150 YEARS OF GLACIER STUDIES IN YOSEMITE NATIONAL PARK: REPRODUCING JOHN MUIR’S 1872 VELOCITY MEASUREMENTS OF THE MACLURE GLACIER

The Lyell and Maclure glaciers in Yosemite National Park are among the longest studied glaciers in North America. These Little Ice Age features, situated at the headwaters of the Tuolumne River, provide critical runoff during the late summer and fall of drought years. Both glaciers retreated substantially since they were first mapped in 1883, with digital reconstructions of historical ice surfaces and subglacial bedrock indicating a 90% reduction in volume. The first scientific study of the glaciers was initiated on 21 August 1872 by the naturalist John Muir and his companion Galen Clark, when they installed five stakes made of whitebark pine in the Maclure Glacier, establishing their positions along a “plumb line made of a stone and a black horsehair”. Returning on 6 October 1872 Muir found that the center stake had moved about 2.5 cm/day. We reproduced Muir’s experiment in 2012, measuring stakes on 21 August and 6 October. Despite being dramatically smaller than when Muir measured it in 1872, we found the Maclure Glacier to be moving at the same maximum rate of 2.5 cm/day, with the substantial decrease in ice thickness since 1872 likely compensated as a driving force by a steepening of the ice surface. In contrast, we found that the adjacent Lyell Glacier had stagnated, likely due to thinning below a critical threshold. In 2022 we repeated Muir’s experiment again, 150 years to the day, finding a maximum rate of 1.9/cm day, 25% slower than Muir measured in 1872 and we measured in 2012. The Lyell Glacier remains stagnant. Although technically the Maclure Glacier is still a “living glacier” as Muir referred to it, it is slowing, with both long term data trends and recent observations indicating that the glacier is reaching a critically thin state.