Spatial and temporal controls on proglacial erosion rates: A comparison of four basins on Mount Rainier, 1960 to 2017

Abstract

The retreat of alpine glaciers since the mid-19th century has triggered rapid landscape adjustments in many headwater basins. However, the degree to which decadal-scale glacier retreat is associated with systematic or substantial changes in overall coarse sediment export, with the potential to impact downstream river dynamics, remains poorly understood. Here, we use repeat topographic surveys to assess geomorphic change in four partly glaciated basins on a stratovolcano (Mount Rainier) in Washington State at roughly decadal intervals from 1960 to 2017. The proglacial extents of the four basins show distinct geomorphic trajectories, ranging from substantial and sustained net erosion to relatively inactive with net deposition.These different trajectories correspond to differences in initial (1960) valley floor gradients, and can be effectively understood as valley floor grade adjustments. Significant erosion was most often accomplished by debris flows triggered by extreme rainfall or glacial outburst floods, though a single rockfall mobilized more material than all other events combined. Year-to-year runoff events had little measurable geomorphic impact. Exported material tended to accumulate in broad deposits within several kilometers of source areas and largely remained there through the end of the study period. Over 10-to 100-year timescales, we find that the impact of glacier retreat on coarse sediment yield may then vary substantially according to the geometry and storage trends of the newly exposed valley floor; the timing of that response may also be dictated, and potentially obscured, by stochastic and/or extreme events.

Predicting probabilities of late summer surface flow presence in a glaciated mountainous headwater region

“…The climate is temperate with cool wet winters and warm, dry summers. Precipitation is rain‐dominated at lower elevations and largely snow‐dominated at higher elevations although the maritime climate can result in fall and winter rainstorms at high elevations (Anderson & Shean, 2022; Frans et al, 2018).…”

“…In Mount Rainier National Park, USA, which includes Mount Rainier, a heavily glaciated stratovolcano, and surrounding high alpine areas, National Park Service (NPS) managers are concerned about surface‐water availability to support park facilities and for drinking water for recreational hikers visiting the park. Climate change within the Pacific Northwest Region is projected to contribute to on‐going glacial retreat (Anderson & Shean, 2022) and reduced annual snowpack as a greater portion of precipitation falls as rain (Gergel et al, 2017; Salathe Jr et al, 2008), which may contribute to decreased water availability that support late‐summer season surface flow in streams (Van Loon & Laaha, 2015; Ward et al, 2020). For example, the 2015 snow drought in the western U.S. was one of the lowest annual snowpacks in Washington as a result of warmer air temperatures despite approximately average precipitation (Mote et al, 2016) and which corresponded to reduced surface‐water flow in streams.…”

Spatial Patterns of Disconnectivity Explain Catchment‐Scale Sediment Dynamics and Transfer Efficiencies

“…The main exception to this finding is the contrasting degrees of functional connectivity between the South Tahoma Glacier meltwater channel (mainstem, RKMs 0–3) and the Tahoma Glacier meltwater channel (primary tributary). In this case, debris flows and outburst floods in the mainstem result in significant geomorphic change and functional connectivity, neither of which occur in the tributary valley (Anderson & Shean, 2021). Perhaps unsurprisingly, structural connectivity indices (Turley et al., 2021) were also unable to differentiate between the contrasting degrees of functional connectivity of these two areas.…”

“…Several researchers have stated that in proglacial settings, the sequencing of extreme events rather than more frequent lower‐magnitude events (e.g., annual rainfall) controls erosion and sediment export over periods of decades and longer (e.g., Anderson & Shean, 2021; Micheletti & Lane, 2016). This conclusion is well supported by evidence from the Tahoma Cr.…”

“…In November 2006, a large atmospheric river dropped around 500 mm of rain (maximum rainfall intensity of 20 mm hr −1 ) on bare slopes over a 3‐day period, an amount far higher than any other event since 1920 when records began (Anderson & Shean, 2021; Legg et al., 2014). This event in turn triggered widespread flooding, debris flows and landslides, factoring heavily into the quantitative sediment budget presented in this study.…”