Final answer:
The article describes the Northern Cheyenne Reservation as an "island" in southeast Montana because it is geographically isolated from other communities, which contributes to the preservation of its unique cultural heritage.
Step-by-step explanation:
The Northern Cheyenne Reservation is described as "an island" in southeast Montana not because it is surrounded by water, but because it is geographically isolated from other communities. This comparison to an island underscores the profound separation and insularity of the reservation in terms of location and access to resources. The geographic isolation has contributed to the preservation of unique cultural practices and traditions among the Cheyenne people, much like how isolated natural islands can lead to the development of distinctive species and ecosystems.