Advancing Earth System Science

Authors: Brown et al., 2023

DOI: https://doi.org/10.1080/15230430.2023.2241279

Brown et al., explored long term changes in river ice formation in southcentral Alaska. For rural Alaskan communities, rivers serve as important transportation routes and provide communities with access to hunting grounds, fishing, harvesting, and access to other communities etc. Impacts from climate change are making these river crossings more vulnerable to rapidly changing ice conditions.

Study area map showing Copper River Basin in southcentral Alaska.

Figure reproduced from Brown, D. R. N., Arp, C. D., Brinkman, T. J., Cellarius, B. A., Engram, M., Miller, M. E., & Spellman, K. V. (2023). Long-term change and geospatial patterns of river ice cover and navigability in Southcentral Alaska detected with remote sensing. Arctic, Antarctic, and Alpine Research, 55(1). https://doi.org/10.1080/15230430.2023.2241279

The Copper River serves as an ideal case study to explore problematic trends in river ice characteristics that have significant impacts on local communities. Most inhabitants live on the western side of the river with traditional harvest lands on the eastern side of the river requiring travel across the river. With early spring break-up, mid-winter breakup leading to thinner ice and persistent open water, Alaska native people (Ahtna people – a northern Dene Athabaskan group) are seeing drastic changes in their way of life, which includes reduced access, safety concerns, fatalities, and changing access conditions to hunting areas. 

Understanding historical changes in river ice cover

Researchers used the historical archive of multispectral Landsat imagery for a retrospective analysis of river ice from 1973-2021. This data was compared with citizen science observations submitted through the GLOBE Observer app and the Fresh Eyes on Ice website. Photos from April 2021-May 2022 were used to understand what ice conditions can be inferred from visual interpretation of the satellite imagery. These photos also help document local river use under different ice conditions.  

452 observations from Landsat archive images were examined, with long time gaps in available images between 1986 to 1995 with almost no winter observations, which make it difficult to quantify what changes in ice were observed during this timeframe. This study also combined citizen science observations which provided ground truth observations of ice conditions such as ice thickness and freeze-up or break-up timing.

Researchers focused on examining the relative proportions and probability of high ice content and the frequency of incomplete freeze-up. They examined interannual variations in weekly ice extent to identify years without development of complete ice cover.

Using Sentinel-2 imagery the researchers assessed river reaches that were likely to develop a potentially navigable ice cover versus open water. Flow energy as a potential control over later-winter open water areas was also examined. Proposed that high energy would slow development of surface ice cover. 

Using Sentinel-1 SAR (Synthetic Aperture Radiation) the study looks at seasonal development of ice and open water occurrence. SAR provides a benefit in not being affected by low light conditions or cloud cover and is effective to explore these differing ice conditions regardless of the weather conditions.

Results

Figure 2 shows ice extent for sections of the Copper River, with collocated time lapsed camera and citizen science photo. 

Figure reproduced from Brown, D. R. N., Arp, C. D., Brinkman, T. J., Cellarius, B. A., Engram, M., Miller, M. E., & Spellman, K. V. (2023). Long-term change and geospatial patterns of river ice cover and navigability in Southcentral Alaska detected with remote sensing. Arctic, Antarctic, and Alpine Research, 55(1). https://doi.org/10.1080/15230430.2023.2241279

The research found substantial variation was observed in the timing, duration and presence of high ice extents. This was complemented with firsthand knowledge of local residents. Logistic regression of changes over time showed significant declines in probability of high ice extents throughout late winter.

Formation of ice cover conductive to travel has shifted to later in the winter. Nonexistent periods of widespread ice cover limits the ability of residents to cross the river near their communities. 

Although difficult to compare results across different areas, the quantitative and qualitative changes in ice cover on the Copper River appear to be particularly pronounced. Much of the patterns observed are impacted by patterns in stream flow energy and river morphology. Open water areas typically formed downstream of large tributaries (fast moving and mixing water inhibit ice formation)

Conclusion

Remote sensing approaches used to identify trends in freeze-up and open water occurrence are useful for local communities. The methodology employed can be applied elsewhere to support local decision making. 

Significance of GLOBE data

Images from the GLOBE observer app and the ‘Fresh Eyes on Ice Project’ provided a comparison with concurrent Landsat satellite imagery. Photos from April 2021-May 2022 were used to understand what ice conditions could be inferred from visually interpreting satellite images. Researchers noted that notes taken by citizen scientists helped to clarify the local river use with differing ice conditions.

Brown, D. R. N., Arp, C. D., Brinkman, T. J., Cellarius, B. A., Engram, M., Miller, M. E., & Spellman, K. V. (2023). Long-term change and geospatial patterns of river ice cover and navigability in Southcentral Alaska detected with remote sensing. Arctic, Antarctic, and Alpine Research, 55(1). https://doi.org/10.1080/15230430.2023.2241279