Pam is in front of an example of one of the tripods used in the Nenana Ice Classic.  The tripod is set in the ice on the Nenana River. Once the ice has thawed to the point where walking on it is no longer safe, a wire is attached from the watchtower on shore to the top of the tripod, and rigged up to a clock mechanism so that the clock will be stopped when the tripod moves 100 feet. When the tripod trips the clock, spring has officially arrived, and Interior Alaska's rivers are once more navigable.