The Arroyo Simi (Spanish for “Small Stream of Simi”, sometimes also referred to as Simi Creek) is a 19-mile-long westwards-running creek, running from the city of Simi Valley and crosses the valley from east to west, before entering the city of Moorpark. It originates at Corriganville Park by the Santa Susana Pass, and streams westwards into Moorpark where it merges with Arroyo Las Posas by Hitch Road. It extends for twelve miles through the Simi Valley, and leaves the city limits by Oak Park at the western end Simi Valley, and continues for seven miles in Moorpark. It is a tributary to the Calleguas Creek, which enters the Pacific Ocean by its estuary at Mugu Lagoon by Naval Air Station Point Mugu. Besides an arroyo, it has been described as a channel, waterway, river, drain, wash, and stream. Arroyo Simi drains an area of 343 square miles in southern Ventura County. In its natural state, it is an ephemeral creek, which is only seasonally filled during winter time and periods of heavy rain. Today it is for the most part a concrete lined water drain that flows year round. Tributaries to the Arroyo Simi includes Aaamos Canyon-, Sycamore-, Dry Canyon-, Tapo Canyon-, Las Llajas Canyon-, White Oak-, Runkle Canyon-, and Bus Canyon Creeks, as well as the Erringer Road- and North Simi Drains.