The spring ice break-up is an important geomorphic driver on many northern rivers, especially on those streams where the break-up is “pre-mature” and capable of generating rapid increases in stage and velocities. (Scrimgeour et. al, 1994) Recent literature identifies several observable features that provide evidence for ice jam flooding, including scars on trees, boulder pavements, ice push marks on channel banks (Burge and Lapointe, 2005), undercut banks, ice-push ridges, island buttresses, and drumlins (Scrimgeour et. al, 1994). In addition to leaving behind those smaller indicators, pre-mature ice break-up events have significant geomorphic influence on channel morphology. They are often associated with anabranch cycles on wandering rivers (Burge and Lapointe, 2005), as well as with nutrient-rich sediment deposits in downstream estuaries (Prowse et. al, 2006).
The Kobuk is a wandering river with several confined, straighter sections interspersed among semi-confined and unconfined meander belts with island complexes, oxbow lakes and backwater sloughs. It experiences a pre-mature ice break-up nearly every spring which can cause the river to rise several meters. During a trip down the Kobuk in mid-August of 2008, we observed a high-water mark in the town of Kobuk that suggested the river might even rise as much as 15 meters there, and locals confirmed that the town (which was about 7 meters above water level) floods every spring. It is worthwhile noting, however, that locals also informed us the river was unusually low at the time. This probably exaggerated our perception of the town’s height above August water levels.
During our float from Walker Lake to the town of Kobuk we also identified several indicator features of break-up flooding. Most striking of these were ice push marks and ridges that appeared on the river’s point bars, as well as significant scouring on both point bars and cut banks. These features occurred with varying density over the river’s longitudinal profile. In the upper reaches of the river, from its confluence with Walker Lake until Aklepik Creek about 100 kilometers downstream, we saw consistent and dramatic evidence of ice scour on almost all surveyed point bars. The scour marks were typically between 0.5 - 2 meters wide and 1 - 3 meters long, and were accompanied by a mound of gravel and/or cobble at the downstream end. They were spotted with greater frequency on the upstream end of bars and further away from the river’s edge, providing some evidence for the high water line during the 2008 spring flood.
After Aklepik Creek, ice push marks and scouring were observed with decreasing frequency, and in the unconfined Kobuk Valley they were even difficult to find. Along the entire stretch of the river, we observed undercut banks on the outer edges of meander bends, but it was less clear whether those were directly attributable to the effects of ice during spring flooding.