Old Mission Dam in San Carlos is a National Historic Landmark within San Diego's Mission Trails Regional Park, located at 1 Father Junipero Serra Trail in the 92119 ZIP. Kumeyaay laborers and Franciscan missionaries constructed the 220-foot-long, 13-foot-wide, 12-foot-high stone-and-cement dam between 1803 and 1816 to impound the San Diego River and deliver irrigation water through a six-mile flume to Mission San Diego de Alcala, making it the first major colonial-era irrigation project on the Pacific coast of the United States. The dam received National Historic Landmark designation in 1963, and an interpretive plaque at the site details the construction timeline, the Kumeyaay labor force, and the 1831 flood damage that led to the system's abandonment during Mexican secularization of the California missions. The Mission Gorge Road corridor that channels traffic to the dam runs through Grantville's commercial district, where Emiliano's Mexican Restaurant and Cantina serves post-hike plates and margaritas roughly three miles south on the same road. A wheelchair-accessible pathway leads from the parking lot to the dam's viewing area, where exposed bedrock and remnant masonry sit above the river channel amid a riparian corridor of coast live oaks and sycamores. The dam serves as a trailhead for hikes into Oak Canyon along the San Diego River, up to the Kwaay Paay Peak summit via the Mission Dam connector trail, and east toward the Kumeyaay Lake Campground along Father Junipero Serra Trail. Birders working the oak canopy and river understory log species including Nuttall's woodpecker, California towhee, and red-shouldered hawk, and the riparian zone supports one of the denser stands of native oaks within the park's 7,220-acre boundary. Parking at the dam is free but limited to roughly 30 spaces, and the lot is accessible via Father Junipero Serra Trail 0.7 miles east of the Mission Gorge Road turn. The site sits four miles northeast of Mission San Diego de Alcala on Mission Gorge Road, connecting the dam to the broader Spanish-colonial mission chain that stretches the length of California. Landscape and nature photographers in San Carlos, including Naomi Renée Films & Photography, use the dam's stone ruins and oak-canopy backdrop for editorial and engagement sessions. Seasonal access runs from 8 to 7 between April and October and from 8 to 5 between November and March, with closures on Thanksgiving, Christmas, and New Year's Day.