A CATASTROPHIC FLOOD CAUSED BY DRAINAGE OF A CALDERA LAKE AT ANIAKCHAK VOLCANO, ALASKA, AND IMPLICATIONS FOR VOLCANIC HAZARDS ASSESSMENT

Aniakchak caldera, located on the Alaska Peninsula of southwest Alaska , formerly contained a large lake (estimated volume 3.7 x 10(9) m(3)) that rapidly drained as a result of failure of the caldera rim sometim e after ca, 3400 yr B.P. The peak discharge of the resulting flood was estimated using three methods: (1) flow-competence equations, (2) ste p-backwater modeling, and (3) a dam-break model, The results of the da m-break model indicate that the peak discharge at the breach in the ca ldera rim was at least 7.7 x 10(4) m(3) s(-1), and the maximum possibl e discharge was approximate to 1.1 x 10(6) m(3) s(-1). Flow-competence estimates of discharge, based on the largest boulders transported by the flood, indicate that the peak discharge values, which were a few k ilometers downstream of the breach, ranged from 6.4 x 10(5) to 4.8 x 1 0(6) m(3) s(-1), Similar but less variable results were obtained by st ep-backwater modeling. Finally, discharge estimates based on regressio n equations relating peak discharge to the volume and depth of the imp ounded water, although limited by constraining assumptions, provide re sults within the range of values determined by the other methods, The discovery and documentation of a flood, caused by the failure of the c aldera rim at Aniakchak caldera, underscore the significance and assoc iated hydrologic hazards of potential large floods at other lake-fille d calderas.