JasonM-
What month of the year does this emergence seem to peak?
If it's some time between mid-June and mid-July, my guess would be Paraleptophlebia memorialis.
Here is a detailed description:
Family: Leptophlebiidae
Scientific name: Paraleptophlebia memorialis
Previously know as: Leptophlebia memorialis, Leptophlebia pallipes (in part), Paraleptophlebia pallipes
Common name: Mahogany Dun, etc.
Locality: W
CAN Regions: NW
MEX Regions: SW
USA Regions: NW, SW
Cent. Amer. Countries:
CAN Provinces: AB, BC.
MEX States: BN.
USA States: AZ, CA, CO, ID, NM, OR, SD, UT, WA
Habitat:
Voltinism:
Emergence (begin) date: mid-Jun
Emergence (end) date: early-Aug
Emergence time of day: afternoon
Spinner fall time of day:
Nymph minimum length: 6 mm.
Nymph maximum length: 7 mm.
Nymph identification keys: gill fork 1/3 from base; abdominal spines on segment 8 & 9; tergites dark, each w/three light dots, a medium-sized one on center-line, and a small-sized one forward on each side of center-line
Nymph body description:
Nymph legs:
Nymph gills: gill/filament length ratio (1:3), trachael branching: (absent)
Nymph tusks: absent
Nymph tails: 3,
Dun minimum length: 6 mm.
Dun maximum length: 8 mm.
Dun identification keys:
Dun body description: brown to dark reddish brown
Dun wings: medium gray, hind wings w/o costal angulation
Dun legs: brown
Dun tails: 3, light to dark brown
Spinner minimum length: 7 mm.
Spinner maximum length: 7 mm.
Spinner identification keys: male claspers w/3 joints
Spinner body description: abdomen whitish
Spinner wings: hyaline, fore wing with attached intercalaries, hind wing w/o costal angulation
Spinner legs:
Spinner tails: 3,