 The Santiago Creek watershed occupies much of the northwestern end of the Santa Ana Mountains, and is located generally north of the city of Irvine.It is bounded on the south by the San Diego Creek, Aliso Creek and Oso Creek drainage areas, on the southeast by the Trabuco Creek watershed, and on the north and west by tributaries of the Santa Ana River.At in size, the Santiago Creek watershed makes up about 3.6% of the entire Santa Ana River watershed - but makes up about 65.7% of the of Santa Ana River watershed within Orange County, and 10.6% of Orange County.