THE SHERMAN CRATER ERUPTIVE PERIOD AT MOUNT BAKER, NORTH CASCADES--A.D. 1843 TO PRESENT--IMPLICATIONS FOR RESERVOIRS AT THE BASE OF THE VOLCANO

Sherman Crater, the active, 0.5 km-wide vent below the ice-filled Carmelo Crater of Mount Baker's summit, has been the site of intense hydrothermal activity since the early Holocene. Its present shape formed with a hydrovolcanic explosion in AD 1843, yielding the main layer of tephra set YP of altered lithics > 15 km northeast. This and smaller subsequent eruptions may have produced juvenile tephra, although no surviving deposit is known. Observations of forest fires and lightning are possible, but we infer that the 1843 event and those of 1858 ("night clouds brilliantly illuminated") and 1859 ("jets of flame") were phreatomagmatic. Collapse of altered rock from the east crater rim yielded a clay-rich lahar inundating the Baker River valley, site of the reservoirs Baker Lake and Lake Shannon, between 1845 and 1847 (dated by dendrochronology). A smaller lahar also reached the valley in 1858(?). Both lahars increased in volume by a bulking factor of at least two by entraining sediment during flow.

The phreatomagmatic phase of the eruptive period ceased before 1891, as the crater was then "ice-filled." Thermal activity continued at reduced levels until approximately the 1930's. Thereafter, aerial photos record loss of ice and snow in the crater, climaxing with the well-documented tenfold increase in aseismic thermal activity in March 1975. Activity declined by 1976, but not subsequently to pre-1975 levels. Continuing significant gas emissions, mainly CO₂, are monitored by the USGS.

Collapse probability will rise with significantly increased activity at Sherman Crater. Volume of the 1845-1847 lahar was 15-25 million m³; volumes of 3 collapse lahars from the same, hydrothermally altered sector of the volcano during the mid-Holocene were 50-200 million m³, substantially greater than believed possible in 1975. Based on these case histories and assessment of any renewed activity, rapid drawdown to reduce spillway-level capacities (total storage of Baker Lake: 390 million m³; Lake Shannon: 200 million m³) is recommended when authorities are advised by scientists. Drawdown, also done in 1975, can accomodate potential lahar inflows, thus minimizing risks of downstream flooding from displaced water as well as those of shoreline inundation by lahar-induced waves.