The SESTRA Project focuses on two sub-Arctic river basins, Kuskokwim and Selenge, located in remote parts of Alaska and Mongolia. Both river basins represent land cover and soil conditions varying from seasonally frozen ground to continuous permafrost and have experienced significant increases in near-surface air temperature over the last 40 years. However, hydroclimatic conditions in these river basins are very different ranging from a very dry climate and scarcity of water resources in the Selenge watershed (with an annual discharge/runoff of ~ 10 km3/36 mm) to an abundance of moisture and high availability of water resources in Kuskokwim river basin (with annual discharge/runoff ~ 37 km3/450 mm). Correspondingly, the observed hydrological changes and their impacts on riverine communities in these basins are drastically different.

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Alaska

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The Kuskokwim River basin encompasses about 130,000 km2 and transitions from mountainous headwaters to lowlands with innumerable lakes and wetlands. The river provides an important transportation route for various types of watercraft, and as an ice road. Most of the basin area is underlain by discontinuous permafrost and snowmelt is a primary source of river runoff. Climatologically, the Kuskokwim basin to a large extent is influenced by the processes in the Bering Sea, which has been losing sea ice, leading to dramatic environmental changes. Significant increases in air temperature (2.5C) and annual precipitation (70 mm) have been observed during the last 50 years in the basin indicating a transition to a more moderate, wetter climate regime. The change in magnitude and pattern of precipitation along with earlier, more intensive snowmelt and permafrost thaw led to a significant increase in spring streamflow (28% since 1951) with a higher risks of catastrophic floods and bank erosion over 30 m/year in some settlements of low Kuskokwim. An increase in water temperature  and a change in discharge seasonality have negative impacts on salmon habitat and the local traditional economy.

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Bethel, a city with a 2021 population of 6,362, plays a crucial role as the regional transportation hub and serves as a government, service, and healthcare center for smaller communities in the Yukon-Kuskokwim region, such as Tuluksak and Akiak. The Inuit villages of Tuluksak (population of 444) and Akiak (population of 462), and McGrath (population of below 300), with roughly half being of Athabaskan, Aleut, Yup’ik, or Iñupiaq descent, rely on subsistence harvesting, including  salmon fishing, hunting and gathering, as an integral part of their local cultures and economies. The village of McGrath is an important transportation and economic center for the upstream Kuskokwim area providing airline, freight & shipping services, and snowplowing. Among the top wage economy occupations in McGrath are public administration, educational services, transportation, and warehousing. Subsistence harvesting remains an integral part of the local culture and economy. McGrath is located on the route of the Iditarod Trail Sled Dog Race and, as such, is illustrative of challenges to traditional transportation due to social, cultural, and environmental changes.

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Mongolia

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Selenge is a transboundary tributary in the Yenisei drainage basin and the largest river draining directly into Lake Baikal. The Mongolian portion of the basin covers 281,000 km2 which corresponds to approximately 20% of the area of Mongolia, yet contributes 52% of the country’s total surface runoff. The Selenge watershed is located in the zone of extreme continental, sub-arctic climate with 41% of the territory underlain by continuous permafrost. The region has experienced a mean annual air temperature increase of more than 2C since 1970, which led to a significant increase in the active layer thickness. There have been no significant changes in total annual precipitation since 1970, however, the number of storm events has almost doubled. These trends are indicative of enhancing climatic continentality which is confirmed by changes in weather patterns such as longer and hotter summers, fewer light precipitation events, and more frequent and intense storms and flooding events. The Selenge basin is highly susceptible to meteorological and hydrological droughts. Continued climate change is likely to significantly increase the probability of dzuds, droughts, flash floods, and other climate-induced hazards in the Selenge basin.

Economically, the Selenge river basin is of great importance in Mongolia as it is home to over 80% of the population, including the country’s capital city of Ulaanbaatar. It also contains most of the country’s cultivated land (~ 2.8% of the basin area) and pastures.Tourism and extractive industrial development also play an important role in the basin’s economy. Although the river basin has considerable restrictions for economic development related to environmental protection, water withdrawals for domestic, agricultural, and industrial needs significantly impact the hydrological regime of the river, partially contributing to the observed decline in stream discharge since 1970. Future development of the basin is uncertain due to a planned large water diversion and two hydropower generation projects, which can seriously impact Lake Baikal, the world's largest freshwater lake.

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Sükhbaatar, with a population of 22,741 people, is the largest city in the lower Selenge basin. It serves as a regional hub connecting rural areas with Ulaanbaatar via railroad and paved automobile road. Its residents are first-hand witnesses of change in the river.

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Bayanzürkh with a population of ~ 4,200 people is home to the Darkhad People. While not considered “Indigenous” by  UN definition, Darkhad is a distinct tribe that maintains traditional subsistence-based nomadic herding practices in the river valleys and forested hills and poses unique culture and knowledge. They rely on cattle, sheep, camel, and yak for food, clothing, and transportation. Darkhad's cash economy is based on animal products (e.g., wool and meat) and subsistence gathering of pine nuts and berries. The mountains and forests around the village belong to the Ulaan Taiga Protected Area causing tensions between subsistence-dependent Darkhad and authorities.

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