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 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.

A series of four lahars (volcanic debris flows and their deposits) occurred in rapid succession about 2,500 radiocarbon yr ago in the river system that drains the northwest sector of Mount St. Helens. The huge initial lahar had an instantaneous peak discharge near that of the Amazon River at flood stage, and the third in the series was the second largest in the history of the watershed. The deposits of the flows form a widespread terrace underlain by as much as 12 m of deposits, but its distribution reflects the fact that the lahars were the middle segments of flood waves beginning and ending as streamflow surges. These flood waves originated as lake-breakout surges analogous to those that would have been released, without engineering intervention, from the outlet-blocked Spirit Lake and several new lakes formed in 1980. The lahar deposits are granular and noncohesive, reflecting the origin of the entrained sediment as stream alluvium beyond the base of the volcano. Megaclasts consisting of blocks probably derived from an ancient debris avalanche are abundant in the initial flow and locally form a diamicton with the laharic diamicton as matrix. Although fine-pebble angularity resulting partly from cataclasis during flow is diagnostic of the lahars, a high content of rounded clasts and a channel facies with zones of clast support may explain why similar deposits have not been more widely recognized. The lahar channel facies superficially resembles alluvium. The depositional record of lahars and their distally evolved, hyperconcentrated lahar-runout flows many tens of kilometers from their source can provide a remarkably detailed history of volcanic activity. It may, in fact, provide the only evidence of some important events, of which the magnitude, frequency, and behavior are vital to the assessment of future volcanic hazards. Lake breakouts, meltwater surges caused by large pyroclastic flows, and forms of catastrophic volcanic ejection are examples of events that can create large downstream lahars but may leave little evidence of their true magnitude preserved on and near a volcano. The largest lahar in the river system, for example, began as a streamflow surge that bulked to a debris flow only after more than 20 km of flow.