Our group expected and prepared for 11 wet cold days on the river since the second peak of the hydrograph in the Kobuk River basin comes in August due to summer precipitation events. The basin gets an average of 21 inches (53 cm) per year; yet, as it turned out, our group saw very little of that.
According to a local pilot in Bettles, it’s been a dry summer with nearby forest fires. The day we arrived in Bettles they received .37 inches of rainy and over the next 10 days they received an additional .15 in. Compare this to their “normal” precipitation of .88 in. for those 11 days according to the National Weather Service.
Overall, only 3 or 4 days had precipitation (mostly of limited duration) during the trip down the Kobuk River from Walker Lake to the town of Kobuk August 11-22. The first evening we arrived it rained through most of the night and into the next day, but after that it cleared up and remained partly cloudy. With a solid overcast the temperatures were moderate and the clouds made the long days of sunlight timeless, with 10 am looking about the same as 3 pm and not much different from 10 pm. By August 15th the days were mostly sunny. The contrast of temperatures and how quickly they can change here is astounding. Air temperature was in the mid 60s or 70s but in direct sunlight it felt like 85 F. The sun’s intense heat beat down uninhibited by cloud cover.
For several afternoons after August 15, thunderheads built up running roughly north-south around 2pm. Dark clouds and rain in the distance was a common occurrence for several days, yet the skies above us were clear and blue. On the 18th just after the confluence with the Pah River, we floated right into one of these fantastic thunderstorms which dropped the temperature and dumped buckets of rain. For a couple hours we made slow progress through the storm and finally settled on a place to camp for the night. Before long, the rain had stopped and the clouds were clearing. Afternoon thunderstorms clearly play a role in the August peak in the hydrograph.
The river’s geomorphology showed signs of falling stage as days past and we moved down river yet, in places such as the Kobuk’s confluence with the Reed River, large woody debris and gravel bar deposits suggested a recent flood. Our observations are confirmed by the hydrograph, available from a gauge downriver from Kobuk at Kianna. Discharge during the month of August has been well below mean daily levels for this time of year, yet a major pulse moved through mid-July. Another lesser peak is present the first week of August just before our expedition and then the hydrograph shows evidence of steady decline in river stage.
Figure 1 shows the river stage at Kianna during the month of August 2008 compared to what level it exceeds 10-50% of the time and more than 50% of the time. This indicates that indeed it has been a below average dry August. Figure 2 from USGS also indicates the below average levels during August and includes the peak in July.