INFLUENCE OF PRECIPITATION GRADIENTS ON LEE-SIDE GLACIER DYNAMICS IN THE OLYMPIC MOUNTAINS, WASHINGTON STATE

The glacial record of the Elwha basin in the lee of the Olympic Mountains is poorly documented in contrast to the well-constrained glacial history of the west side. The broad headwaters of the Elwha are at high elevations and receive considerable spillover precipitation in today’s climate. Subsequently, the valley narrows substantially at middle elevations. If an Elwha glacier extended through this notch, it would likely have been fast-flowing and highly erosive. Any distal record of this glacier would be complicated by interaction with and overprinting by the Cordilleran Ice Sheet, which flowed through the Strait of Juan de Fuca across the outlet of the Elwha. We ask if the climatic and glacial history of the west side can be leveraged to predict the scale of glaciation in the Elwha. A prominent feature of the climate of the Olympics is the large rain shadow. Using numerical weather modeling and 100-year records of river discharge we document the strength of this precipitation gradient and assess the potential for it to have been significantly different during glacial periods. Modern basin-averaged annual precipitation in the Elwha is ~50-60% of that in the Quinault on the west side. This ratio does not vary significantly with regional climate patterns such as ENSO and PDO or wind direction. Cooler temperatures are weakly associated with a flatter or reversed precipitation gradient. Our analysis does not suggest a mechanism for increasing the precipitation gradient, but overwhelmingly indicates spatially-coherent variability in precipitation across the peninsula. As such, we built a 1-D flowline model of the Quinault that matches observed moraine positions driven by temperature and precipitation consistent with paleoclimate data. If we assume a precipitation gradient similar to today’s, the corresponding flowline model for the Elwha glacier suggests that glaciation of the notch was possible. Future mapping and erosion rate measurements in the Elwha basin could assess this possibility and motivate a reinterpretation of interactions between climate, erosion and tectonics in the Olympic Mountains.