Santiago Creek is a major watercourse in Orange County in the U.S. state of California. About long, it drains most of the northern Santa Ana Mountains and is a tributary to the Santa Ana River. It is one of the longest watercourses entirely within the county. The creek shares its name with Santiago Peak, at the highest point in Orange County, on whose slopes its headwaters rise. The Santiago Creek watershed covers about in northern Orange County. The upper part of the creek is free-flowing, while the lower section is urbanized and includes parts of the cities of Tustin, Orange, and Santa Ana. Below the Villa Park Dam the creek is mostly channelized and flows only during heavy winter storms. Historically the Santiago Creek provided water for the Tongva people, whose territory extended over much of northern present-day Orange County and into the Los Angeles Basin. Native Americans have inhabited the Santiago Creek and Santa Ana River watershed for up to 12,000 years. The creek was named by the Spanish Gaspar de Portolá expedition of 1769, which crossed the Santa Ana River near where it meets the Santiago Creek. In the 1870s there was a short-lived silver boom along the tributary Silverado Creek. In 1929 the Santiago Dam was built to form Irvine Lake, to supply irrigation water. Pipelines from Irvine Lake still contribute a small amount of water to the municipality of Villa Park. Course Santiago Creek rises in the Cleveland National Forest, between Santiago Peak and Modjeska Peak, which together form the prominent Saddleback of the Santa Ana Mountains. The creek runs south-southwest toward Portola Hills before turning northwest. Once out of the national forest it passes through the town of Modjeska and meets the first major tributary, Harding Canyon Creek, from the right. Downstream, it receives Baker and Silverado creeks, both from the right. Past the first Santiago Canyon Road crossing, the gorge widens to a broad alluvial plain, where the valley walls pull away and decrease in height. The creek's perennial surface flows are limited to this upper stretch; below here the water flows underground except during the wet season of winter and early spring. The creek then empties into Irvine Lake, which is also fed by Limestone Canyon, a left-bank tributary. The reservoir is formed by the Santiago Dam, located at its north end. Irvine Lake provides water to Villa Park and Orange via a