Henry's Fork Bug Report

What's living on the river bottom — reach by reach (2025) and across the last decade (2016–2025). From the Foundation's macroinvertebrate sampling program. The nymphs counted here are next season's hatches.

~1.7 billion

PMD nymphs per river-mile at Last Chance (2025)

~3,300 / sq ft

Invertebrates packed onto every square foot of riverbed — a city underwater

~33,000 / m²

Avg bug density across all six reaches

53% EPT

Mayflies + stoneflies + caddis — "off the charts" for a trout stream

45% → 59%

%EPT rose over the decade — water quality improving

What's "EPT"?

The three aquatic-insect orders anglers live for — and the ones biologists grade water quality by:

E — Ephemeroptera = mayflies (PMDs, BWOs, Green Drakes, Tricos)
P — Plecoptera = stoneflies (salmonflies, golden stones, yellow sallies)
T — Trichoptera = caddisflies (spotted sedge, Mother's Day caddis)

These bugs are sensitive to pollution and warm, silty water, so the % of all bugs that are EPT is a quick health grade: under 30% = marginal, over 50% = excellent. The bonus for fishermen: EPT bugs are exactly what you tie on — high %EPT means clean water and good hatches. The Henry's Fork runs ~53%.

How much food in each reach?

Total invertebrate density (individuals/m²), upstream → downstream.

The good bugs (%EPT)

% mayflies, stoneflies & caddis. Above 30% = good water; the Henry's Fork is double that.

The hatch lineup, reach by reach

Density (individuals/m²) of the bugs anglers actually fish — by site.

Where are the PMDs?

Pale Morning Dun (Ephemerella) nymph density — the river's signature hatch.

Salmonfly water

Giant salmonfly (Pteronarcys californica) nymphs — Box Canyon/Ashton reaches.

PMDs at Last Chance, by year

The famous Ranch hatch. Best modern years: 2025, 2017, 2020. Note: bug numbers don't track perceived hatch quality.

Water quality climbing (%EPT)

Watershed-average %EPT by year — a steady improvement in the bug community.

Total bug abundance, by year

Watershed average. Drifting down ~4%/yr — midges/non-insects replaced by EPT bugs.

PMDs at Flat Rock (cold, pristine reach)

By far the most PMDs of any site — but the one reach showing a concerning decline.

What the decade says (HFF tech report, 2015–2025)

Water quality is improving: %EPT 45→59%, EPT species +2.5, Hilsenhoff index 4.4→3.9 ("good"→"very good").
Total numbers down ~4%/yr — mostly midges & non-insects replaced by mayflies/stoneflies/caddis (a better community).
Trout aren't food-limited — even tough years have huge bug numbers; perceived hatch decline tracks trout numbers, turbidity & warming, not bug counts.
Flat Rock (coldest, most pristine) is the lone reach with concerning declines — most to lose in a warming climate.