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Date of Award
Master of Science (MS)
Clark, Douglas H., 1961-
Mitchell, Robert J.
The Holocene glacial history of the North Cascades is poorly understood, in part because most existing records rely on moraine remnants and are therefore discontinuous. To develop a more complete record of Holocene fluctuations of North Cascades glaciers, we collected and analyzed glaciolacustrine sediments (i.e., rock flour) deposited over the past ~7800 years in Lyman Lake by the upstream Lyman Glacier. We combined these results with equilibrium-line altitude (ELA) reconstructions and glacier-climate modeling to quantify the climatic conditions that drove these fluctuations. Finally, we compared the Lyman Glacier’s continuous fluctuation record to existing glacier and climate records of the North Cascades.
Our results indicate that the Lyman Glacier was absent in the early Holocene, from before 7.8 ka until ~4.9 ka, when it experienced an early Neoglacial advance that persisted until at least ~3.8 ka. Following an extended non-glacial interval, the glacier experienced significant advances between ~2.6 – 2.25 ka, ~1.8 – 1.3 ka and ~1.1 – 0.9 ka. An advance starting ~ 0.8 ka (1150 CE) culminated at the glacier’s maximum Holocene extent between ~0.6-0.5 ka (~1350 – 1450 CE), from which it retreated and disappeared entirely by ~0.35 ka (~1600 CE). After ~200 years with no significant glacier presence in the cirque, the glacier reformed and rapidly advanced to its maximum Holocene extent (~1800 – 1900 CE). Following this event, the glacier retreated steadily throughout the 20th and early 21st centuries and as of 2014, has approached its minimum viable extent. Paleo-ELA reconstructions of the glacier’s maximum Holocene extents suggest that summers were ~2.6 °C cooler than modern (l981 – 2010 CE); alternatively, glacier-climate modeling indicates that annual temperatures ~1.5 °C cooler than modern would result in maximum glacier extents.
Combining these new results with existing North Cascades glacial records indicates that: 1) the earliest Neoglacial advances in the region (starting ~6 ka) occurred asynchronously, with higher latitude and more maritime sites experiencing earlier advances; 2) Neoglacial advances remained small, infrequent and asynchronous until the last millennium; 3) Beginning at ~1.0 ka, glaciers throughout the North Cascades advanced synchronously, signaling the onset of the Little Ice Age (LIA); 4) North Cascades glaciers reached their maximum Holocene extents during the 15th and early 16th centuries (~0.55 – 0.45 ka), followed by apparent regional retreat and a final smaller 19th century (~0.15 – 0.05 ka) re-advance. The asynchronous early-to-mid Neoglacial fluctuations followed by synchronous LIA behavior suggests that local climate factors drove glacier fluctuations until the regional climate signal became strong enough to induce synchrony ca. 1.0 ka. Although the inferred regional retreat remains uncertain, the disappearance of the Lyman Glacier in the mid-LIA (~0.45 – 0.15 ka) is consistent with the precipitation record at Castor Lake (~100 km to the east), which indicates unusually dry winter conditions between ~1450 – 1850 CE (~0.5 – 0.1 ka).
Western Washington University
Lyman Lake (Chelan County, Wash.); North Cascades (B.C. and Wash.), Washington (State)
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Wershow, Harold N., "A Holocene Glaciolacustrine Record of the Lyman Glacier and Implications for Glacier Fluctuations in the North Cascades, Washington" (2016). WWU Masters Thesis Collection. 513.