The land surface of the Kodiak-Shelikof subregion, an area of approximately 11,000 sq. mi. (28,500 sq. km.), is characteristically steep, rugged, and extensively glaciated (Figure 40a). The Aleutian Range dominates the southern coast of the Alaska Peninsula. Rounded, east-trending ridges rise from 1,000 to 4,000 ft. (300 to 1,200 m.) in altitude, creating an abrupt and rugged coastline. Volcanic peaks, some rising to over 7,000 ft. (2,100 m.), and the frequent occurrence of earthquakes are indicative of current land-building processes. This subregion is only a small part of the narrow, continuous seismic zone that rims the entire Pacific Ocean basin (see section on Earthquakes). The drainage divide between the Bering Sea and Pacific Ocean is generally within 10 mi. (16 km.) of the southern coastline along the highest ridges. Streams flowing into the Pacific are short and steep.
The Kodiak Mountains form the Kodiak Island group, Barren Islands, Trinity Islands, and Chirikof Island. Kodiak Island has a rugged northeast-trending divide with Summit altitudes between 2,000 and 4,000 ft. (600 and 1,200 m.). The topography southeast of the divide has a strong northeasterly rain which parallels the divide; the western part of Kodiak Island is cut by many broad valleys. The coastline is extremely irregular with many deep and beautiful fjords and islands. The north side of the island is characterized by long, narrow fjords, while the eastern coastline is characterized by wide, short estuaries. Afognak Island is located just northeast of Kodiak Island and shares a similar topography; however, the northern section is a hilly lowland.
The islands of the Kodiak group are drained by short, swift, clear streams. There are several lakes of more than a mile in length in the southwestern part of Kodiak and Afognak Islands. Small ponds and chains of lakes are scattered over the glacially sculptured topography; only a few icy remnants of past glacial systems continue to scour the land surface.
Cook Inlet Subregion
The grandeur of the country surrounding Cook Inlet is impressive—from the Inlet to the Susitna, Matanuska, and Kenai lowlands and on to the snow-capped mountain ranges surrounding the basin. The physiographic sections within the subregion (Figure 40b) include the Aleutian and Alaska Ranges, Talkeetna Mountains, upper Matanuska Valley, Kenai-Chugach Mountains, Cook Inlet-Susitna lowland, and a small portion of the Copper River plateau. The total land area is about 38,000 sq. mi. (98,500 sq. km.), more than half of which is in the Susitna River basin.
The Kenai Peninsula forms the east shore of the Inlet, where a glaciated lowland of outwash plains extends along the Kenai and Chugach Mountain ranges. That portion of the peninsula east and south of Kachemak Bay is quite rugged with a fjord-indented coastline. The higher ridges of the two mountain ranges are covered by glaciers and ice fields which support streams, build floodplains, and work continuously to modify the topography.
Separated from the Alaska Range to the north by the broad valley of the Nenana River and from the Chugach Mountains on the south by the Matanuska Valley, lies a rugged, radial mountain range—the Talkeetna Mountains. These ice-carved mountains sustain several glaciers between 6 and 10 mi. (10 to 16 km.) in length and numerous smaller ice masses. Summits are strikingly even, rising to elevations of 6,000 to 7,000 ft. (1,800 to 2,100 m.) and decreasing northward and westward.
The western margin of the subregion is formed by the snow-capped Aleutian Range merging imperceptibly with the Alaska Range to the north, creating a lofty horizon dominated by Mounts Iliamna, Redoubt, Foraker, and McKinley. Great glaciers radiate out from these central peaks. Aside from these isolated giants, the Alaska Range includes only a small number of very high peaks. Fewer than twenty peaks exceed 10,000 ft. (3,000 m.) in height, and the crest of most of the range averages 7,000 to 9,000 ft. (2,100 to 2,700 m.) in altitude (Williams 1958).
The Cook Inlet-Susitna lowland, with an elevation of from sea level to 500 ft. (150 m.), is a broad basin with local relief of 50 to 250 ft. (15 to 75 m.). The retreating glaciers which occupied the Susitna Valley left a topography dominated by such glacial features as ground moraine, drumlin fields, eskers, outwash plains, and kettles. At one time this glacier may have been 4,000 ft. (1,200 m.) thick in the valley center, and 50 mi. (80.5 km.) wide (Williams 1958). In addition to glacial deposits, stream and terrace gravels are prevalent along drainages west of Cook Inlet. Permafrost exists only sporadically adjacent to the Alaska Range. The areas adjoining Cook Inlet are not well-drained, and lakes and swamps are plentiful.
The lowland has two branching arms which extend into the mountains; the largest is the Matanuska Valley south of the Talkeetna Mountains. The second is the Susitna River valley which extends east from the center of the basin and occupies a floodplain which varies from one to eight mi. (1 to 13 km.) in width.
Cook Inlet is a large tidal estuary which flows into the Gulf of Alaska just east of the base of the Alaska Peninsula. Knik and Turnagain Arms at the head of the Inlet are 45 and 43 nautical miles long, respectively. This shallow estuary is generally only 200 ft. (61 m.) deep and is an underwater extension of the lowland to the north. The entire Cook Inlet-Susitna lowland is over 200 mi. (320 km.) long and averages 60 mi. (96 km.) in width. The land surface of deltas of the Susitna, Matanuska, and Knik Rivers and the head of Turnagain Arm are rapidly encroaching on the tidal flats, testifying to the great volume of sediments carried by the rivers to the sea. Five active volcanoes—Mounts Augustine, Spurr, Redoubt, Iliamna, and Douglas—border the Inlet on the west side.
The region is both the major population center and the most agriculturally developed area in the state.
Copper River-Gulf of Alaska Subregion
The main physiographic sections in the Gulf of Alaska subregion are the Alaska Range, Copper River lowland, Gulkana upland, Wrangell Mountains, Kenai-Chugach Mountains, Prince William Sound, and the Gulf of Alaska coastal section (Figure 40c).
The Gulkana upland, located in the northwest corner of the subregion, and the Copper River form an intermontane basin which drains an area of 24,000 sq. mi. (62,000 sq. km.). The eastern part of the lowland is a relatively smooth plain 1,000 to 2,000 ft. (300 to 650 m.) in altitude, trenched by the valleys of the Copper River and its tributaries. Major tributaries include the Chitina, Gulkana, Gakona, Slana, Chistochina, Tazlina, Klutina, and Tonsina Rivers. Stream valley walls are steep and generally 100 to 500 ft. (30 to 150 m.) high. The Copper River is joined by its main tributary, the Chitina River, at the town of Chitina and continues southward through the Chugach Mountains to the Gulf of Alaska. These two river valleys contain longitudinal moraines and ice-scoured bedrock ridges rising above outwash plains. Large lakes occupy deep ice-scoured basins in the Gulkana upland, and thaw lakes are abundant in the eastern plain of the lowland.
The lowland is bounded on the north by the Alaska Range and on the east by the Wrangell Mountains. The latter are a compact cluster of volcanic peaks rising more than 10,000 ft. (3,000 m.) above the Copper River plateau. The higher peaks range from 10,000 to 16,000 ft. (3,000 to 4,900 m.). Large ice caps and glaciers forming in the mountains rim the interior basin and feed glacial waters to the main stem of the Copper River. Many of these streams have braided upper courses characteristic of glacial streams having heavy sediment loads and extreme seasonal variations in runoff.
The Kenai-Chugach Mountains, varying in width from 30 to 110 mi. (48 to 175 km.), comprise the 450 mi. (725 km.) central segment of the mountain chain bordering the Gulf of Alaska. The range consists of extremely rugged east-trending ridges rising 7,000 to 13,000 ft. (2,100 to 4,000 m.), and massive, broad mountains of lesser altitudes. The higher peaks in the range are draped with ice fields which feed valley and piedmont glaciers.
The St. Elias Mountains are desolate, icebound, and largely inaccessible. Massive, isolated peaks from 14,000 to nearly 20,000 ft. (4,300 to 6,600 m.) high rise at intervals of 5 to 30 mi. (8 to 50 km.) from a myriad of narrow ridges and sharp peaks 8,000 to 10,000 ft. (2,400 to 3,000 m.) high. The range is drained by a network of glaciers 4 to 15 mi. (6.5 to 24 km.) wide and up to 80 mi. (125 km.) long. The Bering-Bagley Ice Field complex stretches from Cape Suckling to the shoulder of Mt. Logan, a distance of approximately 130 mi. (210 km.).
Streams throughout the southern part of the subregion are typically short and swift with large sediment loads.
The Bering Glacier coastal area is a massive unit of ice fields and glaciers, cut by one major estuary—Icy Bay. The coastal mountains are separated from the sea by a narrow strip of lowlands crossed and modified continuously by numerous outwash streams.