Pilarcitos Creek (Spanish for: Little Pillars or Pillaries Creek) is a 13.5-mile-long (21.7 km)[3] coastal stream in San Mateo County, California, United States, that rises on the western slopes of the Santa Cruz Mountains and descends through Pilarcitos Canyon[4] to discharge into the Pacific Ocean Half Moon Bay State Beach.

It is a source of clean drinking water for residents of the central coast and San Francisco Bay Area and supports several natural-resource based economies – including agriculture and recreational tourism.

The name Arroyo de los Pilarcitos meaning "creek of the little pillar-like rocks" was recorded in land grant papers as early as 1836.

[1] In recent history, public and private stakeholders in the watershed have been working together intermittently since 1992, when two separate pollution discharge events occurred.

The Pilarcitos Creek watershed is host to a number of plant and animal species, including anadromous steelhead trout (Oncorhynchus mykiss) which are listed as "threatened" by the Federal government.

Over recent history, physical and biological impacts resulting from human activity have degraded the overall watershed condition, threatening native plant and animal species including steelhead.

The key watershed management issues include: -Instream flows to support aquatic resources during critical summer and fall periods.

-Other ecosystem factors, including fish passage, instream habitat, watershed erosion, channel maintenance, riparian vegetation, exotic invasive species, and the spring estuary/summer lagoon.

[11] The Pilarcitos Creek watershed holds a considerable biodiversity and is confirmed habitat for the endangered San Francisco garter snake.

[12] arroyo willow (Salix lasiolepis) and red alder (Alnus rubra) are the dominant riparian trees in the lower watershed, with coast Douglas fir (Pseudotsuga menziesii var.

The purpose of the IWMP is to promote balanced solutions to effectively manage the Pilarcitos Creek watershed that satisfy environmental, public health, domestic water supply, and economic interests.