Whether Colter reached Yellowstone Lake at all is disputed. The 1814 Clark map shows him at 'Eustis Lake' — an early name that corresponds, geometrically, to Yellowstone Lake — but the geography Clark drew from Colter's testimony is famously distorted. This is one of the route's defining low-confidence anchors.
Yellowstone Lake and its surrounding country were a known seasonal use area for Shoshone, Bannock, Crow, and (further north) Blackfeet. The Tukudika (Sheep Eater) Shoshone had a year-round presence in the surrounding high country well into the nineteenth century, recognized by NPS only in recent decades.
The geography around the lake was peripheral to the main fur-trade transit routes of 1807-08; the high interior of what is now Yellowstone was too rugged and too cold to be a primary trapping zone before the 1820s rendezvous era expanded operations.
The 1814 Clark map labels a lake on Colter's traced route 'Lake Eustis' (after William Eustis, then U.S. Secretary of War). The lake on the map is large, oriented roughly correctly, and located approximately where Yellowstone Lake sits. But the map's distances are compressed and several other features are mislocated; reading 'Eustis Lake' as a precise visit-by-Colter claim has been contested for over a century.
Mattes (1962) accepted Yellowstone Lake. Harris (1952) was more cautious. The map renders the lake-reach as a low-confidence segment.
Whether Colter saw the lake at all. Which shore he saw if he did. Whether the West Thumb thermals (which are real, and which he plausibly described to Clark) were the source of the broader 'Colter's Hell' confusion, separate from the actual Cody hydrothermal field.
Yellowstone Lake is accessed via the East Entrance road or the Grand Loop south. West Thumb Geyser Basin is the most-visited interpretive feature on the lake. Lake Village, the Lake Yellowstone Hotel, and Fishing Bridge are the developed visitor areas. Yellowstone Forever interpretive programming covers the lake heavily.
Phase 3 target: West Thumb 360° set, framed as Colter's plausible hydrothermal anchor (separate from the Cody-field Colter's Hell), with explicit historiographic caveat.
Sources & attribution: Yellowstone Forever · NPS Yellowstone · Merrill J. Mattes (NPS, 1962) · Burton Harris (1952)