the subject.

As a river warms from its winter lows in the mid-30’s to its summer highs in the 60’s there is a general, and consistent,  sequence of hatches that occur on our rivers year after year. For simplicity, let’s consider the Central Rocky Mountain Rivers near and around our fly shop in Silverthorne, Colorado. So we are talking about such rivers as:  the Blue River, the Frying Pan River, the Colorado River, the South Platte, the Arkansas River and the Roaring Fork River. The annual January through December hatch sequence goes like this: Midges, Blue Winged Olives (Baetis), Caddis, Salmon Flies, Golden Stones, Green Drakes, Yellow Sallies, Pale Morning Duns, Rusty Duns, Tricos, Blue Winged Olives and sporadic, larger late season Mayflies, October Caddis and back to Midges. The corresponding temperature sequence goes something like this:



Found in almost any water temperature above 32 degrees.  But at around 42 degrees expect to see the first significant, consistent hatch of the year.  Midges hatch year round and are very often a trout’s primary food source.



These babies like to hatch when the water temperatures are in the 46-56 degree range. You will see them in both spring and fall as the water warms from the winter’s frigid months and as the water temperatures come down from the summer’s heat. Overcast, drizzly conditions are also famous for triggering a significant BWO emergence.



Caddis will hatch spring through fall. The magic water temperature to see the first significant emergence for the year is right around 56 degrees.



Perhaps the most anticipated, and most difficult to anticipate, hatch of the year. This giant insect likes a sustained water temperature in the middle 50’s. However, and perhaps more than any other insect, the time of year plays a significant role in the Salmon Fly emergence. In Colorado, the Salmon Fly usually hatches in late May to early June. Further, it does not live in all Colorado rivers. And if it isn’t already confusing enough, the Salmon Fly hatch generally occurs during run-off. So getting a water temperature of 57 degrees in April—it could happen, maybe---before run-off starts typically won’t trigger a Salmon Fly emergence. 



The bigger Goldens usually follow shortly on the heels of the Salmon Fly. And the Yellow Sallies begin to be seen shortly after, or in conjunction with, the larger Golden Stones. The Sallies will hatch through much of the summer months and long after there are many larger Goldens on the water.



Green Drakes are also notoriously difficult to time. 58 degrees is about the temperature you should begin looking for them on the water. However, on some rivers, Green Drakes are infamous for only hatching at specific times of day. For example, on the Roaring Fork River, they favor the late evening into the darkness of night. On the Upper Arkansas, Green Drakes seem to like to hatch around 11 am.  While on the Blue River, a Green Drake hatch could occur at almost any time of day (or night) in late July into late August.



PMD’s usually like to see daily water temperatures peak out at near 60 degrees before gracing us with their presence. Though these bugs can hatch into the late summer, especially on tailwaters, PMD’s are usually thought of as the July bug. In July, it is not uncommon to see yellow sallies in the morning and PMD’s in the afternoon; or even both simultaneously!



Look for the watertemperature to return to the mid-50’s. BWO’s should continue to hatch until the daily high water temperatures fail to reach the mid-40’s.



So as it began, so shall it end. Midges will hatch year round but it is in the fall that they become very important again to the fly fisherman. Midges will be often be hatching in good numbers along with the fall BWO’s. But eventually the water will cool down enough that the BWO hatch stops and they are longer a food source for the trout.  Midge patterns will be then be your go to flies.

I want to leave you with a few final thoughts. First, when talking about water temperatures in relation to a specific insect, I am referring to the river’s maximum water temperature for the day. Second, this is by no means a comprehensive list of insect hatches in Colorado. There are lots of hatches we didn’t cover. We were only trying to outline the important relationship between water temperature and insect hatches. Third, with the exception of Midges and BWO’s, once a river “runs through” a water temperature it also “runs through” the insects that hatch at that water temperature. For example, if a river’s maximum temperature goes from 62 degrees back into the middle 50’s, you wouldn’t expect a previous hatch to resume; you wouldn’t expect to see Green Drakes, Salmon Flies or large Golden Stones hatch again. In most cases, once a hatch is over, it is over until next year. The primary exceptions to this are the Midges and the BWO’s: As summer slides into fall and winter, the BWO’s will make a fall appearance as the water temperatures again fall into the low 50’s. Likewise, when the fall water temperatures fail to reach the high 40’s, thereby also failing to trigger a BWO hatch,  the Midge will once again become the dominant insect for your day of fishing. This will last until the water becomes too cold for even the mighty Midge to hatch with any consistency. This generally occurs when the river temperatures don’t rise above 38 degrees. Even so, they still gotta eat sometime……so get out there!

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