River Ice Breakup
Monitoring River Ice Breakup in Alaska
by Melanie Engram
Each spring, Alaska rivers ‘breakup,’ the local term for the yearly melting, shifting, breaking and ‘running’ of tons of river ice. ASF provides SAR data to the National Weather Service (NWS) to help monitor this dramatic phenomenon.
Sudden warm temperatures can quickly melt heavy winter snow, dumping large volumes of water into iceclogged waterways. Since SAR data is exceptional for imaging both ice and flood events, it’s a natural choice for monitoring river ice breakup.
Ice sheets and chunks can jam in a river, forming ice-dams that impede the flow of water, causing flooding. For the inhabitants of Alaska’s fly-in rural villages, seasonal flooding during breakup can mean inconvenience, property damage or even evacuation.
Arleen Lunsford of the NWS uses 30 m resolution standard beam Radarsat-1 imagery each spring to detect the location and condition of river ice. “We look for many different clues (in SAR imagery),” says Lunsford.
“Does the ice appear to be getting rotten? Has it cleared out of a given reach? Is the ice moving? Is the main channel clear, but is ice remaining in side channels or sloughs? Is there an ice-run upstream of ice that hasn’t moved?”
Lunsford, a staff member of the Alaska-Pacific River Forecast Center in Anchorage, is one of a team who provides river breakup forecasts, flood warnings and other hydrometeorological products during the breakup season.
The center began using ERS-2 and Radarsat-1 data from ASF in 1997 to monitor spring breakup. Information derived from the imagery regularly contributes to the river analyst team’s daily ‘breakup discussion’ and the ‘breakup map for Alaska.’
Alaska’s widest rivers, the Yukon and the Kuskokwim, are the two main targets for ice analysis with SAR imagery, but Lunsford says that an ice/no-ice determination can usually be made from SAR data for narrower rivers as well. “A huge advantage of the SAR data,” claims Lunsford, “besides the (high) resolution of the standard scale imagery, is the fact that it doesn’t matter if we are cloud-covered or if it is night. We still get the image, unlike high-resolution visible satellite imagery, which is of no use at night or when the area of interest is fully overcast.”
Breakup for spring 2004 was mild with minor flooding in only a few areas. Villagers were cautioned, however, to keep watch for late ice-runs coming from the upriver tributaries of a few northern rivers, incidents that could leave small boats capsized or crushed.