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On May 4, 2009, the Yukon River ice below Eagle, Alaska, had jammed and the town was flooding; one local resident called it the worst flood since 1937. That night the water rose another 10 feet (3 m), and ice chunks the size of bulldozers moved ashore and crushed homes. This article by Scott Lindsey of the National Weather Service's River Forecast Center, in Alaska Park Science, uses that flood and a later one to explain how river ice breaks up.
The Yukon at Eagle, May 7, 2018. Before May 2009, the island in the left foreground was covered with mature trees. Photo courtesy of Scott Lindsey, National Weather Service.
The river and its seasons
The Yukon enters the United States just upstream of Eagle and winds through Yukon-Charley Rivers National Preserve, past Circle and Fort Yukon, to the Bering Sea, about 1,200 miles (1,930 km) of wilderness. In summer it is a highway between villages; in winter its ice, often more than 5 feet (1.5 m) thick, carries snow machines and dog sleds. Spring breakup interrupts that access and can bring catastrophic floods. Through the interior winter snow builds from October to April at temperatures as low as -60°F. As days lengthen in April and May, snow melts and the ice starts to move, and how it moves decides whether residents can relax or must flee to higher ground.
Map of Yukon-Charley Rivers National Preserve. Image from the National Park Service's page.
What decides breakup
Ice thickness and snowpack shape the timing and the chance of jams. Freezing levels decide how much of the watershed sends snowmelt to the river. Rare early rain adds runoff. Ice with fresh snow on top reflects the Sun's energy and stays stronger than bare ice exposed to days of sunshine. Rivers that froze at high water form sheets that do not fracture easily when runoff pushes them. The most important factor is the temperature pattern from early April to mid-May.
Air temperatures at Eagle Airport, spring 2009. The caption says flooding began on May 3, 2009 after four days of record high temperatures. Data source: xmACIS.
Two kinds of breakup
| Thermal breakup ("mushout") | Dynamic (mechanical) breakup | |
|---|---|---|
| Ice | Becomes rotten and weak before ice from upstream arrives | Stays hard and intact until pushed |
| Typical conditions | A slow shift from cold to warm, or low snowpack | Normal to above-normal snowpack and thick ice, then a sudden warm-up |
| What happens | Breaks into small pieces, often at many places at once | Sheets half a mile or more wide move and jam at bends and constrictions |
| Jams | Persistent jams are very unlikely | Jams can flood upstream; the front marches downstream |
| Examples | Quiet breakups on the upper Yukon since 2013 | Kenai River, January 1969 and January 2007; Yukon, May 2009 and 2013 |
In 2009 an ice jam 10 miles (16 km) downstream raised the water at Eagle more than 30 feet (9 m) in 48 hours. The breakup front then stalled repeatedly downstream, and Circle, Stevens Village and Tanana also flooded. Ice shoved up onto land levelled the trees on an island in front of Eagle and left little of the buildings in Eagle Village.
Eagle Village, May 5, 2009. Photo courtesy of Scott Lindsey, National Weather Service.
Tributaries flood too. When the main river is high, a stream such as Hess Creek near Rampart cannot drain into it, and water backs up the tributary.
Backwater flooding where Hess Creek meets the Yukon near Rampart, May 10, 2009. Photo courtesy of Scott Lindsey, National Weather Service.
Watching the river
The Riverwatch program, run since the 1970s by the Alaska-Pacific River Forecast Center and the state's Division of Homeland Security and Emergency Management, sends a hydrologist and an emergency management specialist along the Yukon and Kuskokwim in a chartered plane. They report ice conditions, land in towns to brief community leaders, and share observations with the NWS and state officials. In 2009 the state had current information and launched relief quickly.
2013, and quiet years since
Four years later cool weather held snow in the interior into mid-May, then temperatures rose quickly toward normal. Ice began moving just after midnight on May 17, 2013, and had stopped by 3 am; within an hour there were 3 feet (1 m) of water on the ground floor of the building where the author was staying. Eagle reached 43.9 feet (13 m), almost 10 feet (3 m) above flood stage and the second-highest on record, well below 55.7 feet (17 m) in 2009. Circle had moderate flooding, Fort Yukon minor, and Galena catastrophic.
Temperatures at Eagle Airport, 2013: not far above normal, but a rapid rise from well below normal with overnight lows above freezing. Data source: xmACIS.
Since 2013, breakup on the upper Yukon has been quiet: slow, uniform warming and thermal breakups, with the earliest breakup on record in many villages in 2016. The first recorded breakup at Eagle was May 10, 1898. Lindsey expects another year when a quick turn from winter to spring means the ice will stop.
Sources
- Scott Lindsey, National Weather Service, River Forecast Center, "Spring breakup on the Yukon: What happens when the ice stops," Alaska Park Science 18(1): 70-75 (2019), National Park Service, https://www.nps.gov/articles/aps-18-1-9.htm; rewritten in hubnx's own words.
In these publicationsYukon - Charley Rivers National Preserve
Licence: CC0 1.0 (public domain) · Adapted from www.nps.gov
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