Warm Spring May Result in Gentle Breakup

He also said that due to cooler conditions and more snowfall along the Koyukuk River, in Northwest Alaska and on Alaska’s North Slope, there is a normal chance of spring flooding there.

In the recent past, many of Alaska’s major rivers would be blue-white and solid in late April. After a historic warm March and early April this year, many Alaska rivers have ice-free segments.

In one strange example, Johnson showed that the Kuskokwim River in mid-April had open water both in Bethel and hundreds of miles upriver, in Nikolai. Ice on big Alaska rivers has in the past broken up like a row of falling dominoes, with ice first melting upstream and then progressing downriver in a predictable fashion.

River ice is weak throughout the state, Johnson said, and people choosing to walk, snowmachine, drive or ski on it should take care.

“This year, you can’t travel to places you normally could during the last week of April,” he said.

Spring air temperatures are the most important factor in river breakup (just ahead of snowpack-depth and thickness of river ice). Temperatures have been been very warm over most of Alaska, encouraging what researchers call a “thermal” breakup, during which ice rots and rivers flow on to the sea with little drama.

“Dynamic” breakups happen when spring temperatures stay cool until they suddenly warm, flushing  snowmelt onto river ice. This pulse can shove chunks of ice together to create a dam that pinches a river’s flow, causing water to ooze over riverbanks. In late May, 2013, an ice dam at Bishop Rock backed Yukon River water into Galena and flooded every building in town.

With many rivers in late April 2019 already starting to mush out, the scientists at the river forecast center predict low flood potential for almost every village in Alaska. They will fine-tune their predictions in early May, when their “River Watch” team of pilots and meteorologists fly small planes above Alaska rivers and eyeball known ice-dam spots.

River transitions from solid to liquid are a big deal in a state that has fewer miles of paved road than Vermont. A longer open-water season means villagers can fire their outboard motors later in the fall and earlier in spring, but Thoman pointed out that a spring like this one might both give and take away boat travel.

“Once snowmelt gets going, you think that’s it,” he said. “But then it backs up and goes from water to snow-covered ice again. The ice is too thin to snowmachine on and you can’t boat. Is this a feature of our new climate?”