ABSTRACT High-resolution light detection and ranging (lidar) data and new stratigraphic, lake sediment, and radiocarbon constraints help to resolve a long-standing dispute regarding the timing and nature of the Everson interstade and the Sumas stade, the last major events of the Cordilleran ice sheet in the Fraser Lowland. The new data indicate that: (1) an early, maximum Sumas advance occurred roughly 14,500 cal yr B.P. (calibrated 14 C years before 1950), extending into the Salish Sea near Bellingham, Washington; (2) ice retreated north of the International Boundary long enough for forests to establish in deglaciated lowland sites; (3) a rapid, short-lived rise in local relative sea level (RSL) of ~20–30 m, possibly related to meltwater pulse 1A or the collapse of a glacio-isostatic forebulge, inundated the U.S. portion of the lowlands up to ~130 m above modern sea level; and (4) directly following this transgression at ca. 14,000 cal yr B.P., ice readvanced across the border to nearly the same limit as reached during the early Sumas period. Distinct crosscutting marine strandlines (erosional and depositional remains of emerged marine shorelines), subaerial moraines, and till plains imaged in lidar data indicate that following the maximum extent of the second Sumas advance, local RSL progressively lowered as the glacier fluctuated and gradually thinned. By ca. 13,000 cal yr B.P., ice had retreated north of the border, and local RSL had fallen to within ~4 m of modern. A layer of possible loess in sediments in Squalicum Lake suggests a possible third and final Sumas readvance between 13,000 and 11,150 cal yr B.P., at which time a moraine was constructed ~8 km south of the border near the town of Sumas, Washington. Together, our results suggest that the concept of a distinct Everson interstade and Sumas stade should be abandoned in favor of a more nuanced “Sumas episode” that encompasses the sequence of events recorded in the Fraser Lowland.

ABSTRACT Recently obtained radiocarbon ages from the southern Puget Lowland and reevaluation of limiting ages from the Olympic Peninsula in the light of new light detection and ranging (LiDAR) data suggest that the Juan de Fuca and Puget lobes of the Cordilleran ice sheet reached their maximum extents after 16,000 calibrated yr B.P. Source areas for both lobes fed through a common conduit, likely requiring that downstream responses to changes in either source area were similar. Dates for ice-sheet retreat are sparse and contradictory, but they suggest that retreat was rapid. Depositional and geomorphic evidence shows that retreat of the Juan de Fuca lobe predated retreat of the Puget lobe. No recessional end moraines have been identified in the Puget Lowland, in contrast to numerous recessional end moraines constructed by the Okanogan lobe east of the Cascade Range, and in contrast to later ice-sheet retreat in western Whatcom County north of the Puget Lowland. These observations lead to the hypothesis that collapse of the Juan de Fuca lobe, hastened by the instability of a marine-based ice sheet, steepened the ice-sheet surface over the eastern Strait of Juan de Fuca and diverted ice flow upstream of the Puget lobe to the west. Starved of ice, the Puget lobe retreated quickly.

Abstract The Middle Fork Nooksack River drains the southwestern slopes of the active Mount Baker stratovolcano in northwest Washington State. The river enters Bellingham Bay at a growing delta 98 km to the west. Various types of debris flows have descended the river, generated by volcano collapse or eruption (lahars), glacial outburst floods, and moraine landslides. Initial deposition of sediment during debris flows occurs on the order of minutes to a few hours. Long-lasting, down-valley transport of sediment, all the way to the delta, occurs over a period of decades, and affects fish habitat, flood risk, gravel mining, and drinking water. Holocene lahars and large debris flows (>10 6 m 3 ) have left recognizable deposits in the Middle Fork Nooksack valley. A debris flow in 2013 resulting from a landslide in a Little Ice Age moraine had an estimated volume of 100,000 m 3 , yet affected turbidity for the entire length of the river, and produced a slug of sediment that is currently being reworked and remobilized in the river system. Deposits of smaller-volume debris flows, deposited as terraces in the upper valley, may be entirely eroded within a few years. Consequently, the geologic record of small debris flows such as those that occurred in 2013 is probably very fragmentary. Small debris flows may still have significant impacts on hydrology, biology, and human uses of rivers downstream. Impacts include the addition of waves of fine sediment to stream loads, scouring or burying salmon-spawning gravels, forcing unplanned and sudden closure of municipal water intakes, damaging or destroying trail crossings, extending river deltas into estuaries, and adding to silting of harbors near river mouths.

Abstract As the Vashon glacier retreated from its terminal position in the southern Puget-Lowland and thinned rapidly, marine waters invaded the central and northern lowland, floating the ice and depositing Everson glaciomarine drift over a wide area from southern Whidbey Island to southern British Columbia. The Everson deposits are characterized by vast areas of massive, poorly sorted stony silt and clay commonly containing marine shells. At Bellingham Bay and elsewhere in the Fraser Lowland, Deming sand is overlain by massive, poorly sorted, Bellingham glaciomarine drift to elevations of 180–210 m above present sea level and is underlain by Kulshan glaciomarine drift. Following deposition of the Everson glaciomarine drift, ice readvanced into northern Washington and deposited Sumas Drift and meltwater channels were incised into the glaciomarine deposits. Four moraine-building phases are recognized in the Sumas, the last two in the Younger Dryas. Rapid deglaciation between 14,500 and 12,500 14 C yr B.P. resulted in lowering of the surface the Cordilleran Ice Sheet below ridge crests in the Nooksack drainage and glacial activity thereafter became topographically controlled. Local valley glaciers in the upper Nooksack Valley were fed by alpine glaciers on Mount Baker, Mount Shuksan, and the Twin Sisters Range that were no longer connected to the Cordilleran Ice Sheet. Remnants of the Cordilleran Ice Sheet persisted in the Fraser Lowland at that time but were separated from the Nooksack Valley glaciers by several ridges 1200 m higher than the surface of the ice sheet. Alpine glaciers deposited drift in the Middle and North forks of the Nooksack drainage 25–45 km down-valley from their sources. Large mega-landslides in the Nooksack drainage are associated with an area of unusually high seismic activity, whereas nearby areas having the same geology, topography, climate, and vegetation have no such mega-landslides, suggesting that the landslides are seismically induced. Five Holocene tephras have been recognized in the region around Mount Baker–Schreibers Meadow scoria, Mazama ash, Rocky Creek ash, Cathedral Crag ash, and the 1843 tephra.

Abstract Holocene volcanic deposits from Mount Baker are plentiful in the low-lying Baker River valley at the eastern foot of the volcano. Tephra set SC (8850 yr B.P.), erupted from the nearby Schreibers Meadow cinder cone, is sporadically present. Exposures of both subaerial and subaqueous facies of the associated Sulphur Creek basalt lava flow are easy to access; the lava, the most mafic product known from the entire Mount Baker volcanic field, entered Glacial Lake Baker, invaded lacustrine sediments, and formed peperites as well as subaqueous block-and-ash flows. A volcaniclastic delta was deposited in the lake above the lava. The peperite and delta can be seen in the walls of Sulphur Creek, and in the banks of Baker Lake when the reservoir is drawn down in winter and early spring. The best exposures of volcaniclastic flank assemblages from Mount Baker are found in the Baker River valley. The Boulder Creek assemblage formed a thick fan between the end of the Vashon glaciation and the deposition of the SC tephra. Now deeply trenched by Boulder Creek, lahar and block-and-ash diamicts can be seen with some effort by ascending the creek 2 km. A tiny vestige is exposed along the Baker Lake Road. Much younger deposits are also accessible. In 1843, tephra set YP, erupted from Sherman Crater, was deposited in the valley. In ca. 1845–1847, the Morovitz Creek lahar swept down Boulder, P.r., Morovitz, and Swift Creeks and inundated much of the current location of the Baker Lake reservoir. This lahar is an example of the most likely future hazard at Mount Baker as well as the most common type of lahar produced during the Holocene at the volcano—clay-rich or cohesive lahars initiated as slope failures from hydrothermally altered rock. They commonly increase in volume by entraining sediment as they flow. When thermal emissions from Sherman Crater increased in 1975–1976, the level of the reservoir was lowered to accommodate inflow of lahars such as the Morovitz Creek lahar. Renewed activity at Sherman Crater will again trigger reservoir drawdown. In 1890–1891, and again ca. 1917–1932, debris avalanches from pre–Mount Baker lavas flowed down Rainbow Creek. The largest, which flowed 10.5 km, can be visited at the Rainbow Falls overlook. Here, the peak discharge of the flow, derived from reconstructed cross sections defined by well-exposed lateral levees and from reported velocities of equivalent modern flows, is estimated to have been greater than the peak discharge of any historic flood in the Mississippi River.