New Mexico Geological Society Annual Spring Meeting — Abstracts

Regionally deglaciating climate coupled with sediment loading of historically stable single-thread glacial rivers on the flanks of Mount Rainier have driven recent extreme fluvial network evolution on sub-decadal timescales. Rapid river channel widening, streambank erosion, braidplain development, and channel-bed aggradation is a transient sediment storage signal that is morphodynamically prograding downstream through major river corridors. Major rivers emanate from steep boulder-clogged meltwater channels beneath retreating glaciers that transition to single- and multi-thread gravel-bedded channels towards Mount Rainier National Park (MRNP) boundaries. Coarse glaciogenic sediment is delivered to high-gradient (>10%) fluvial systems through high-ratio sediment:water flows. Flows intermediately deposit sediment along riparian corridors as channel gradients transitionally decrease from transport to depositional slopes (~5–10%) as rivers approach MRNP boundaries (~1–2%). Historical photo analysis of MRNP river channels suggests a lag in the onset of long-term paraglacial sedimentation processes (post-1880) that has been rapidly accelerated by the effects of modern climate change and the increased frequency of extreme weather (i.e., atmospheric rivers) since ~1990. Current aggradation along downstream reaches of the Carbon River is contrary to historically stable surveys done pre-2009 due to an avulsion (ca. 2009) that relocated the full mainstem to an adjacent lower-elevation late-seral stage conifer forested floodplain (i.e., old growth 200–400+ years old) which initiated incision into the sediment accumulation historically restricted to headwater reaches proximal to glaciogenic sediment sources. Large wood recruitment and accumulation along high gradient reaches exacerbates deposition and transient storage of coarse sediment, observed to force >10 meters of deposition along headwater river corridors. Aggrading channels, despite widening, have diminished flow capacity. Flooding within MRNP during historically frequent river discharges is strongly associated with the downstream progradation of transient in-channel sediment and wood storage. High-magnitude precipitation events have increased in frequency across the western United States, from Washington to southern California. New Mexico and the desert southwest are particularly sensitive to the effects of changing climate on regional precipitation regimes.