Field guide to Mount Baker volcanic deposits in the Baker River valley; nineteenth century lahars, tephras, debris avalanches, and early Holocene subaqueous lava

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, Park, 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.