The river usually freezes over during the months of Oct. and Nov. The ice continues to get thicker throughout the winter with the average thickness being 42" on April 1. Depending on the temperature snow cover, wind, ect., the ice may freeze slightly more and then start to melt. The ice melts on the top due to the weather and on the bottom due to the water flow.
River: The tripod is planted two feet into the Tanana River ice between the highway bridge and the railroad bridge at Nenana, just up-river from the Nenana river tributary. It is 300 feet from the shore and connected to a clock that stops as the ice goes out.
Prize: In 1917 railroad engineers bet $800 guessing when the river would break up. Last year, the winners shared the prize money of $303,895. Over $10 million has been paid during the past 92 years. Payoff will be made June 1st, 2009.
This contest surely cannot be accused of fraud. So plotting the breakup time from 1st January you get:
Not much happens until 1965 when a steady decline begins
Interestingly it shows the early 40s to be warm.