The contractor used a loader to remove sediment from the dammed side of the island, switching to hand excavation “within 10 feet of the dam to prevent damage by mechanized equipment.”
One sign of the dredging of Old Mission Dam in Mission Trails Regional Park is an approximately 435-cubic-yard mound of sediment in a Santee parking lot. Dredging started February 17, and the City of San Diego’s $324,121 contract calls for Miramar General Engineering to remove 870 cubic yards of sediment, according to a March 1 email from Chris Zirkle, deputy director of the San Diego Park and Recreation Department’s Open Space Division.
Sediment removed from the 200-year-old San Diego River dam is drying in a dirt lot on the corner of Father Junípero Serra Trail and Bushy Hill Drive. The lot is across the street from the Kumeyaay Lake entrance; the dam is about 0.6 mile to the south. Once the sediment is dried, “it is hauled off-site to an approved disposal location of the contractor’s choice,” Zirkle wrote.
He provided the 2002 mitigated negative declaration for the dredging, a document Zirkle said had not been updated. The document described a process that includes construction of a temporary dam and some dredging by hand.
The dredging project area encompasses approximately 2.97 acres upstream and adjacent to the dam, and the 0.56-acre parking lot. Zirkle said the dam was last dredged in 2007–2008, and 750 cubic yards of sediment were removed.
He confirmed something I heard about the lengthy permit process. The current project involved establishing “the off-site mitigation required by state and federal resource agencies for the last and subsequent dredging. Then there were delays in obtaining an Army Corps of Engineers permit for the current dredging due to differences of opinion between the Corps and the U.S. Fish and Wildlife Service concerning the need for the City to post an endowment to ensure maintenance of the mitigation site.”
Asked about the need for dredging, Zirkle wrote, "As a significant historic resource, it needs to be protected. Dredging protects the dam because built-up sediment puts more pressure on the dam than does water."
The dam, built between 1813 and 1816, is on the National Register of Historic Places and the California Register of Historical Landmarks lists. Additional information about the dam is on the National Park Service website.
In 1769, Serra founded Mission San Diego de Alcalá on Presidio Hill. Five years later, the mission was moved to its present location. The website for Mission San Diego (a Catholic parish) also describes the dam.
Both sites refer to a drought lasting from 1800 until 1802. While the mission history said Diegueño Indians worked on dam construction in 1803, the park service said members of the Kumeyaay Nation built the aqueduct and dam structures that year.
The dam built about five miles east of the mission was about 224 feet long, 13 feet wide, and 12 feet high, according to the parish website. The aqueduct at the dam “consisted of tiles, resting on cobblestones in cement, and carried by gravity flow a stream one foot deep and two feet wide to mission lands. It was built through the north side of a dangerously steep gorge, impassable on horseback."
According to the mitigation document, the dam, “also referred to as Padre Dam,” has two low-flow tiers with a control elevation of 276 feet.
Zirkle said once mitigation measures “were in place for biological flagging, cultural resources monitoring, and storm water pollution reduction measures, the contractor installed a dam” about 250 feet “upstream and on one side of the island.”
The contractor used a loader to remove sediment from the dammed side of the island, switching to hand excavation “within 10 feet of the dam to prevent damage by mechanized equipment.” Sediment was removed to reach “a prescribed bottom elevation,” and then the “dam is moved to the other side of the island, and the process repeats.”
Asked how often the dam is dredged, Zirkle wrote, “Dredging frequency depends on sediment accumulation and obtaining permits to do the work.”