Ahtanum Creek is a tributary of the Yakima River in the U.S. state of Washington. It starts at the confluence of the Middle and North Forks of Ahtanum Creek near Tampico, flows along the north base of Ahtanum Ridge, ends at the Yakima River near Union Gap and forms a portion of the northern boundary of the Yakama Indian Reservation. The name …Ahtanum Creek is a tributary of the Yakima River in the U.S. state of Washington. It starts at the confluence of the Middle and North Forks of Ahtanum Creek near Tampico, flows along the north base of Ahtanum Ridge, ends at the Yakima River near Union Gap and forms a portion of the northern boundary of the Yakama Indian Reservation. The name Ahtanum originates from the Sahaptin language, which was spoken by Native Americans in the region.