Classified by substrate composition

Community zonation on steep rocky beach.

The Alaska Shorezone Project spatial database classifies intertidal beaches in many ways. One of the most useful is substrate size. Beaches can be subdivided into 3 groups: 1) exposed bedrock; 2) unconsolidated beaches with coarse material such as boulders, cobbles, or gravel; and 3) sand beaches. Note that estuaries are discussed on a separate page.

Habitats arrayed along the land-sea interface are important far out of proportion to their areal extent. Because of its narrowness, the beach comprises relatively few acres, but its linear extent is impressive. Southeast has about 19,000 miles of marine shoreline, and the archipelago is so intricately dissected that nearly all resident vertebrates can make use of the coastal fringe at some point in their life cycles. Extremely high productivity makes this fringe a critical foraging zone for both herbivores and predators. And all but the steepest beaches provide well-used travel corridors.

Within the coastal fringe habitat division, the linear beach habitats are separated from those of estuaries. Although estuaries comprise only about 2% of the total land area of Southeast, their ecological importance merits further subdivision into several tidal and barely supratidal zones.

1) bedrock beaches  Outcroppings of bare bedrock are found on many Southeast beaches. They sometimes form sheer cliffs, as in the granite walls of Ford’s Terror. Elsewhere, as on the gently sloping basalt flows of Shandák’w T’áak Aan (below), bedrock beaches can be as broad and flat as the surf-pounded sands of Yakutat.

Beach gradient determines suitability to the coastal fauna. A beach too steep for wolf, bear, or river otter to use for travel may, for that very reason, serve as a secure nesting habitat for pigeon guillemots.  Because of its complex geologic history of successively accreted terrains, Southeast has an enormous diversity of bedrock types. Rock texture, friability, and chemistry affect plant and animal community composition in all coastal and inland habitats of Southeast, but nowhere are these influences more starkly apparent than on exposed bedrock beaches.
For example, on the volcanic sea cliffs of Kanasx’éey, island of stunted spruce (St Lazaria Island) right-angle fractures create an abundance of perches for sea birds, resulting in some of the highest nesting densities of murres and cormorants in Southeast. These abundant birds in turn attract rare nesters like peregrine falcons.

Carbonate rocks such as limestone and marble have equally distinctive shapes and erosional patterns within the intertidal zone. Soluble rocks are eaten into ‘swiss cheese’ by contact with sea water. Resulting pockmarks, fissures, and tunnels are expressed at many scales, from cubbyholes for marine invertebrates to giant karst caves bearing ancient human pictographs and artifacts.

Shandák’w T’áak Aan, creek inside seal-clubbing reef (Shoals Point, Kruzof Island). Shelving basalt formations create unusual, gently sloping bedrock beaches. When molten, the basalt was highly mobile and created long, thin flows. Alaska Shorezone photo

2)  boulders, cobbles & gravel beaches  Unconsolidated surfaces are the commonest substrate on the beaches of Southeast. The greater structural complexity of these habitats, compared to raw bedrock or sand beaches, results in richer assemblages of algae, vascular plants, and macroinvertebrates. With a tidal range of more than 24 ft in some areas, Southeast beaches have well-expressed zones.

Rocky intertidal invertebrate communities are much richer on the outer coast of Southeast. Creatures that live in a fully marine environment have adapted to ocean salinity of about 30 parts per thousand, and many are inhibited where input from freshwater rivers dilutes the salt content. River influence on the inside waters also lowers temperatures and increases turbidity, reducing light penetration and photosynthesis

3) sand beaches  Apart from estuaries, where sand and silt deposition is common, examples of extensive pure-sand beaches are rather uncommon in Southeast. The largest is an almost continuous sand beach extending from Icy Point 130 miles northwest to Yakutat Bay. Even here, great rivers such as the Alsek and Dangerous are the ultimate sources of the sand, but these sediments have been carried far from the parent river mouths by ocean waves striking at angles to the beach, creating barrier spits, and “smearing” the sands northwestward at rates of about 160 ft per year. The Situk River mouth migrated 1.5 miles northwest between 1948 and 1989, based on measurements from aerial photography.

Ten miles of sandy beach forms the southern edge of the apron of Little Ice Age glacial outwash at Gustavus. Much smaller sand beaches occur on the outer coast of Kruzof Island in places like Shelikof Bay and Sea Lion Cove. On the archipelago, most outer-coast sand beaches occur in association with granitic bedrock. The granite of the outer coast is relatively youthful (Tertiary age) and more friable than the older granite of mainland terranes, perhaps more prone than older rocks to rapid weathering.

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