Over the course of our late summer trip on the Kobuk River, we surveyed numerous tributary sites. In contrasting these different confluences and their downstream effects on the Kobuk, some seemed markedly more dominant than others. While a few of the steeper gradient, northern tributaries had a significant effect on the Kobuk River’s downstream channel(s), the Kobuk seemed to “win the battle” with equally large, yet lower gradient, southern tributaries.
Two of the Kobuk’s larger tributaries, the Reed and Pah, demonstrate this contrast very well. The Reed enters from the north in a relatively straight channel that is controlled and confined by bedrock. Large woody debris deposited on the gravel bar at its confluence indicate that the Reed River dominated the direction of flow there in the last high water event. Trees were pointed at an almost 90 degree angle to direction of Kobuk River flow. Directly downstream from this tributary, the Kobuk branches around several large islands, beginning an island complex several kilometers in length. It is likely that this is caused by a large influx of sediment from the Reed River, seemingly deposited en masse during episodic, high flow events. Looking in the opposite direction, upstream at the Reed River, the Kobuk seems to have very little effect on the channel and/or vegetation.
In contrast to this, the Pah River enters the Kobuk from a lower-lying plateau south of the river, joining the Kobuk in the middle of an island complex. Like the Reed River, the Pah drains a large area and flows in a relatively straight channel down to the Kobuk. Unlike the Reed, it has a low gradient from headwaters to confluence. The Pah enters the Kobuk in a very wide channel, composed mostly of shallow mudflats. Whereas the woody debris at the Reed confluence indicated dominance of tributary flow, the woody debris on the gravel bar across from the Pah confluence display a visible downstream slant in the direction of Kobuk River flow.
Also different from its interaction with the Reed, the Kobuk River does not change in any significant way downstream from the Pah confluence, while the Pah undergoes significant changes just upstream of the Kobuk. There is a small resistant stand of spruce vegetation on the river left side of the Pah confluence, and maps show that the location has been incredibly stable over the last 30 years. Nevertheless, the Pah River above the Kobuk changes abruptly from a single-channel river to a multi-channel anabranching river that eventually enters the Kobuk in two or three distinct channels (depending on stage). It is possible that this sudden change in character is due to the Kobuk River. The Pah enters the Kobuk at the inside of a wide meander bend, and because of its low gradient may become susceptible to sediment deposition by the Kobuk in times of higher flows. During spring flood events on the Kobuk, the river has the potential to rise 4 meters, giving it a significantly large floodplain, and thus the ability to heavily influence a tributary like the Pah.
This comparison is not to imply that all northern tributaries with large gradients had significant effects on the Kobuk, or that gradient and latitudinal origin alone can be used a reliable predictor of tributary influence. Rather, it was interesting to note the apparent battle between the Kobuk and its tributaries, with some creating major change for the river, and others themselves being changed by the Kobuk.