Geomorphology
The Kobuk River is a periglacial river, fed by a remnant glacial lake (Walker Lake) and mountain snowmelt in the Brooks Range. It cuts a channel through a landscape otherwise dominated by permafrost. The Kobuk’s current form and structure is a direct result of several stages of erosion and channel formation following the last glacial retreat.
As the glacier first retreated and melted, large amounts of highly erodible, fine-grained sediment dropped out in relatively high mountain valleys. The availability of this fine-grained, loose sediment combined with a high gradient to turn a newly forming Kobuk River into a fast-working sediment transport system. The river picked up glacial till from its upper reaches and transported it downstream until the gradient diminished. When the river encountered flatter ground, it deposited its sediment load resulting in the creation of broad, flat floodplains, alluvial fans, and meander bends through aggradation.
After the first stage of aggradation and sediment transport, the Kobuk began a new phase of erosion and landform development. The river exhausted its supply of easily erodible sediment upstream, thus decreasing its sediment load and increasing its load carrying capacity downstream. With more capacity downstream, the river began to incise into the alluvial fan it previously created, moving sediment stored for a long time on its original floodplains to newer floodplains even further downstream.
Further down in the Kobuk watershed, the river worked in concert with the wind to create one of the more famous landforms in Alaska: the Kobuk Dunes. These large sand dunes are the modern ancestor of alluvial deposits that became shaped and dominated by an exchange of aeolian and fluvial processes. In moister climates, the river has more water, and thus more power, and cuts down through a sandy bed. In times of drier climate, wind dominates and blows a lot of sediment into a weaker fluvial system, leading to aggradation and floodplain re-distribution.
Currently, the Kobuk River in its middle and lower reaches is an anastomizing stream, with several braided channels in places, wide migrating meander bends, and oxbow lakes. It is controlled by yearly cycles of freeze and thaw, much like its surrounding landscape. For six months of the year, the river is largely still, frozen on top by a thick layer of ice. In the spring, warmer temperatures cause this ice to begin to melt. But the resulting process is not gradual. Rather, the pressure of melting ice from upstream builds up upon ice “dams” in the river’s channel, eventually causing an “ice break-up” event, in which a flood of ice and water moves powerfully downstream.
These annual spring break-up events have several important consequences. First, the river has deeply undercut and eroded banks, caused by large, fast-moving chunks of ice carving out the river’s channel before it begins spilling out onto its floodplain, which is 1 to 6 miles (1.6-9.7 km) wide except at the confluence of major tributaries. Second, the river moves laterally very quickly and dramatically, re-inventing side channels every year as its secondary streams become drowned in each yearly flood by an overwhelming amount of sediment. Lastly, since the river is surrounded mostly by permafrost and because during the spring break-up event there are still large parts of frozen ground close to its banks, floods often transports large amounts of fine sediment across broad expanses of floodplain in thin sheets of water that slide easily across the frozen ground. These characteristics also translate into a relatively variable habitat for the Kobuk’s native species.
Farthest west, the Kobuk empties into Hotham Inlet, the easterly arm of Kotzebue Sound. During recent geologic times however, when sea level was lower, the Noatak, Kobuk, and Selawik Rivers were joined. Now, they have separate deltas with many lakes and swamps and intricately webbed channel systems. The deltas are composed primarily of silt, sand, and gravel.
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