Copper River Ice: Satellites Show a Shrinking Season for Winter Travel

People near Copper Center, Alaska, can cross the Copper River on ice for fewer weeks than they once could. A satellite study of the river puts numbers on it: across nine late-winter and spring weeks, the probability that the river had a widely travel-ready ice cover fell by an average of 53.3 (± 6.6) percentage points between water years 1973 and 2021. The study, in Alaska Park Science, was written by Dana R. N. Brown, Christopher D. Arp, Todd J. Brinkman, Melanie Engram and Katie V. Spellman of the University of Alaska Fairbanks and Barbara A. Cellarius and Mark E. Miller of Wrangell-St. Elias National Park and Preserve.

Open water on the Copper River, February 14, 2023, Wrangell-St. Elias National Park and Preserve (NPS/Barbara Cellarius).

Why the ice matters

For many Alaskans rivers are travel corridors, in open water and over winter ice, that lead to hunting, fishing, trapping and gathering areas. Across the state, climate change has worsened river ice for travel, with later and less predictable freeze-up, thinner ice, more open-water leads and earlier break-up, which narrows the window for winter travel and raises safety risks.

In the Copper River Basin, small communities lie along roads mostly west of the river, and there is only one bridge. Residents must cross the river to reach land to the east, which is the traditional territory of the Ahtna people and today holds Wrangell-St. Elias National Park and Preserve, with inholdings owned by Ahtna, Incorporated and Chitina Native Corporation and Native allotments owned by individuals. Other rivers cross the park too. Residents recall a time, decades before the study, when they could cross ice-covered rivers easily and predictably. The study had two aims: to document how the river's ice seasonality has changed, and to map where open-water hazards occur along it.

Fifty years of Landsat

The researchers used a roughly 50-year archive of Landsat satellite images of a 30-km stretch of the Copper River near Copper Center, for water years 1973 to 2021 (each running October 1 to September 30). They sorted weekly ice extent into classes by looking at the images. "High ice extent," the class that suggests widespread river ice travel is possible, meant open water affecting less than 25% of the stretch's length. Photos from citizen scientists and time-lapse cameras helped check their readings. Large open-water leads show up easily in the images, but not every water feature smaller than 60 m, or every other ice hazard, can be identified.

• Year to year. Winters with incomplete freeze-up, or with only one week of high ice extent, were very common in water years 2013, 2014, 2015, 2018, 2019 and 2020. They also occurred in early years of the record, including 1976 and 1983, although in many years gaps in suitable imagery made it impossible to tell.
• Two periods. In water years 1973 to 1997, an eight-week stretch, from the first week of February through the last week of March, had high ice extent in a majority (more than 50%) of weekly observations. In 1998 to 2021 only two weeks did, the first and second weeks of March.
• Trend. A logistic regression for the weeks from the second week of February to the second week of April found a significant decline in the probability of high ice extent in every week tested.

For water year 1973, high ice extent was most likely (68% to 89% probability) in eight of the nine weeks tested; for 2021 it was unlikely (7% to 40%) over the same weeks. Local air temperature points the same way: accumulated freezing degree days (each day's mean air temperature subtracted from 0°C, summed over the season) from October to April fell by 15% from water year 1943 to 2021 despite high variability among years. Together, the authors say, the results suggest that travel-ready ice now forms later in winter, incomplete freeze-up is more common, and break-up comes earlier.

Where the river stays open

For the second aim the team mapped open water in late winter from Sentinel-2 images for water years 2018, 2020 and 2021, covering approximately 217 miles (350 km) of the river from Slana to south of Chitina, and used Sentinel-1 radar images for 2018 to 2020 to follow how freeze-up progresses. Open water varied from year to year, but clear patterns emerged:

• The upper river between Slana and Gakona had consistently low open water in late winter and may be the most reliable place for community-accessible ice travel and crossings.
• Between Kenny Lake and Chitina, open water was low to moderate.
• Between Gulkana and Willow Creek, including near Tazlina and Copper Center, extensive open water and large leads tended to last late into winter.

The authors attribute much of this to flow energy, river shape and distance from the places where ice jams. High discharge, a steep channel and a narrow channel each add energy, and high-energy reaches stayed open longest, while low-energy, braided reaches froze earlier. Freeze-up usually starts as ice along the banks, which has less distance to span in narrow, winding or braided reaches, and river bends give floating ice places to jam; once an ice front forms, floes pile up and fill open water upriver. Ice often bridges a narrow, high-energy stretch at Wood Canyon below Chitina in early winter, and the front may then advance quickly upriver. In warmer winters that advance is held back, and some reaches above Willow Creek, including near Copper Center, stay partly open all season.

What it means

The authors conclude that the Copper River's season for ice travel is severely diminishing as air temperatures rise, which matches what residents say about no longer being able to reach hunting and gathering areas, private land and public land across the river predictably and safely. They expect the duration and continuity of the ice cover to keep declining, given projected increases in air temperature, precipitation and river discharge in the basin over the next century. Their maps pick out likely crossing areas and areas prone to open water, which they say may help with access and with preparing for hazardous conditions.

Sources

• National Park Service, Alaska Park Science 22(1), Using Satellite Imagery to Detect the Changing Seasonality of River Ice, by Dana R. N. Brown, Christopher D. Arp, Todd J. Brinkman, Melanie Engram and Katie V. Spellman (University of Alaska Fairbanks) and Barbara A. Cellarius and Mark E. Miller (Wrangell-St. Elias National Park and Preserve, National Park Service), 2023; the article's maps, charts and image panels are left out; rewritten in hubnx's own words.