(CNN) -- An ice jam on the Yukon River in Alaska sparked flooding in the city of Galena, where some 300 people have been evacuated, authorities said.
Most of the people who left their homes were being housed in either Ruby or Fairbanks, said Kerry Seifert, operations section chief at the Alaska Division of Homeland Security and Emergency Management.
Some are at shelters, while others were with friends and relatives.
"We expect the flooding to become worse before the waters start to recede," said Alaska Gov. Sean Parnell, who surveyed the damage this week.
"Ensuring the safety of those impacted by the flooding remains my top priority."
According to Seifert, the cause of the jam was a late break up of ice.
Colder temperatures meant there was no melting until a rapid warming put lots of water in the river basin.
A wall of ice got hung up at a steep curve in the river near Galena, he said.
Water appeared to be beginning to move around the sides of the ice Wednesday, Seifert said, which could be good news for people living in Galena.
When the jam breaks, however, downriver communities of Koyukuk and Nulato may be at risk.