The flies will range from size #18 to #24. The baetis nymphs will begin to move about in early March so fishing a sparkle baetis or other nymph pattern down deep with weight will produce results early.
Typically the Baetis hatch starts lower down valley on the Colorado in later March or early April and then proceeds up valley arriving in Basalt in late April or early May. BWO's will start to appear in the upper reaches of the Fork and the Pan early to mid-May.
The primary period for this hatch therefore is from May through to October with September and October being the best.
The baetis hatches on the Colorado can be tremendous if the run-off holds back long enough for the hatch to develop. With a colder spring and a little later run-off there will be a great opportunity to enjoy the baetis in the light green water. However, if the spring is warm and the run-off starts before the baetis hatch gets into full swing the Colorado will be difficult to fish well.
Like most mayflies the Baetis loves to hatch on gray overcast days between 11:00 a.m. and 4:00 p.m.. Some time between noon and 3:00p.m. is usually the best. They will hatch in waves, so it is best to be prepared to stay out despite the weather to catch the hatch. Some great hatches occur during a downpour so while you let your buddies go in for a break stay out and fish on through the wettest, coldest and snowiest times.
Fish this hatch with a high floating dry fly such as a BWO comparadun with an olive biot emerger or a baetis nymph as a trailer. If you like, apply some sink or spit on the trailer to hold it below the surface film. The recommended dropper is usually 12" to 18" long.
If the water it boiling with fish, don't both with the dropper as it will interfere if the fish are taking the duns off the surface. The dropper will tend to twist around the fighting fish and make releasing it more difficult.
Fish a three or four weight rod and a 6x or 7x tippet. I find that a softer rod is preferable when mending. A fast action rod will tend to exert too much power during the mend and therefore will drag the single dry fly or worse turn the mend into a roll cast. A soft rod will not tend to produce this result. If you are fishing with a dropper, the problem will not be as evident as the dropper will tend to hold back the line in the water during the mend.
Furthermore, a light softer rod gives a more delicate presentation and doesn't pop the fine tippet as easily during casting. 6x or 7x tippet is a must on the upper Frying Pan while fishing small flies. I like to fish 8x but one loses a lot of flies!
Nymphs. Fish the nymph near the bottom. As it is a swimmer there will be something moving most of the time, falling prey to a fish. Fish them deep with shot. They can be fished at any time so there is no need to wait for a hatch. Let the nymph drift. Allow the full drift high sticking to minimize drag. Sometimes the trout will take them on the swing at the end of the drift so don't recast too quickly. Near riffles, cast towards the end and let the fly drift down to the slower deeper water.
Sometimes, I will fish the nymph as a dropper with a 2ft distance from the lead dry fly. In the shallower water, the fish will opportunistically take them tumbling through the current.
I have also found when the water flow is kicked up in the Pan in summer and the fish spread out, putting a sparkle baetis behind a larger fly such as a stimulator will work well. I suspect that with the fish being displaced by the faster water, they will opportunistically take a nymph near the surface after being attracted by the stimulator.
Emergers and cripples. The nymphs are helpless as they struggle to release themselves. Therefore look where debris is most likely to gather on the surface of the water. Eddies with froth are good places. Just identify where the greatest accumulation of debris caught in the film of the moving water will drift to and hold. That is where the helpless emerger or cripple will coincide with the hungry trout.
Fishing eddies with different cross currents can be a little tricky. But a longer rod with the ability to reach and create slack line will help. In any case pile the line so it will give you a longer uninterrupted drift.
Duns. The first thing to remember is to not get too excited about the hatch and put on a dry straight away. Often the fish might appear to be taking the duns when in fact they are taking the emergers and cripples trapped in the film. So be wary about ignoring emerger patterns. You will know when they are taking mostly duns because the principal rising fish will be picking them off the surface.
A good way to get the best of both worlds is to use a dry fly and a nymph or emerger dropper. Use the lightest tippet you are comfortable with and grease the line to help keep the dry up. If you use fluorocarbon to connect the dropper it will sink a little better and be less visible.
When matching the hatch, sometimes it can be a problem if you are too perfect. In a multitude of almost identical insects, it might be difficult to identify your fly so you won't know if it has been sipped. Therefore something slightly darker might help. However, be mindful that the fish must still be fooled. If there is a choice err on the side of caution and go lighter tippet and smaller fly.
Remember that the fish just sip the fly and have a long time to inspect and decide whether to take it. If you watch carefully you will note that sometimes a fish will follow a fly for some distance before it decides to take it or leave it. As soon as the fly acts unnaturally, the fish will turn away. In addition the slower water affords greater inspection opportunities to the wary fish.
The most important thing though is to get a good drift. The best fly is useless if it acts unnaturally. Take your time. Move into position and aim for the best drift. In this area one can fish across and even down to a feeding fish without disturbing it so it is not too difficult to spend time getting into a good lateral position to make the cast.
Spinners: Where the spinner lays eggs underwater, the fish can be addressed with a soft hackle pattern. It will be drifted in the current. Otherwise if fishing in the surface film, the fly is exhausted so it will be drifted with the lightest possible tippet and left to the current.