Bishop Rock's oversized effect on breakup

Bishop Rock's oversized effect on breakup

A few weeks ago, as my friend Forest and I rode our bikes on the vast white sheet of the frozen Yukon River downstream of Galena, the river forced us into a 90-degree hard left. There, the channel suddenly necked down from being almost a mile wide to just a quarter mile.

A 300-foot outcrop known as Bishop Rock sits at this pinch point on the middle Yukon. Its name - bestowed by someone in remembrance of an Oregon missionary who was murdered there in 1885 - comes up at this time every year when people start talking about river breakup and the potential for ice-jam flooding.

Kyle Van Peursem of the Alaska Pacific River Forecast Center mentioned Bishop Rock during a recent presentation on the potential for spring floods in communities along the state's rivers.

Though the Yukon, Kuskokwim, Koyukuk and other rivers in central and northern Alaska are all very solid and white as of this writing, that will soon change. River breakup happens when the power of the sun melts feet of snow from the landscape and rots the ice of the river that was hard as iron for so dang long.

Predicting when breakup will occur at any of the dozens of villages along river systems is an inexact science. The most important variable is air temperature. Warmer Aprils are good, Van Peursem said, because they allow the snow and ice to melt at a more gradual rate that won't overwhelm river channels.

The biggest driver of the dynamic breakups that flood villages is a cold April that "compresses the time to get rid of snowmelt," he said.

Alaska villages on rivers most often flood in springtime due to ice jams. Jams happen when meltwater shoves chunks of recently broken ice sheets together.

"I think of these as like a dam in the river," Van Peursem said. "The breakup front (a conveyor belt of ice chunks) stops, water has no place to go and piles up behind it."

Constrictions in rivers like Bishop Rock are common places for ice jams. In 2013, a pileup at Bishop Rock swelled the river upstream like a python and flooded Galena. The same happened in 1945, when U.S. Air Force bomber pilots dropped more than 75 bombs on the ice jam in front of Bishop Rock. They failed to dislodge the mass of ice.

Bishop Rock will soon loom large in the windows of a single-engine aircraft in which Van Peursem will fly. He will monitor that portion of the Yukon River on flights from Galena as part of the Riverwatch program.

Van Peursem said the part of the Yukon he is monitoring is trending toward a dynamic breakup due to a cold April (Galena's low temperature on April 22, 2026, was in the single digits Fahrenheit), but "hopefully we can slowly warm up as we go into May."

A note to my readers: This, friends, is the second-to-last Alaska Science Forum I will write. After 31 years in the saddle, I am retiring from my science-writer job here at the Geophysical Institute on May 1, 2026. Though I have planned this for a while, the date sure has snuck up. I will sum up the whole adventure in my final column next week.

And - fear not - my boss and other leaders at the Geophysical Institute are committed to continuing the Alaska Science Forum after I leave.

Since the late 1970s, the University of Alaska Fairbanks' Geophysical Institute has provided this column free in cooperation with the UAF research community. Ned Rozell is a science writer for the Geophysical Institute.