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Location Canada, Northwest Territories
Central coordinates 119o 45.12' West  74o 0.06' North
IBA criteria A4i, A4iii
Area 68,000 ha
Year of IBA assessment 2008

Bird Studies Canada/Nature Canada

Site description The Thomsen River is located in north-central Banks Island, in the western Arctic. The site covers the river from its mouth at Castel Bay to a point about 60 km upstream. The river flows through a broad valley surrounded by low rolling hills. These hills are dissected by gullies and underlain by sandstones and shales. The river eventually empties into Castel Bay and is surrounded here by deeply cut badland topography. Polar desert and arctic tundra communities dominate the vegetation, with the main types being dwarf shrub, willow-sedge and lichen-legume tundra. Extensive wet sedge meadows are found in the valley bottom of the river. A polynya exists in MClure Strait and open water can appear as early as January or February, that then disappears by July. Muskoxen are commonly found year-round, while Peary Caribou migrate north in late spring, moving to calving grounds in the northeast and northwest parts of the island. Arctic Foxes are common and Polar Bears are occasionally spotted on the northern coast.

Key Biodiversity Together, the Thomsen River and Castel Bay have hosted up to 5,000 moulting Black Brant (subspecies nigricans) during late summer. This number exceeds 3% of the population of this subspecies, and is also over 1% of the North American Brant population.

Also during the late summer, up to 25,000 Lesser Snow Geese have been reported moulting in the Thomsen River valley. However, it is unknown how consistently the geese use this area for moulting year to year. Surveys in the mid-1970s recorded a maximum of only 1,200 birds in early August, while later reports in 1988 and 1989 found that numbers varied from several hundred to none. Clearly, more surveys are needed to elucidate this situation. Two hundred and fifty Canada Geese have also been recorded moulting on the river.

Populations of IBA trigger species

Species Season Period Population estimate Quality of estimate IBA Criteria IUCN Category
Brent Goose Branta bernicla non-breeding  1975  5,000 individuals  A4i  Least Concern 
A4iii Species group - waterbirds non-breeding  1965  20,000-49,999 individuals  unknown  A4iii   

Protected areas

Protected area Designation Area (ha) Relationship with IBA Overlap with IBA (ha)  
Aulavik National Park 1,203,368 protected area overlaps with site 68,000  
Banks Island No.2 Migratory Bird Sanctuary 17,000 protected area overlaps with site 68,000  


IUCN habitat Habitat detail Extent (% of site)
Forest Boreal deciduous woods  -
Shrubland Scrub  -
Grassland Edaphic grassland; Tundra  -
Wetlands (inland) Rivers  -
Sea Open sea  -

Land use

Land-use Extent (% of site)
not utilised major

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Recommended citation  BirdLife International (2016) Important Bird and Biodiversity Area factsheet: Thomsen River. Downloaded from on 27/10/2016

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