Summary of Significant Floods in the United States, Puerto Rico, and the Virgin Islands, 1970 Through 1989

Alaska is located northwest of the conterminous United States and Canada. Alaska is bordered on three sides by different bodies of water--the Bering Sea, the North Pacific Ocean, the Chukchi Sea, and the Arctic Ocean. The moisture in Alaska is mainly controlled by storms originating over the North Pacific Ocean or the southern perimeter of the Bering Sea (Paulson and others, 1991). These storms travel northeastward from their point of origin toward southeastern Alaska and the Gulf of Alaska during most of the year. During summer, these storms typically change path and move more northward over the Bering Sea and Bering Strait. These storms partly regulate the geographic distribution of seasonal and annual precipitation, and the change in direction of the storms results in storms occurring in the interior of Alaska more during the summer than during the winter. Mean annual precipitation ranges from 320 in. in southeastern Alaska to less than 5 in. in northern Alaska near the Arctic Ocean (Paulson and others, 1991).

Alaska can be divided into four climatic zones. This division was necessary because of the great climatic differences resulting from the size of the State, the topography, and the mechanisms by which temperature and precipitation are affected by the oceans on three sides. The zones are defined progressively northward and are characterized as follows. The areas along the southern coast have small temperature variations, large annual precipitation, relatively cool summers, and relatively warm winters that have short periods of below-freezing temperatures. The southwestern and western coasts have greater daily and seasonal temperature variations and less precipitation than the southern coastal areas. The interior of the State has a continental climate with large daily and annual temperature ranges and a small quantity of annual precipitation. The northern coast has less temperature variation, less precipitation (especially near the Arctic Ocean), and lower mean annual temperature than the interior part of the State. Precipitation is greatest during August along the northern coast and in continental areas, and during October in the southern coastal area.

When the temperatures are mild, local flooding is caused by convective storms. Warm temperatures also bring the probability of the melting of the snowpack and associated flooding. Serious flooding in Alaska is difficult to determine due to the extensive area of the State that is undeveloped.

On the basis of regionalization procedures that relate flood characteristics to watershed and climatic characteristics, the magnitude of maximum discharges for streams in Alaska depends largely on the drainage area of the basin, mean annual precipitation, and minimum January temperature. The percentage of the drainage area that is forested and the lake storage areas also affect runoff (Jennings and others, 1994).

The flood of October 1986 was the most extensive in Alaska during 1970-89. In south-central Alaska, almost 18 in. of rainfall during August 9-11 caused flooding around Seward, and a separate but concurrent storm in the Susitna River Basin caused floods having recurrence intervals greater than 100 years. Landslides and landslide-dam failures and resultant floods, debris flows, alluvial-fan aggradation, and flooding in and around Seward caused about $20 million in damages, but short-term streamflow records and unstable channels precluded accurate discharge or recurrence-interval determinations in the most extensively damaged area (Jones and Zenone, 1988; Lamke and Bigelow, 1988).

In August 1971, flooding occurred in the Matanuska and Susitna River Valleys and along the western Cook Inlet in south-central Alaska. Rainfall ranging from 3 to 9 in. was recorded in early August, compounding flood conditions created in July. Discharges having recurrence intervals greater than 100 years occurred in several streams in the sparsely populated area, and flood damages amounted to more than $10 million, mostly to highways in the Matanuska Valley (Lamke, 1972).

Jennings, M.E., Thomas, W.O., Jr., and Riggs, H.C., 1994, Nationwide summary of U.S. Geological Survey regional regression equations for estimating magnitude and frequency of floods for ungaged sites: U.S. Geological Survey Water-Resources Investigations Report 94-4002, 196 p.

Jones, S.H., and Zenone, C., 1988, Flood of October 1986 at Seward, Alaska: U.S. Geological Survey Water-Resources Investigations Report 87-4278, 43 p.

Lamke, R.D., 1972, Floods of the summer of 1971 in south-central Alaska: U.S. Geological Survey Open-File Report 72-215, 88 p.

Lamke, R.D., and Bigelow, B.B., 1988, Floods of October 1986 in south-central Alaska: U.S. Geological Survey Open-File Report 87-391 (revised 1988), 31 p.

Paulson, R.W., Chase, E.B., Roberts, R.S., and Moody, D.W., compilers, 1991, National water summary, 1988-89-Hydrologic events and floods and droughts: U.S. Geological Survey Water-Supply Paper 2375, 591 p.