Heavy rain, snow melt, and sometimes ice jams sometimes result in spring flooding in the lower 48 states. The primary threat for flooding in the spring along Alaskan rivers is ice jams caused by the breakup of ice. Snowpack and precipitation also contribute, but ice breakup on the rivers in Alaska is when residents along the rivers collectively hold their breath.
River-ice break is a major annual event, and the primary ice breakup window is April and May. Thick ice that formed during the long cold winter begins to soften and melt as the weather warms and the sun climbs higher in the sky during the spring. Breakup tends to occur earlier when temperatures and river flow are above normal.
Ice jams form when ice accumulates at bends in the rivers, causing more ice to pile up at that spot. Water backs up behind these jams, and the river can leave its banks, flooding communities along the river. Flooding might also be caused by the rapid release of water from the breakup of an ice dam, causing more of a flash flooding situation. As you might expect, flooding risks increase when river levels are higher than normal. Even in years where water levels may be low, ice jams can quickly turn a low flood potential into a high flood potential.
|Map showing locations of Eagle and Nenana, AK.|
|A huge slab of ice sits across a road near Eagle, AK in May 2009.|
|Ice and damaged homes in Eagle, AK in 2009.|
Credit: National Park Service
There are two main processes for ice breakup on the Alaskan rivers, thermal and dynamic. Dynamic breakups tend to produce the worst flooding. These occur when the river levels rise due to runoff from rapid snowmelt or heavy precipitation. The changing water level helps break up the ice, usually into large sheets. These large pieces of ice can easily be jammed up at bends or narrow portions in the river, causing an ice jam. During a thermal breakup the ice melts in place because there is not enough flow to move the ice downstream. It becomes thinner and thinner, and when it does break up it usually is in smaller pieces rather than massive sheets of ice. However, ice jams and flooding are still possible.
After a warm and snow-starved winter, Alaska experienced its 9th warmest and 11th wettest April on record. The heavier April precipitation was offset by the lack of snow and warm weather during the winter and early spring, reducing the flood threat. This year the flood potential is on the low side for most Alaskan rivers as a thermal breakup is likely on most rivers.
|It has been much warmer than normal the past 60 days across central and eastern Alaska.|
A number of communities have contests to predict when ice breakup will occur. The best know of these is the Nenana Ice Classic. This began in 1917 when railroad engineers bet $800 guessing when ice on the Tanana River would break up, and it has been conducted each year since. People from all over the world participate. A tripod is set two feet into the river ice 300 feet from shore. It is connected to a clock on shore that stops once the ice breaks up and the tripod tips over. This year the ice breakup occurred on April 24th, and the winner walked away with over $300,000.
|The tripod being installed in the river ice for the 2015 Nenana Ice Classic. |
Credit: Nenana Ice Classic
The National Weather Service River Forecast center in Alaska and the Alaska Department of Homeland Security and Emergency Management have begun a Riverwatch program to monitor the breakup of river ice and keep communities informed about river conditions and flood potential.
You can read about ice breakup conditions on the NWS Alaska River Forecast Center web site.