Flood in Upr Tanana Vly Fortymile, Alaska

An ice jam formed on the Mosquito Fork of the Fortymile River approximately 1.5 miles downstream from the Taylor Highway bridge. A NWS automated river gage on the bridge recorded a rise in water levels of 10 feet in 48 hours. Ice rose to the base of the bridge and lower part of the guardrail but damage to the bridge was not observed. Water also went overbank and flooded portions of a wayside at the bridge but the Taylor Highway was not impacted by floodwaters and the road remained open.

A prolonged period of well-below-average spring temperatures combined with an above-average winter snowpack created a dynamic breakup of the Mosquito Fork of the Fortymile River. This resulted in ice jams and flooding. April temperatures were well below normal, and when temperatures began to warm in early May, it melted the low-elevation snowpack quickly. Because of the relatively quick addition of snowmelt onto strong ice, the ice was more susceptible to jamming and floods.