Floods in the summer of 1971 in south-central Alaska

Abstract

Floods and high water occurred throughout the summer of 1971 in south-central Alaska. Snow cover, 150 percent of average, in the mountains on May 1 caused local snowmelt floods from mid-May to mid-July. The peak discharge of 265,000 cfs (cubic feet per second) on July 15 at Copper River near Chitina was the highest in 22 years of record. However, the major flood period was August 8-11. Precipitation totals of 3 to 9 inches during August 5-11 were recorded in an area extending northeastward from Iliamna Lake to Palmer, Talkeetna, and Paxson Lake. The principal flood areas were the upper Copper, Matanuska, Susitna, Chakachatna, and Kvichak River basins. Flooding in the Anchorage area was not severe. Total damage caused by the August flood is estimated to be 8 to 10 million dollars of which 6 million dollars occurred in the Matanuska Valley. Extreme floods occurred in the Matanuska Valley. A lake near Sutton on an unnamed tributary to Granite Creek breached its embankment, which released a peak discharge in Granite Creek on August 10 of 58,600 cfs, 23.4 times the probable 50-year flood. Other streams near Sutton, tributary to the Matanuska River, had peak discharges 1.8 to 8.9 times the probable 50-year flood. The Matanuska River at Palmer had a peak discharge of 82,100 cfs, 1.2 times the probable 50-year flood. Downstream near Bodenburg Butte, an area was inundated when the Matanuska River overtopped a dike. Another extreme flood occurred at the Chakachatna River near Tyonek on August 11 where the peak discharge was estimated to be 470,000 cfs. The peak was caused by lateral erosion of a channel constriction formed by Barrier Glacier at the outlet of Chakachamna Lake. Hydrologic data for planning, discussions of antecedent conditions, and meteorology along with a description of the floods and flood damage are included. Tables of storm precipitation, peak discharge data, sediment data, and discharge for the June-August period are also included.