This term is a tautological formation which does not refer to an Indian of any specific tribe.
1889  . . . a Negro, a Chinaman, and a Siwash Indian prepared dinner on the steamer for a company representing many countries, provinces and States.
1926  The Mounted Police handled the Government stores and to facilitate their transport a large number of Siwash Indians were employed.
1934  The Nisqually and other Puget Sound Siwash Indians with whom trade was established were friendly and helpful.
1957  The water-trail ended at a camp of Siwash Indians at the northern end of the lake.