The Yakama Indian Reservation is a United States Indian reservation located on the east side of the Cascade Mountains of the state of Washington. According to the United States Census Bureau the size of the reservation is 2,187.56 mi² (5,666 km²) and the population in 2000 was 31,799. It was created in 1855 by a treaty signed by Washington Territory Gov. Isaac Stevens and represenatives of the Yakama tribe. A dispute over the treaty led to the Yakima war of 1855 .