Original:  They are the indigenous people of the Wlastk River (Saint John River) valley and its tributaries , and their territory extends across the current borders of New Brunswick and Quebec in Canada, and parts of Maine in the United States.
Revised:  They are the indigenous people of the Wlastk River (Saint John River) valley and its tributaries . Their territory extends across the current borders of New Brunswick and Quebec in Canada, and parts of Maine in the United States.
Edit intent: coherence