By Jenny Neyman
Breaking up is hard to do. Nowhere is that more literally true than on Alaska’s major rivers in the springtime, when the ice cover cracks and fractures into car-sized chunks that scour their way downstream.
In a good year, riverbanks remain protected by a hardened shell of ice and those ice boulders just grind against themselves until they flush out into the sea. And the whole progression happens as smoothly as the movement of a massive jumble of watery rubble can be.
In a great year, breakup is even gentler, with ice warming and thinning in place, until it shatters like glass and is docilely swept downstream. The winter of 2015 might not have been good for skiers, snowmachiners and dog mushers, but it’s been a great one for breakup on the Kenai River.
“The edge of banks pretty much stayed frozen. Where there wasn’t running water that ice was pretty thick, and it was somewhat shaded, so we still had that protection on banks themselves, so that was a good thing,” said Tom Dearlove, director of the Donald E. Gilman River Center on Funny River Road.
The center houses multiple agencies tasked with monitoring and managing development on the river, including issuing permits for any work being done on or near its banks, like installing fishing stairways and docks. Dearlove said he hasn’t heard of anyone needing to replace structures from ice damage this year.
That tracks with what the Alaska-Pacific River Forecast Center has seen statewide. Crane Johnson, senior hydrologist with the forecast center, gave a prediction April for how Alaska’s annual breakup with winter might go this year.
So far, things are looking good. The annual Spring Breakup Outlook for Alaska, released April 10, notes warmer than usual temperatures, thinner than usual ice conditions and drier than usual snowpack for much of the state so far this April, particularly in Southcentral Alaska. That’s shaping up to cause river breakups that are thermal, rather than dynamic.
Dynamic breakup is the rowdy one, and ice jams are typical, where ice remains hard and only moves downstream when it’s shoved from upstream, either by other ice or by a surge of water, for instance, if a glacier-dammed lake happens to release. That was the cause of the two worst ice-jammed floods on the Kenai River, in 1969 and 2007.
As Crane explained, thermal breakup is a kinder affair.
“Ice becomes very rotten, is weak, and has less resistance to breaking up. Generally there’s no significant ice jams associated with thermal breakups,” he said.
That’s not to say flooding can’t happen with a thermal breakup, however.
“The flood threat, while maybe low or below normal, during spring breakup we never can consider it zero. There’s always some chance that there’ll be a local ice jam,” he said.
As for timing, dynamic breakup happens all at once in dramatic, sometimes traumatic fashion, whereas thermal breakup can extend over a longer period.
“We can get multiple breakup fronts. You also see generally less erosion of riverbeds and banks during a thermal breakup, primarily due to the weaker ice conditions. The ice melts or rots in place, or commonly the ice that’s referred to as a mush out, where the ice melts in place,” Crane said.
The Kenai River has had more of a thermal breakup this year — actually, a couple of them, with warm periods causing river ice to thin a few times over the winter. Dearlove was concerned that this could actually cause some problems.
“It was one of those I was a little bit worried about the river in terms of having the freezing and thawing that that might cause us more ice jams, but it looks like we’re through the thick of things,” Dearlove said.
Crane noted that Southcentral Alaska, in particular, is lacking the amount of snowpack it typically has this time of year. The water level in the Kenai River is low at the moment, but not abnormally so, Dearlove said. And he doesn’t expect the river level to be particularly affected by the peninsula’s lack of snow for most of the summer, since glacial melt and whatever precipitation the area gets will still trickle into the waterway. But come fall, especially if it’s a dry one, the river could feel the effects.
“We’ll still get the glacial melt so we’ll still have water coming through, and it may be late fall that we see some lower water levels that are abnormal but that all depends on our amount of rainfall we get this summer, so it’s kind of hard to predict that one,” Dearlove said.
The Alaska-Pacific River Forecast Center’s Spring Breakup Outlook for Alaska can be found online on the National Weather Service Alaska Region website, and the forecast is updated regularly as breakup occurs around the state.