The cold oxygenated streams of Colorado produce an abundance of stoneflies. These insects spend from one to three years as a nymph becoming readily available to trout immediately prior to and during their emergence. As a consequence, stoneflies of varying size are always present for trout.

The two largest and most important stoneflies found on western waters are the Giant Black Stonefly, (Pteronarcys californica) and the Golden Stonefly, (Acroneuria pacifica). Wherever these two stoneflies are found in good numbers, big trout are also found. Stoneflies require unpolluted, fast flowing waters with bottoms paved with large rocks.

Giant Stoneflies
Pteronarcys californica, larva: 38-43 mm excluding tails; occurs under rocks with accumulated debris or in beds of aquatic plants in streams and rivers in mountainous western North America.

Pteronarcys californica, adult (the Salmon Fly): 38-41 mm to wing tips; mid-April through early-August emergence, with largest numbers flying in June.

Perlodid Stoneflies
Isoperla bilineata, larva: 9-11 mm excluding tails; common inhabitant of medium-size to large rivers of central and eastern North America.

Isoperla bilineata, adult (the Little Yellow Stonefly): 13-15 mm to wing tips; this species has been restricted by water pollution.

Aquatic Entomology: The Fishermen's Guide and Ecologists' Illustrated Guide to Insects and Their Relatives by W. Patrick McCafferty

Prior to hatching stoneflies migrate towards shore across the river bottom, exposing themselves to trout looking for an easy meal. Once out of the river, stoneflies seek shelter in the branches of willows, rocks or trees. Upon reaching the bank they allow their skins to air-dry, to become brittle and to split open introducing the winged adults. After mating, the female stonefly sails over the river, depositing her fertilized eggs. Clumsy fliers under the best conditions, stoneflies frequently crash into the surface and become hopelessly cauth in the current.

When conditions on the West's great rivers are ideal, anglers frequently will travel from all parts of the country to fish this hatch.

As other hatches do, stonefly hatches progress upstream, starting first in the lower reaches of the river, playing itself out far upstream. As with all aquatic insects, stonefly hatches are triggered when water temperatures reach optimum degree. What that temperature happens to be depends on the river. Day by day, stoneflies will hatch progressively farther upstream until the hatch is over for the year.

Colorado River, Colorado - Mid-June between Grand Junction to Lake Granby. Best stretches are from Kremmling to Hot Sulfur Springs. Black stoneflies in sizes 4 to 10.
Gunnison River, Colorado - Mid June, depends on water discharges from dams. Black Stonefly.
Dolores River, Colorado - Variable, spring, depends on water releases from McPhee Reservoir. Golden Stone, size 6-12.
Blue River, Colorado - Hatch erratic due to water level fluctuations. Golden Stone, size 6-12
Frying Pan/Roaring Fork Rivers, Colorado - Early June to mid June. Black stonefly.