Observations of fall freeze-up and spring break-up, important to subarctic hydrology and to local economy, were made at the Yukon River town of Beaver, Alaska, September 1949 to June 1950. On October 15, with river temperature at 0° C., the freezing together of floating ice crystals formed thin ice pans that gradually thickened. Falling river level and increasingly heavy ice concentration choked off all but main channels. On 25 October ice jammed downstream and produced a continuous ice cover and a temporary rise of about 1 m. at Beaver. After 26 October the river resumed its drop in level until April. Freeze-up appears governed by local channel, current and weather conditions and lacks systematic progression either upstream or downstream.
Spring thaw beginning in late April thawed snow cover and weakened river ice. In early May the river began to rise slowly, but at an increasing rate, until 13 May when ice was nearly free from shore. On 14 May, after a rise of about 3 m. in 24 hours, the ice broke and moved downstream as the flood crest passed Beaver. Downstream progression of break-up is delayed by local ice jams, the chief cause of disastrous river floods, and is advanced by early break-up of large tributaries.