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A view of Downtown Adak, overlooking Sweeper Cove

Physiography

Adak Island was formed by extreme geologic events, including the tectonic collision of large sections (plates) of the earth’s crust and resulting volcanic eruptions. Advancing and receding glaciers, frequent rainfall, and high winds have shaped Adak Island into a dramatic landscape of hills, valleys, cliffs, and floodplains. Very few areas of the island are flat, and grading to create flat areas could not be done easily.

The highest point on Adak Island is Mt. Moffett (elevation approximately 3,875 feet). Some coastal cliffs on the island rise 2,500 feet above sea level.

Geology

Adak Island was created during the last 60 million years by a complex set of geologic processes resulting from the collision of the North American and Pacific crustal plates. The resulting rock sequences consist primarily of volcanic rocks with some sedimentary rock. A relatively thin layer of unconsolidated material (generally less than 10 feet think) covers the entire island. Only the downtown area is known to have a thick sequence of unconsolidated material (greater than 100 feet). The northern region of Adak is dominated by the remnants of three volcanoes.

Hydrogeology

Groundwater occurrence on northern Adak Island is predominantly associated with high-permeability deposits including beach, alluvial, and eolian deposits in the low-lying areas of the island (i.e., downtown). These materials were used extensively to fill marshland in the downtown area during construction of the base in World War II. This area has the only notable aquifer with high groundwater yield.

Groundwater also occurs in discontinuous, though relatively permeable, deposits of lahar (poorly sorted angular cobble and boulder gravels) in the upland areas. Less permeable volcanic ash deposits, volcanic and intrusive fractured bedrock, and glacial till deposits, which are also associated with upland topography, may contain some groundwater locally. While the quality of these groundwater sources is unknown, their yield and frequency of occurrence are negligible in comparison with the downtown area.

Groundwater, which occurs from 5 to 30 feet below ground surface (bgs) in the downtown area, likely overlies denser saltwater that intrudes the aquifer from adjacent Kuluk Bay and Sweeper Cove. Although groundwater occurrence is relatively significant in the downtown area, groundwater has not been used for drinking water supply. Currently and historically, all water supply for the island has been obtained from surface water sources.

Surface Water Hydrology

Salt Water

The steep slopes that characterize the Adak coastline continue below the water’s surface. Water is extremely deep off shore. The island is ice-free and open to navigation all year. A breakwater at the entrance to Sweeper Cove creates a protected harbor. Depths within the cove range from 132 feet at the entrance to 36 feet about 300 yards from the shore.

Fresh Water

Short, steep-gradient streams draining radially from Mt. Moffett, Mt. Adagdak, and other upland areas characterize the surface water hydrology of the northern portion of Adak Island. Perennial flow is maintained by snowmelt in the mountains and seepage from the shallow surficial soils. Numerous lakes and sediment deposits occur along stream courses.

Surface water provides the only drinking (potable) water on Adak. The Lake Bonnie Rose system is operated as a Class A public water system.

Climate

The maritime climate on Adak is characterized by persistently overcast skies, high winds, and frequent, often violent, cyclonic storms originating in the northern Pacific Ocean and Bering Sea. Weather can be localized, with fog, low ceilings, precipitation, and clear weather all occurring within a distance of a few miles. Storms can occur during any season, although the most frequent and severe storms occur during the winter.

Wind conditions are typified by local shifts and rapid changes in velocity. Average wind velocity is 15 knots, with gust in excess of 100 knots during winter storms. High winds are also frequent during the summer months, with gusts over 50 knots not uncommon. The prevailing wind direction is from the southwest.

Mean annual precipitation for Adak Island is about 54 inches, most of which falls as rain. Average monthly precipitation varies from a low of 3 inches in June and July to a high of 7 to 8 inches in November and December. Snowfall averages over 100 inches per year. Because of the relatively warm temperatures, snow rarely exceeds 1 to 2 feet in depth and is concentrated in the mountains.

Mean monthly temperatures vary from a low of 32.9° F in February to a high of 51.3° in August. The highest temperature recorded on Adak is 75° F (August 1956), and the lowest temperature is 3 degrees F, recorded in January 1963 and February 1964.