How did Henry Rowe Schoolcraft identify Lake Itasca as the Mississippi River's source?

In 1832, American explorer and ethnologist Henry Rowe Schoolcraft led an expedition to the upper reaches of the Mississippi River to locate its source. During this journey, he identified Lake Itasca in present-day Minnesota as the river's true starting point.312

Schoolcraft is credited with coining the name "Itasca" by combining the Latin words veritas ("truth") and caput ("head"), signifying it as the "true head" of the river. While some historians note that fur trader William Morrison may have visited the lake as early as 1803 or 1804, Schoolcraft’s 1832 expedition is the one that formally proposed and documented the lake as the source. His claim was later confirmed by the survey conducted by French explorer Joseph Nicollet in 1836.21