The Ashnola River is a tributary of the Similkameen River, rising in the northeastern part of the North Cascades in Washington, United States, and flowing north into British Columbia, Canada, to join the Similkameen River about halfway along that river's course between the towns of Princeton and Keremeos. The river crosses the international boundary at 49°00′00″N 120°19′37″W / 49.00000°N 120.32694°W and transits Cathedral Provincial Park. It has one main tributary, Ewart Creek, which is about 25 kilometres (16 mi) long and begins virtually at the border and is entirely within Cathedral Park.