Yellowstone Lake drains north from its only outlet, the Yellowstone River, at Fishing Bridge. The elevation of the lake's north end does not drop substantially until LeHardy Rapids. Therefore, this spot is considered the actual northern boundary of Yellowstone Lake.
Paul LeHardy, a civilian topographer surveying the Yellowstone River, met trouble at these rapids. In 1873 his makeshift raft was sucked to the bottom, forcing him to salvage what he could and tramp downriver to rejoin the Jones Expedition.
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