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Rating depth structure for brook trout.
Recent public catches load as you reach this section.
219-acre lake in Park County, Montana. The lake tops out around 100 ft. The depth chart comes from published agency bathymetric survey data, so the contours are survey-derived rather than computer-modeled. Every species below gets its seasonal read from the same numbers driving the chart.
Traced from the shoreline, Rock Island Lake runs 219 acres with 2.2 miles of edge. A shoreline index of 1.1 says round and simple: fish relate to depth changes here, not shoreline features. Roughly 22% of the water is 15 ft or less, the band where the food chain lives. Figure 0.8 mi of water from one end to the other, 0.7 mi at the beam. You'll find it in Park County. For a second option, Fox Lake (Park) lies 1 mi northeast.
Come spring, look for cutthroat trout tight to the bank in 3 to 15 ft. Expect a summer slide to 10 to 35 ft, then a winter hold in 10 to 40 ft. Start at Main-lake point. It's a point holding about 1 ft of water and rates prime come fall. Prime time lands in early morning and the evening. A trolled spoon (small) or an inline spinner is the honest summer answer here.
| Season | Depth | Best window | Start at | Go-to baits |
|---|---|---|---|---|
| Spring | 3 to 15 ft | dawn & dusk | Main-lake point (1 ft) | Inline spinner (silver/gold), Small spoon |
| Summer | 10 to 35 ft | dawn & dusk | Main-lake point (1 ft) | Trolled spoon (small), Inline spinner |
| Fall | 5 to 25 ft | dawn & dusk | Main-lake point (1 ft) | Casting spoon, Inline spinner |
| Winter | 10 to 40 ft | dawn | Main-lake point (1 ft) | Small jigging spoon, Marabou jig |
In spring, brook trout run shallow here, mostly 2 to 15 ft. Summer pushes them deeper, 10 to 30 ft, and winter fish settle into 3 to 20 ft. Main-lake point is the standout, a point at roughly 1 ft, rated prime for spring brook trout. Time it for early morning and the evening. You won't need more than a small spoon fished deep or a trolled lake-clear wobbler + worm most summer days.
| Season | Depth | Best window | Start at | Go-to baits |
|---|---|---|---|---|
| Spring | 2 to 15 ft | dawn & dusk | Main-lake point (1 ft) | Small inline spinner (gold), Tiny spoon |
| Summer | 10 to 30 ft | dawn & dusk | open water | Small spoon fished deep, Trolled lake-clear wobbler + worm |
| Fall | 2 to 15 ft | midday & dawn | Main-lake point (1 ft) | Small spoon (orange), Spinner |
| Winter | 3 to 20 ft | dawn | Shallow bay flat (1 ft) | Small jigging spoon, Tungsten jig + plastic |
Rock Island Lake carries 6 mapped structure features, topping out (or rather bottoming out) at Deep basin, about 100 ft down.
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 Rock Island Lake specifically, so confirm the current rules before you fish.
Check the official Montana fishing regulations →
Rock Island Lake is about 100 ft (30 m) deep at its deepest point. The depth chart comes from published agency bathymetric survey data, so the contours are survey-derived rather than computer-modeled.
Anglers target cutthroat trout, brook trout on Rock Island Lake.
Main-lake point is the top-rated area: a point in about 1 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.
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.
Fishing on Rock Island 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.
Fox Lake (Park) 1 mi · Vernon Lake 1 mi · Widewater Lake (Carbon) 2 mi · Diamond Lake (Carbon) 2 mi · Big Moose Lake 2 mi · Big Moose Lake 3 mi, WY · Canyon Lake (Carbon) 3 mi · Ivy Lake 3 mi, WY · Aquarius Lake 3 mi · Russell Lake 3 mi · Broadwater Lake 4 mi · Mariane Lake 4 mi
See how Rock Island Lake stacks up: best brook trout lakes in Montana · best cutthroat trout lakes in Montana
More water nearby: Park County fishing maps (31 charts). Browse all Montana fishing maps. See every water on DepthScout. Bathymetry: published agency survey data. 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.