Changing Ice in the Copper Basin
Dana Brown used Landsat images to calculate the amount of open water present on the Copper and Chitina Rivers over time. Landsat images are administered by NASA and the USGS.
Allison Sayer - CRR Staff
Two recent studies have explored changing ice conditions in the Copper Basin. Many locals have already observed changes in winter river conditions over time, including later freeze-ups and earlier breakups.
The impetus for the research was the hope it could inform both personal safety decisions for locals and management decisions around winter subsistence opportunities. The original discussions relating to this need came from the Wrangell St. Elias National Park Subsistence Resource Commission.
Anthropologist Odin Miller, working at the time under the auspices of the Ahtna Intertribal Research Commission (AITRC), interviewed local trappers about their land use patterns and their observations of river ice.
Ecologist Dana Brown, a research assistant professor at the International Arctic Research Center at the University of Alaska Fairbanks, used satellite images of the watershed to make comparisons of ice coverage across space and time on the Copper and Chitina Rivers.
Where are the best and worst places to cross the Copper and Chitina Rivers in the current climate? Can changes of ice coverage in space and time be quantified? Have people altered their use of the landscape in response to changes in the river?
Miller’s study sought to gather first hand knowledge from people with an intimate and longstanding relationship with the watershed. Not all respondents were in agreement, but the majority stated that in prior decades, the Copper River used to be less “treacherous” to cross, and could be crossed earlier in fall and later in spring.
Miller said it was “unremarkable” to cross the Copper River in the past. Some long-time residents even recalled large log trucks driving across the river in the 1990s in areas they reported are now tricky to cross with a snowmachine.
The ease of Copper River crossings facilitated trapping, firewood harvesting, and family visits to settlements on the east side of the river. Those settlements are no longer occupied. According to Miller, the decline in settlement on the east side of the river is not related specifically to ice conditions.
Respondents interviewed by Miller reported trapping less on the east side of the Copper River than they used to, and at least one person said he had given up trapping altogether. Some of this can be attributed to later freeze up and earlier break up, or generally less reliable river crossings.
Unreliable snow and an increase in shrub cover were cited as environmental factors that discouraged trapping as well. However, Miller emphasized changes in lifestyle are also important factors in the decline of trapping in that specific area.
Brown looked at two types of satellite images to learn more about the ice conditions on the Copper River: Landsat and synthetic aperture radar.
Landsat images, similar to visual photographs of the earth going back to 1973, help to investigate change over time. The record has holes in it due to the images’ vulnerability to clouds, darkness, and other factors, but it is still a valuable tool.
Synthetic aperture radar uses microwaves instead of visual light to create an image based on the surfaces the microwaves hit. The images are more resilient to changes in light or weather, but more difficult to interpret.
An example of one of many images of the Copper River showing relative ice extent. This one is from February 1, 2019. Image created by Dana Brown using synthetic aperture radar (SAR). Images courtesy of the European Space Agency. NASA’s home for SAR data and imagery is the Alaska Satellite Facility Distributed Active Archive Center (ASF DAAC).
Brown identified open water leads in hundreds of Landsat images. She then used that to quantify the extent of ice on the Copper River over time. There was quite a bit of variability between years. For example, the river did not completely freeze even back in 1976 or 1983.
Despite the interannual variation, you would be more likely to be standing on a mostly frozen Copper River during the period from 1973-1997 than you would between 1998 and the present. These changes were found from fall through spring, and suggest that freeze-up is occurring later, is less complete, and that break-up is occurring earlier.
Brown used the synthetic aperture radar images to create detailed images of the Upper Copper and Chitina Rivers for the winters of 2018, 2019, and 2020. These images show some areas where someone might be more likely to find river crossings, and areas where there is more likely to be substantial open water.
What Brown found in her image analysis is similar to what Miller heard in his surveys: The area from Gulkana to Willow Creek often has a lot of open water. Miller noted there are also quite a few villages and other settlements along that stretch of river.
The area north of Gulkana is more reliably frozen than it is to the south. Right below Chitina, the ice conditions were inconsistent from year to year. This could create a dangerous situation for crossing.
Brown looked at environmental and hydrological data to examine what is most likely to be driving the variability in ice conditions. There are a lot of different ways to look at winter weather, including taking averages of temperatures.
The weather pattern that best predicted the ice extent on the Copper River from year to year was the “accumulated freezing degree days” recorded in Gulkana. The accumulated freezing degree days for a given winter is a relatively simple calculation based on how much the average daily temperature is above or below zero.
A breakdown of the percentage of the number of “high ice extent,” meaning less than 25% of the river length containing open water, observations on the Copper River over two time periods. The y-axis shows the calendar dates for the observations.
Brown also said that the best predictor of where the ice was most likely to be inconsistent on the Copper and Chitina River was the energy of the river on a given stretch. The energy of a portion of the river can be estimated from the slope, flow accumulation, and channel width.
It is too early to say whether these studies will lead to changes in management.
For a wealth of information regarding snow and ice conditions across Alaska, including a variety of cameras and ground measurements from the Copper Basin, visit http://fresheyesonice.org/. Copper River School District students, and Upstream Learning students in particular, have actively participated in collecting local data.
The National Park Service, National Science Foundation, and NASA were contributors to this research.
Disclosure: The National Park Service is a Copper River Record advertiser.