But it is water from the Stewart and White Rivers that finishes off the job, and sometimes causes flooding. Once breakup has started in Dawson, staff from Water Resources keep a close eye on conditions both upstream and downstream of Dawson, keeping track of where the ice is running and how snowmelt is progressing. They also use satellite images to monitor conditions in the area. Every day they fly over the section of river between the Chandindu and White Rivers. The White, which drains the western St. Elias Mountains, is warmed by coastal flows of milder air from the Pacific, and can open up as much as two weeks before the Yukon River.
Thicker ice is more likely to form ice jams, which in turn can cause flooding.
Around Dawson ice jams come in two different forms. The most common ones form downstream of Dawson, causing the water to back up for a time. But ice jams also form upstream when water from the White and Stewart Rivers flows into the Yukon, causing the ice there to decay. These upstream jams can cause surges in the water flowing down the Yukon River. Eventually a wave of water a metre high can form.