BIG THERRIAULT LAKE Depth Map & Fishing Spots (MT)

Hatch outlook

Grasshoppers (east-side summer)Active window

Typical here: Jul 1 – Oct 15 · until the hard frosts of mid-October; on the Missouri, August hoppers fish mid-river, not the banks

adultall day in high summer, best when afternoon wind knocks bugs onto the water

Pulls fish to the windward grassy banks and edges for months — fish the bank the wind blows ONTO.

The friendliest crossover of the summer: on lakes, drift the windward bank with a bubble-and-foam-hopper — the bubble's splash doesn't hurt a hopper presentation. On rivers, the same rig dead-drifted along the undercut grass banks, or small gold spinners under the banks between rises.

The event happens over grassy banks, undercut edges and windward shorelines

Spruce MothActive window

Typical here: Jul 20 – Aug 31 · a conifer-forest event — it needs fir and lodgepole on the banks, so it's a Rock Creek / Blackfoot / upper Bitterroot specialty

adultmorning through midday — moths don't enter the water willingly — landings are sporadic and near the conifer banks; fish the imitation LOW in the film with minimal floatant, even fully drowned, with a little movement

When the moths show, just about every run produces — large trout key on them so hard that one pattern carries the whole box for two weeks.

Honest limit: a small drowned-surface moth has no lure analog. The workable rig is a casting bubble with a drowned moth pattern along the timbered banks in the morning; otherwise fish spinners early and late and leave the moth window to the film.

2–3 weeks at best, and day-to-day — fish key on the moths when they show and vanish with them

The event happens over banks under conifers — the moths rarely reach mid-river

Recommendations come from depth structure and seasonal species behavior. Treat them as a starting point, not a guarantee. Contours here are interpolated from real agency depth surveys (state DNR / USACE). Never use for navigation. Always check local regulations.

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Recent catches at BIG THERRIAULT LAKE

Shared by DepthScout anglers. Exact spots stay private, always.

No shared catches here yet — be the first.Log a catch from the map above and share it to put BIG THERRIAULT LAKE on the board.

Fishing BIG THERRIAULT LAKE: what the data says

51-acre lake in Lincoln County, Montana. The lake tops out around 80 ft. The depth chart is built from real state fisheries GPS surveys, so the humps, channels and drop-offs you see are measured, not guessed. Every species below gets its seasonal read from the same numbers driving the chart.

Traced from the shoreline, BIG THERRIAULT LAKE runs 56 acres with 1.2 miles of edge. Its 1.1 shoreline index means few bank features; the contours below do the pattern work. About 13% of the surface sits shallower than 15 ft — that's the weed-and-bait zone. The basin spans about 0.4 mi end to end and 0.3 mi across at its widest. You'll find it in Lincoln County. For a second option, Little Therriault Lake lies 1 mi northwest.

Cutthroat Trout fishing at BIG THERRIAULT LAKE

Early season cutthroat trout use the shallows, holding in 3 to 15 ft. Expect a summer slide to 10 to 35 ft, then a winter hold in 10 to 40 ft. Put Main-lake point first on the milk run — a point near 16 ft that grades prime for fall. In summer the feeding windows fall in early morning and the evening. Tie on a trolled spoon (small) or an inline spinner and you're in the game.

Season	Depth	Best window	Start at	Go-to baits
Spring	3 to 15 ft	dawn & dusk	Main-lake point (16 ft)	Inline spinner (silver/gold), Small spoon
Summer	10 to 35 ft	dawn & dusk	Main-lake point (16 ft)	Trolled spoon (small), Inline spinner
Fall	5 to 25 ft	dawn & dusk	Main-lake point (16 ft)	Casting spoon, Inline spinner
Winter	10 to 40 ft	dawn	Main-lake point (16 ft)	Small jigging spoon, Marabou jig

Notable structure

The chart marks 5 structure features on BIG THERRIAULT LAKE; the deepest is Deep basin at about 79 ft.

Deep basin: deep basin, around 79 ft
Break off the point: drop-off, around 58 ft
Main-lake point: point, around 16 ft
Shallow bay flat: feeding flat, around 7 ft
Back-bay flat: feeding flat, around 7 ft

BIG THERRIAULT LAKE fishing regulations

Season dates, license requirements, and size and bag limits for Montana waters are set by the Montana Fish, Wildlife & Parks — and special regulations can apply to BIG THERRIAULT LAKE specifically, so confirm the current rules before you fish.

Check the official Montana fishing regulations →

Common questions

How deep is BIG THERRIAULT LAKE?

BIG THERRIAULT LAKE is about 80 ft (24 m) deep at its deepest point. The depth chart here comes from real state fisheries GPS surveys, so the contours reflect measured depths.

What fish are in BIG THERRIAULT LAKE?

Anglers target cutthroat trout on BIG THERRIAULT LAKE.

Where is the best fishing spot on BIG THERRIAULT LAKE?

Main-lake point is the top-rated area: a point in about 16 ft of water that rates prime for cutthroat trout in fall. Open the map above to see every rated spot for each species and season.

When is the best time to fish BIG THERRIAULT LAKE?

For cutthroat trout, the summer bite is best in early morning and the evening. Each species page section below lists the seasonal windows, and the map shows a live NOW badge when you're inside one.

What are the fishing regulations for BIG THERRIAULT LAKE?

Fishing on BIG THERRIAULT LAKE is regulated by the Montana Fish, Wildlife & Parks. Check the official Montana regulations for current license requirements, season dates, and size and bag limits — special regulations can apply to this specific water.

Nearby waters

Little Therriault Lake 1 mi · Glen Lake 6 mi · Rock Lake 9 mi · Long Lake (Lincoln) 9 mi · Moran Lake 11 mi · Frank Lake 11 mi · Tetrault Lake 11 mi · Sophie Lake 11 mi · Shotnana Lake 12 mi, BC · Martin Lake (Lincoln) 13 mi · Murphy Lake 14 mi · Edwards Lakes 15 mi, BC

See how BIG THERRIAULT LAKE stacks up: best cutthroat trout lakes in Montana

More water nearby: Lincoln County fishing maps (28 charts). Browse all Montana fishing maps or see every water on DepthScout. Bathymetry credit: state fisheries survey programs. How we build these maps. Fish here with a sonar unit that logs? Send us your depth data and we'll improve this chart, credited to you. For fishing guidance only, never navigation.