In Winters v. United States (1908), the Supreme
Court held that the right to use waters flowing through
or adjacent to the Fort Belknap Indian Reservation
was reserved to American Indians by the treaty
(5) establishing the reservation. Although this treaty did
not mention water rights, the Court ruled that the
federal government, when it created the reservation
intended to deal fairly with American Indians by
reserving for them the waters without which their
(10) lands would have been useless.
This is the part of RC question from OG 13
edition. My concern is what does the " when it created the reservation" subordinate clause plays here? i mean what is the role of this clause, is it modifying something? if so can a subordinate clause modify a noun?