41. Anderson (2013) Tahoma Creek: Aggradation and resource management
    Aggradation directly beneath the Tahoma Creek Bridge has required repeated dredging to maintain adequate freeboard. Given that this span is an integral part of the Parks main access route, damage to or destruction of this structure would be a major disruption to Park functioning. This research was initiated with a goal of documenting and understanding the aggradation within Tahoma Creek, with the hope that such information would provide a better sense of how such hazards may evolve in the future and how best to manage these hazards. Specifically, the research attempts to a) document the rates of vertical channel change along the length of Tahoma Creek, b) determine, if aggradation is observed, whether this represents a systemic or transient disturbance, and c) discuss the potential options for mitigating these hazards. Using LiDAR surveys flown in 2002, 2008 and 2012 to directly measure aggradation and incision over the entirety of the basin, I find that the lower reaches of Tahoma Creek, as a whole, have not aggraded over this period of record. While over 10⁶ m³ of sediment was transported through the lower five kilometers of the creek, the net change in storage within these reaches was about -5x10⁴ m³. Averaged over this area, this represents around 10cm of incision. Local exceptions exist, including a zone of net aggradation between the bridge and the Nisqually confluence for the 2008-2012 period. Alder stands growing on the bare-gravel surfaces within the channel were observed to have established primarily in the years immediately following the debris flows of the early '90s, indicating that the lower channel does respond to such upstream sediment loading. However, the vertical position of these stands within the channel places a low upper limit on the extent of aggradation that may have occurred during these years, and further indicate that the modern channel has not risen above the high-stand position obtained in the mid-'90s. Taken together, it appears that debris flows do cause increased channel activity in the lower reaches, but that this activity largely manifests as increased lateral mobility and sediment transport rates, with only minor associated aggradation. The increased activity appears to subside within several years of the end of significant upstream sediment loading. A variety of sources were used to investigate channel activity over the past 100 years. The methods are not exact, but broadly suggest that the channel has seen several intervals of increased activity or aggradation over this period, while maintaining a long-term stability. Tree-ring records were used to reconstruct debris flows over the past 500 years. This records shows that there was a suite of events, similar in extent to the modern debris flows, in the mid-19th century. This coincides with the onset of glacial retreat out of the Little Ice Age (LIA). This suite of debris flows, along with several other isolated events that occurred during earlier periods of retreat, suggest a connection between negative glacial mass balance and debris flow frequency. However, the exact mechanisms of this connection are unclear, making it difficult to predict how this frequency will evolve in the coming decades. Regardless, these records show that the modern debris flows are not without precedent, increasing the odds that Tahoma Creek is already in an equilibrium defined by semi-regular intervals of such elevated sediment loading. That being said, the extent of forest mortality in the upper basin, and particularly above the old campground site, does appear to be unprecedented since the forests were last cleared by the Tahoma lahar c. A.D. 1500. Taken together, these findings indicate that the lower channel has been dynamically stable over the period of record, and the recent debris flows do not appear to be significantly more frequent or intense than those of the past 500 years. The aggradation at the bridge appears to be a function of unique local conditions, which may include a) the narrowness of the bridge opening relative; b) a dynamic overshoot of aggradation as the stream infills the dredged reach; c) augmented local sediment influxes from dredging spoils placed on local gravel bars; or d) an increased local base-level caused by aggradation at the Nisqually-Tahoma Creek confluence. All four options are plausible readings of the available data, nor are they mutually exclusive. While uncertainty remains, it is my belief that dredging provides little benefit to the long-term maintenance of the Tahoma Creek Bridge, and likely plays a role in the persistence of the local aggradation. Near the bridge, Tahoma Creek transports an average of 45,000 m³/yr of bed-material, and may transport an order of magnitude more than this during a single large flood. In contrast, dredging efforts generally reposition between 2,000 and 25,000 m³ of material. As such, even if the dredged material was removed from the channel, the channel transports enough material to re-obtain the pre-dredging profile in, at most, several years. The lower local channel slope created by dredging causes material to preferentially deposit in this reach, and may cause transient aggradation above the equilibrium profile as the channel attempts to dynamically re-obtain balance. This situation is exacerbated by the practice of placing dredging spoils on top of gravel bars within the active channel. This sediment is readily re-entrained by the river, creating a lateral sediment influx of a magnitude that greatly exceed the natural lateral inputs that are derived from the erosion of vegetated floodplain banks. These results put the fate of the bridge in something of a grey area. While there is no evidence to suggest that the recent aggradation will continue unabated, it is very likely that periodic, local conditions will reduce the freeboard of the bridge below acceptable margins of safety with some regularity. This is simply the nature of dynamic, mountain streams. Given the regularity with which channel maintenance has been performed near the bridge reach, and the very short-term nature of the improvements, it still seems reasonable to consider an investment in a longer-term solution that will reduce the need for emergency operations. There exists a suite of techniques designed to increase local sediment transport rates, which theoretically could reduce the potential for aggradation at the bridge. However, none of these methods are likely to be effective in Tahoma Creek, given the high energy of the stream and coarse sediment being transported. Modifying the bridge to accommodate the creek is a more reasonable solution, and seems most inline with the mission of a National Park. While the entire Park is classified as a National Historical Landmark District, the Tahoma Creek Bridge itself is considered a non-contributing structure, reducing the bureaucratic overhead needed to modify it. The major drawback to this solution is the cost involved. This cost must be weighed against that of repeated dredging, which provides only marginal benefits in the short-term and may actually exacerbate the situation in the long run. Regardless of which mitigation strategy is used, simple long profile surveys of the low-flow wetted channel, taken annually between the Nisqually-Tahoma confluence and a point somewhat upstream of the bridge, would provide valuable insight into the processes at play. Such surveys could be easily obtained within the parks current framework of resource management.
538. Wall et al. (2026) Origin and evolution of mafic volcanism associated with 3 m.y. of andesite production at the Goat Rocks volcanic cluster, southern Washington Cascade Range
     More than 3 m.y. of mafic volcanism near the Goat Rocks volcanic cluster in the southern Washington Cascade Range, USA, lends insight into the evolution of basalts and the subarc mantle at a long-lived, major arc volcanic locus. We contribute field observations, 40Ar/39Ar dates, paleomagnetic directions, and bulk rock and mineral compositions to characterize nine mafic units that erupted in association with the Goat Rocks volcanic cluster. The time frame of mafic volcanism, ca. 3.6 Ma to 60 ka, encompasses the lifespan of the central volcanic cluster (3.1 Ma to 115 ka), with a lull from ca. 2.7 Ma to 1.4 Ma. A climactic period of voluminous mafic activity and far-traveled lava flows, including construction of the Hogback Mountain shield volcano, coincided with voluminous andesite eruptions from the central volcanic cluster. The basaltic rocks in the Goat Rocks area are calc-alkaline to barely tholeiitic and have high field strength element depletion relative to large-ion lithophile elements characteristic of calc-alkaline basalts (CAB) of the Cascade volcanic arc. Unlike at neighboring andesitic volcanic centers (Mounts Adams, St. Helens, and Rainier), no other mafic end members such as high-aluminum olivine tholeiite (HAOT) or intraplate-type basalt (IPB) are present at or near the Goat Rocks volcanic cluster, although some of the calc-alkaline basalts in this study have IPB-like affinities. The Goat Rocks mafic units exhibit two main temporal trends in composition: (1) the most primitive basalts erupted earlier, compared to less primitive and more evolved compositions later, and (2) high field strength element concentrations are higher in the younger basalt units relative to the oldest two. In contrast to these temporal trends, the mafic units define two compositional groups that recur through time, a low-Sr and a high-Sr group, each with distinct trace element and Sr and Nd isotope ratios. Although radiogenic isotope ratios are generally aligned with High Cascades CAB and HAOT, some extend toward IPB of Mount Adams and Simcoe Mountains volcanic field. Olivine-dominated crystal fractionation at shallow pressure from a small range of parent magma compositions accounts for much of the variation among the basalts and basaltic andesites. A high-pressure fractionation model is plausible for only one of the youngest basalt units (basalt of Walupt Lake volcano). Mafic recharge and crustal assimilation accounts for the incompatible-element enriched composition of basaltic andesites erupted during construction of the largest andesitic centers, further supporting sustained basalt mass flux and thermal energy driving andesite genesis. We model the most primitive members of the Goat Rocks mafic units as partial melts of successively less depleted mantle in time. Variable degrees of fluxing with fluids and melts from subduction explain the distinction between high-Sr and low-Sr groups. We propose that mantle metasomatism by ancestral subduction and fluid-flux melting is heterogeneously distributed through the local subarc mantle and played a greater role in the genesis of the high-Sr basalt group. The limited range of primitive basalt types around the Goat Rocks volcanic cluster contrasts with the much greater diversity of basalts throughout the southern Washington to northern Oregon Cascade arc. On the other hand, the central volcanic cluster encompasses nearly the entire diversity observed at neighboring composite volcanoes. In the case of the Goat Rocks area at least, and perhaps attributable to the entire region, this means that the genesis of diverse intermediate magmas is independent from and does not require vastly different parental basalt compositions.
537. Black et al. (2026) Forest-floor burial in 1507 by the largest Mount Rainier lahar of the past millennium
     New dating of lahar-killed trees underscores volcano hazards in the Puget Sound metropolitan area. Beginning as a landslide from the west flank of Mount Rainier, Washington, USA, the Electron Mudflow, which was the largest lahar of the last millennium, swept more than 60 km down the Puyallup River drainage into areas now densely populated. Wiggle matching of seven radiocarbon ages from buried, bark-bearing Douglas-fir (Pseudotsuga menziesii) trees brackets the mudflow’s age between 1477 and 1522 CE with 99.7% certainty. To narrow this date, we applied dendrochronology crossdating on samples collected from 21 trees killed by the lahar, measuring 86 time series for statistical verification. The four bark-bearing trees died the same year while the final rings in all other trees had decayed, exposing rings formed in earlier years. When averaged together, the crossdated measurements form a 475 yr master chronology that was correlated against absolutely dated tree-ring chronologies in the region. The Electron chronology best matched with chronologies from low-elevation sites, especially a Douglas-fir chronology from Vancouver Island, Canada, to show that the Electron trees died in 1507 CE. Latewood in the final ring was beginning to form, indicating the mudflow likely occurred in the late-summer months. What caused the Electron Mudflow is unknown, but this precise date will help to assess possible relationships with other events, assist in interpreting Indigenous narratives about the mudflow, and increase awareness of potential lahar hazards.
536. Koepfli et al. (2025) Discovering spatial variability of critical zone processes at Mount Rainier using DAS
     Mount Rainier (4392 m a.s.l.), an active stratovolcano located ~95 km south-east of Seattle, WA, USA, poses hazards due to its steep glaciated slopes and highly porous volcanic surface. The combination of snowmelt, rainfall, and unstable surface materials frequently triggers debris flows and lahars, threatening downstream communities. At the same time, Mount Rainier’s glaciers play a crucial hydrological role, storing water that sustains rivers and therefore agriculture across the heavily populated lowlands during dry summer months. To better understand the shallow subsurface (critical zone) and its connection to the surface, we collected data using Distributed Acoustic Sensing (DAS) along a ~40 km fiber-optic cable that spans over ~1000 m elevation and crosses diverse lithologies. We analyze ambient seismic noise by using auto- and cross-correlations to image and monitor near subsurface conditions and compare our results with data from nearby weather stations, river gauges, and soil pits. We identify various coherent fiber sections and link the frequency content of seismic noise sources to local hydrological settings. We also find an increased signal-to-noise ratio for specific lithologies. Observed seismic velocity changes (dv/v) align with nearby ground moisture measurements but vary along the fiber. To explain these spatial variations, we investigate hydrological processes that connect surface conditions and subsurface responses
535. Kenyon (2025) Behind the curtain: Characterizing the Nisqually Watershed of MORA as a means to explore the use of non-contact data sources in mountain hydrology
     Impacts from a changing climate are affecting the hydrology, geomorphology, and overall variability of rivers around the world. Upland water especially prone to these effects. Mountainous rivers are experiencing significant shifts in precipitation patterns and the storage of snow and ice in source areas, resulting in stark changes to hydrologic variability, sediment transport, and fluvial morphodynamics. Most hydrology methods have been developed for use in rivers with a slope of <0.001 m/m, and the advancement of knowledge relevant to steeper rivers with has followed slowly in comparison. This research aims to address gaps in mountain hydrology associated with the measurement of discharge and bedload sediment transport in mountain rivers with a slope ≥0.02 m/m, seeking means to improve our ability to observe hydrologic trends and morphodynamics. Containing widely distributed low-resilience infrastructure, significant increases to precipitation intensities, and glacial recession rates greater than 0.1 m/day, the Nisqually River within Mount Rainier National Park (MORA) exemplifies a nexus of modern land management issues driven by climate stressors of the Pacific Northwest. With this study we seek to further characterize observable surface processes in the Nisqually watershed within MORA, and begin considering new methods and frameworks enabling reliable monitoring of steep mountain rivers. We consider the use of seismic, infrasound, and video analysis data as non-contact methods to measure discharge and sediment transport in steep mountain rivers. The primary non-contact data series can then be supported by remote LiDAR products and Sentinel-1 data to assess changes in the source areas and their potential impacts on observable behaviors. Initial data shows signals in the seismic/infrasound that seem to correlate to both water flow and bedload transport. We hypothesize there will be observable correlations with topography and snowmelt timing seen though remote sensing analysis, but also anticipate site-to-site variability based on substrate and local morphology
534. Conner et al. (2025) Characterizing surges from a debris flow induced by a glacial outburst flood at Mount Rainier, USA
     On 15 August 2023, a small debris flow occurred in Tahoma Creek on the southwest side of Mt. Rainier National Park, Washington, USA. The debris flow originated from an outburst flood from the South Tahoma Glacier. Multiple instruments installed in the Tahoma Creek drainage recorded evidence of the debris flow, including nodal and broadband seismometers, infrasound sensors, a laser rangefinder located about 3.4 km downstream of the glacier, and a timelapse camera that captured images of the glacier terminus. In particular, nodal seismometers with a sampling rate of 500 Hz were deployed roughly every 350 m along approximately 2 km of the stream. After initiation of the debris flow, we find evidence in the seismic data of at least three debris flow surges due to either additional small outbursts from the glacier or the debris flow separating into multiple surge fronts caused by wave development from flow instability. Though the arrivals of the surge fronts are often obscured by higher-frequency signals contributed by the full debris flow, we find that the surges can be tracked as they travel downstream. From the seismic data, we are able to approximate where and when the surges merged or separated from the main flow and estimate the flow velocity of each surge front. As the fronts of debris flows generally contain the largest and most damaging materials in the flow, each surge front increases the hazard associated with an event. The dense instrumentation in the Tahoma Creek drainage allows for an in-depth analysis of the evolution of debris flow surges, providing information on how similar debris flows may behave in the future and contributing to the overall understanding of how debris flows evolve over time.

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The U.S. Geological Survey Cascades Volcano Observatory and the University of Washington Pacific Northwest Seismic Network continue to monitor Washington and Oregon volcanoes closely and will issue additional notifications as warranted.

Website Resources

For images, graphics, and general information on Cascade Range volcanoes: https://www.usgs.gov/observatories/cvo
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Jon Major, Scientist-in-Charge, Cascades Volcano Observatory, jjmajor@usgs.gov

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