Final answer:
The Dawes Act most directly led to the Suquamish/Port Madison reservation having a "checkerboard" appearance.
Step-by-step explanation:
The legislation that most directly led to the Suquamish/Port Madison reservation having a "checkerboard" appearance is the Dawes Act. The Dawes Act of 1887 reversed the policy of isolating Native Americans on reservations and instead forced them onto individual properties that were intermingled with White settlers. This resulted in the reservation having a "checkerboard" appearance, with Native American-owned plots of land scattered among non-Native-owned plots of land.