SUMMARY OF HYDROLOGIC CONDITIONS

Surface Water
Alaska contains more than 40 percent of the Nation's surface-water resources. The highest runoff rates per unit area are mainly in southeastern Alaska and other areas influenced by the maritime climatic effects of the northern Pacific Ocean and the Gulf of Alaska. Farther north and in the interior parts of the State, runoff rates decrease sharply, although runoff generally increases with altitude throughout the State. Year-to-year runoff variability also increases from south to north.

Seasonal runoff characteristics also differ in southern and northern Alaska. Areas with a maritime climate usually have two periods with high runoff volumes: a spring snowmelt period and a fall rainfall period. High water can occur throughout the year, but high instantaneous peak discharges are more prevalent in the fall months; low-water periods usually occur in late spring and also in mid-summer prior to the rainy fall period. Farther north, most of the total runoff and floods are concentrated in May through September; low-flow periods usually occur from mid-winter to shortly before spring snowmelt begins.

During the first 3 months of water year 1996, snowfall was significantly less than normal in large parts of interior and south-central Alaska. The lack of an insulating snowpack caused both ground and rivers to freeze deep, resulting in lower than normal streamflow through the winter. Although late winter snowfall amounts were at or above long-term averages, they were too little and too late to replace the deficit. By May 1, the snowpack was significantly less than normal in the Central Yukon, Copper, and Matanuska-Susitna basins, and in Southeast Alaska, and at record lows in the Northern Cook Inlet and Kenai basins. Consequently, streams in southeast, southcentral, and interior Alaska had lower than normal flow during the summer (fig. 1). The lowest mean annual flow since the early 1970's occurred in several streams having more than 30 years of record, and many streams having less than 25 years of record had the lowest mean annual streamflow recorded. Minimum or near-minimum recorded monthly mean streamflow occurred on the Kenai Peninsula, and in Southeast Alaska, in the Matanuska-Susitna Valley, the Tanana River basin, and the Yukon River basin during May, June, July, and August.

Near the end of September, remnants of Typhoon Tom produced record rainfall in parts of southeastern Alaska; more than 8 inches of rain fell in a 24-hour period in Elfin Cove, about 70 miles west of Juneau. A peak flow of 44 years of record occurred in Gold Creek at Juneau. Numerous mud slides and flooding all around Juneau and Sitka closed roads and flooded homes and businesses.

Figure 1. Monthly discharge of the Yukon River near Stevens Village, Alaska, and the Little Susitna River near Palmer, Alaska, during water year 1996 and mean, maximum, and minimum monthly discharge for the period of record.

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Last modified: Friday, June 29, 2001