During the first year he discovered Lake Tear-of-the-Clouds, often considered the source of the Hudson River. He directed surveying parties throughout the Adirondacks and determined the altitudes of most of the highest peaks, becoming obsessed with his task. Determined to fix the precise altitude of Mount Marcy (having decided that the barometric method of determining altitude was insufficiently accurate) he established a series of 111 elevation benchmarks along the course of a forty mile trek from Westport on Lake Champlain to the summit of Marcy, each intermediate altitude being calculated to a precision of one-thousandth of a foot. As the crew approached the summit of Marcy, they encountered an October snow storm with ice and freezing rain; despite urging by his guides and assistants to wait for better weather, Colvin pushed on despite the danger of becoming trapped in Panther Gorge. At 4:50 PM on November 4, 1875, Colvin took the final observation on the summit of Mt. Marcy and determined its elevation to be 5,344.311 feet above mean tide level in the Hudson. Upon his return to Panther Gorge later that evening, Colvin wrote Joy reigned in the camp to-night, for we had accomplished the work in the face of the greatest difficulties; when the most experienced guide had ventured to say that it was for this season impossible. In honor of the event we had a choice supper of bear's meat fried, boiled and stewed, and hard bread and biscuit; then, unmindful of the snow storm beating in and around our open shanty, fell fast asleep.