Updated August 2026
Deepest Lake in Montana: Tally Lake (495 ft)
Tally Lake is Montana's deepest lake at 495 ft, per the Montana Fish, Wildlife & Parks. Reclamation reservoir on the South Fork Flathead, entirely within Montana; USACE NID gives Hungry Horse Dam a hydraulic height (depth of water at the dam at full pool) of 515 ft, which exceeds Tally Lake. Excluded because no agency publishes a bathymetric maximum depth for the reservoir - 515 ft is a dam-engineering figure, not a lake sounding - and the pool is drawn down 100+ ft seasonally, so its depth is not a stable lake statistic. If you want the strictly-deepest-water answer including reservoirs, it is Hungry Horse, not Tally. None of Montana's lake pages carries a surveyed maximum depth yet, so there is no ranked table or map.
Browse all Montana lakes for the water-quality data that does exist, or open Tally Lake's report card.
No ranked-set map
A map here plots lakes with a surveyed maximum depth at their own coordinates. None of Montana's lake pages carries one yet, so there is nothing to plot.
Sources and method
- Deepest lake: Tally Lake, 495 ft, per the Montana Fish, Wildlife & Parks, 2011-07-18 (FWP Tally Lake Bathymetry Survey Map). Tally Lake report card
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Frequently Asked Questions
What is the deepest lake in Montana?
Tally Lake, at 495 ft, in Flathead County. Reclamation reservoir on the South Fork Flathead, entirely within Montana; USACE NID gives Hungry Horse Dam a hydraulic height (depth of water at the dam at full pool) of 515 ft, which exceeds Tally Lake. Excluded because no agency publishes a bathymetric maximum depth for the reservoir - 515 ft is a dam-engineering figure, not a lake sounding - and the pool is drawn down 100+ ft seasonally, so its depth is not a stable lake statistic. If you want the strictly-deepest-water answer including reservoirs, it is Hungry Horse, not Tally. Source: Montana Fish, Wildlife & Parks, 2011-07-18 (FWP Tally Lake Bathymetry Survey Map).
How are lake depths measured?
Maximum depth is the deepest point at full-pool water level, from state agency bathymetric surveys and, for reservoirs, dam-design records. The ranking uses surveyed depths only, never modeled estimates. Everyday water level can sit below the surveyed figure during drawdowns or drought. Mean depth, the average across the whole basin, is listed where a survey recorded it.
Why are some Montana lakes so much deeper than others?
Lake depth depends on how the lake formed. Glacial lakes (common in MN, WI, MI, NY) were carved by ice sheets and can exceed 200 ft. Reservoir lakes (common in IA, MO, OK, KS) are shaped by river valley depth at the dam site — typically 50-150 ft at the dam, much shallower upstream. Flooded mine pits can be deeper still. Oxbow lakes and pothole lakes formed by river meandering or glacial pitting tend to be shallower, usually under 30 ft.
Does deeper mean cleaner?
Not directly, but deeper lakes tend to be cleaner on average. Depth creates thermal stratification in summer, isolating cold deep water from surface algal blooms. Deep lakes also resist nutrient resuspension from wind-driven sediment mixing. That said, many deep lakes still suffer from impairment if the watershed loads heavy phosphorus — each lake's page carries its grade and EPA impairment status.