Most of The Night Watchman takes place at Turtle Mountain Reservation in North Dakota, the homeland of the Turtle Mountain Chippewa People. The reservation is rich with history, ancestry, and significance; and it's full of natural beauty and animals, though less than in the past. It's also a place full of economic hardship, powerful community bonds, and the pain of loss. The complicated history of Turtle Mountain and the Chippewa people is a major factor in the book, both in its influence on characters and its relevance to the conflict over the Termination Bill. 

Thomas articulates the spiritual importance of Turtle Mountain in one of the last passages of the novel:

The bones tipped and staggered, assembling into forms, and took on shaggy flesh. The grass rippled and billowed like a green robe and the animals crossed vast and vaster numbers. The earth trembled in a serpentine rush, blew away, and vanished into the sky.

Here, Thomas envisions the landscape covered with bison, as it was centuries ago. Though Turtle Mountain has undergone a difficult history of violence, colonization, legal changes, and identity collisions, the presence of the past is still important to the Chippewa who live there.

Some of the book, though, takes place in Minneapolis, which acts as a foil for Turtle Mountain. The city is threatening, violent, and dangerous, particularly for Native women like Vera, who are often exploited and sexually abused in horrific ways. The contrast with Turtle Mountain is stark, as demonstrated when Patrice goes looking for Vera:

2214 Bloomington Avenue was a brown three-story house, peeling white paint, broken windows blanked out with cardboard. An assembly of mailboxes hung by the front door. Next to the mailboxes, what looked like a list of inhabitants. Patrice paced questingly in the dead yard. [...] Patrice had the sudden sense that the house had warned her. She shook off the feeling, knocked on the window next to the door. Thought she heard a scuffling sound inside. A dog started barking. Its bark was rough, high, desperate to live. She froze. Tears started into her eyes.

The "dead yard," the sense of being "warned," and the "dog... desperate to live" all differ greatly from Turtle Mountain. Minneapolis is cruel and abusive toward nature and people alike. Though Patrice is able to navigate the city surprisingly well, she leaves it as quickly as possible to return home. 

The contrast between Turtle Mountain and Minneapolis highlights the violence of collisions between cultures, aligns with the contrast between the present and the past, and also makes clear the stakes of the events in the final setting of the book: Washington, D.C. Patrice, Thomas, and a few other Chippewa go to Washington briefly to argue against the Termination Bill. Though Washington is not as severe or horrific as Minneapolis, the power it represents is enormous, and the toll the trip takes on Thomas is heavy. 

In the end, both Minneapolis and Washington, D.C. contrast with Turtle Mountain in ways that reveal the enormous importance to the Chippewa people of the nature and history of their homeland. This use of setting helps transform the conflict over the Termination Bill from a legal dispute into an existential matter.