Colorado fly box: what to carry by season

A fly box is a set of answers to questions the river asks. This guide is about which questions come up on Colorado water through the year, and what kind of fly answers each one.

One thing to be clear about up front, because a lot of fly writing is not. Nobody can promise you a hatch. Insect emergence depends on water temperature, flow, weather and daylight, all of which shift year to year, and a river that produced a heavy hatch in the second week of April one season may do nothing of the kind the next. So this is organised by what the river is doing rather than by dates, and where a specific timing matters it says to check locally rather than trusting a calendar on a web page.

Month-by-month detail for named Colorado rivers is in the Colorado Playbook, which carries an edition date so you know how current it is. What follows is the reasoning that sits behind any such list, and it does not go out of date.

The principle: quantity before variety

The most common fly box mistake is variety at the expense of quantity. Twenty patterns in one size each looks like preparedness and behaves like scarcity: the moment the one that works starts getting eaten, lost to trees or destroyed by fish, you are out of it and back to guessing.

Five patterns in three sizes and two weights is a far more capable box than twenty singles. And when one of a pair of nymphs accounts for most of your fish, the useful response is to put a version of it on both droppers, which you cannot do if you own one.

The food types worth understanding

Almost everything in a Colorado box is imitating one of a handful of things. Knowing which is which is more useful than memorising pattern names, because it tells you what to reach for when conditions change.

• Midges. Small, present year-round, and the backbone of winter tailwater fishing. Carried as larvae, pupae and the occasional adult.
• Mayflies. Nymphs, emergers, duns and spinners, with distinct shapes and sizes depending on species. The classic dry fly fishing on most rivers.
• Caddis. Larvae, pupae and adults. Often the most abundant food in freestone rivers, and a fly that moves rather than drifts inert.
• Stoneflies. Large nymphs that live among rocks for years before emerging, which is why a big stonefly nymph is worth carrying regardless of whether anything is hatching.
• Terrestrials. Ants, beetles, grasshoppers. Summer and early autumn, when wind and grass put land insects on the water.
• Eggs and worms. Not insects, but genuinely present when fish are spawning upstream or when high water dislodges aquatic worms.
• Streamers. Imitating small fish and other larger prey. A different game entirely, fished actively rather than drifted.

Winter

What the river is doing: cold and mostly stable. Freestones are low and locked up or close to it; tailwaters below dams are running clear, cold and predictable, which is why they are where people fish in January.

What the fish are doing: holding deep and slow, feeding with much less enthusiasm, and not moving far for anything. Their metabolism is down, so the window you have to put a fly into is small.

What that means for the box: small, subsurface and deep. Midge larvae and pupae in small sizes do the majority of the work. Eggs can be worth carrying where fish are spawning. A weighted attractor nymph as the top fly on a pair gets the rig down and occasionally gets eaten itself.

Depth control matters far more than pattern selection in winter. If you are not occasionally touching bottom, you are fishing above the fish, and no fly fixes that. See nymph rigs and why yours is not getting deep enough.

Spring, before runoff

What the river is doing: warming slowly, still relatively clear, and rising gently on the warmer days as low-elevation snow begins to move.

What the fish are doing: becoming genuinely active as water temperature climbs. This is one of the better windows of the year, and it is short.

What that means for the box: midges still, but mayfly nymphs and emergers become worth carrying in numbers, and this is often the first honest dry fly fishing of the year. Small dark mayfly patterns cover a lot of ground. Keep the winter depth rig available, because a cold snap puts things straight back.

Runoff

What the river is doing: on freestones, rising hard, running cold and carrying enough sediment to cut visibility to inches. Tailwaters are largely unaffected, which is the whole reason they matter in Colorado.

What the fish are doing: moving out of the main flow to the edges, behind structure and into slower water, because holding mid-river at four times normal volume is not worth the calories.

What that means for the box: bigger, heavier and easier to find. Large stonefly nymphs, worm imitations, and anything with contrast or flash that a fish can locate in dirty water. This is not a subtlety season. Heavier tippet too, since nothing is inspecting it and you need to pull rigs free.

The more important adjustment is where you look rather than what you tie on: the fish are within a few feet of the bank, in water you would walk past in July. Fishing a freestone through runoff covers the rest, including when not to go.

Summer

What the river is doing: dropping and clearing after runoff, then warming through the season. High country opens up. Tailwaters stay cold.

What the fish are doing: feeding well, and moving toward oxygenated water as things warm. In genuinely hot low water they become stressed, which is a planning consideration and an ethical one.

What that means for the box: the widest range of the year. Caddis in all stages, a variety of mayflies, terrestrials once the grass is up and the wind is working, and small nymphs for the middle of bright days. Attractor dries earn their place on freestones and in the high country.

Watch water temperature. When it climbs, playing a fish can kill it even if you release it carefully, and CPW publishes voluntary and mandatory closures for exactly that reason. A cheap stream thermometer is the relevant piece of equipment here.

Fall

What the river is doing: flows stabilising and dropping, water cooling, and considerably fewer people on the bank.

What the fish are doing: feeding hard as water cools into their preferred range, and in some waters moving in preparation for spawning, which changes where they are and brings its own responsibilities about where you tread.

What that means for the box: smaller mayflies come back into their own, caddis taper off, terrestrials hang on until the first hard frosts, and streamers become genuinely worthwhile as larger fish get aggressive. Eggs matter around spawning fish.

If you fish over spawning fish, avoid walking on gravel that is obviously a redd. That is not a gear point, but it belongs on any honest autumn page.

Organising it so you can use it

What this page will not tell you

Exact patterns, sizes and quantities for a named river in a named month. Not because it is a secret, but because it is the most perishable information in the sport and a frozen list on a web page is how people end up fishing last year's answer.

For that, three things, in increasing order of specificity: our river guides tell you what kind of water each Colorado river is, the Colorado Playbook carries a twelve-month hatch calendar and fly box checklists for fifteen rivers with an edition date, and a local fly shop will tell you what is working this week for free. Use all three.