On our expedition down the Kobuk River from Walker Lake to the village of Kobuk, we encountered many stretches of river dominated by the presence of islands, or complexes of islands. However, mid-channel islands are not pervasive throughout the system, nor through comparison with historical maps, do they seem to be stable features. The presence of islands and island complexes on the Kobuk River is often determined by topographical confinement and sediment input from tributaries. Island formation due to each of these factors seems to produces a unique set of characteristics.
Stretches of river confined by topographical features are less likely to form meandering (stream with sinuous shape) or bifurcating streams and therefore have fewer islands. From our observations, islands that formed in these confined stretches of river are often stable features, appearing on historical maps. Unconfined stretches of the river had greater numbers of islands, which seemed to be shifting location. Island formation in these stretches is most likely due to the meandering nature of the river. The meander bend of a river has an outer cut bank where erosion occurs and a point bar where deposition occurs on the rivers inner bank. It is the formation of the point bar in a meandering river that can lead to island formation. Meander bends are continuously migrating downstream, and in the process reworking and building point bars in the direction of migration. At some point, usually during high flows, the river forms a cut off channel that in rough terms used for visualization, redirects some of the channels flow from the inflection point of one sine wave to the inflection point of the next downstream wave. This process leaves a mid-channel island that begins at the point of bifurcation and ends at the point of reconnection, or the confluence. As the river continues to meander and shift in the downstream direction, old islands are reworked, new islands form from point bars and at times of high flow the river forms new cut off channels. This dynamic process forms unstable islands and island complexes. Observations of this type of island formation were prevalent in the unconfined portion of the river flowing through the Kobuk Valley.
The second type of island formation observed on the Kobuk River is due to the input of sediment from tributary streams. In some cases, tributaries carrying large sediment loads empty into the main channel of the Kobuk and deposit sediment. It is thought that sediment deposition occurs only in stretches of the Kobuk where the input from the tributaries exceeds the competence and capacity of the Kobuk. Here, sediment builds up and mid-channel islands form. Islands formed in this manner change rapidly as flow from the both the Kobuk and the tributary fluctuate and remove or deposit additional sediment. A clear example of this type of island formation occurs at the confluence of the Reed and Kobuk Rivers.