Landform Development in an Arctic Delta: The Roles of Snow, Ice and Permafrost

Abstract

Among the distinctive characteristics of arctic deltas is the presence of snow, ice and permafrost. In many other aspects arctic deltas differ little from deltas found elsewhere, i.e. they have distributaries, sand bars, mudflats, sand dunes, subaerial and subaqueous portions and various kinds oflakes. Included among the unique forms in arctic deltas are ice wedges, ice-wedge polygons, pingos and thermokarst lakes. The 600 km2 Colville River delta in northern Alaska illustrates well the forms and processestypicalofarcticdeltasandalsoillustratestherelationshipbetweenanarcticdelta and its drainage basin, the ocean, the atmosphere and the biosphere. The drainage basin, which occupies about 113 of the North Slope of Alaska, is confined entirely to the continuous permafrost zone north of the Brooks Range. It is a region where snow cover (albeit generally thin) and river, lake and sea ice last for eight to nine months. During that period oftime, geomorphic activity is minimal. Hydrologically, the sea is connected with the river in the deeper parts of the river's channels so that sea water penetrates beneath river ice some distance upstream during winter. In contrast fresh water flow during winter from the basin is virtually nil because ofthe frozen surface ofwater bodies and a frozen active layer which eliminates ground water as a source. Snow melt, which generally begins in May, gradually leads to breakup. A pre- breakup flood period is followed by breakup and post-break flooding; combined they last about three weeks. During that period, flood water flowing against riverbanks can cause thermoerosional niching which often leads to bank collapse. The character of the bank collapse depends to a large extent on the texture and type of frozen bank material. During breakup flooding, water flows over the bottom-fast ice of the river, into connected lakes and out to the sea. Much of the river's suspended load is deposited in these overflow areas. As floodwater progresses out over the sea ice it eventually flows down through pressure ridge cracks and scour holes to progress seaward beneath the floating ice. Being a low-lying area and being situated in the Arctic, arctic deltas are vulnerable to climatic and sea-level changes. Changes in temperature, snow fall, duration and extent o f sea ice, and active layer thickness are among the many changes that would have major impacts on deltaic processes and forms.