Ice on the river accumulates each winter to an average maximum thickness of at Nenana. The Nenana Ice Classic, begun in 1917, is an annual guessing game about the date of the ice break-up. In October or November, after the freeze has begun, a tripod is planted in ice in the middle of the river. The tripod is connected to an on-shore clock that stops when the tripod begins to move during the spring thaw. Over the years, the break-up date has varied from April 20 to May 20. Betting on the exact time of the break-up takes the form of a lottery, called the Nenana Ice Classic.