From the state plaque:
The City of Santa Barbara Mission Historical Park
Santa Barbara Mission was
founded December 4, 1786. Portions of five units of its
extensive waterworks, built by Indian labor and preserved
in this part, are a filter house, Spanish gristmill, sections
of aqueducts, and two reservoirs, the larger of which,
built in 1806, is used today as part of the city water
system. The fountain and lavadero are nearby, in front
of the mission, and a dam built in 1807 is located in
the Santa Barbara Botanic Garden, one and one-half miles
up Mission Canyon. Only ruins remain of the mission's
pottery kiln, guard house, and tanning vats.
Plaque placed by California State Park Commission April 21, 1957.