All the missions were located near a source of fresh water. They needed water to drink, to cook, to do the laundry, to bathe etc. Some of the missions were located near major rivers. Santa Clara was located on the banks of the Guadalupe River, for example.
In many cases the source of water was a stream or creek. The padres dammed the stream, then built an elaborate water system, typically with reservoirs to hold the water and an aqueduct to carry the water to the mission, which was usually a few miles away.
Mission San Diego built what was called the Padre’s dam, six miles above the mission and remnants of the original dam are still visible. Mission Santa Barbara damned a creek two miles above the mission and built a stone aqueduct to carry the water, parts of which are still standing. At some missions like La Purisima springs were a major source of water.
Sometimes an expanding city covered an original water source. San Francisco de Asis was located near a stream named Dolores, which ultimately became a street and gave the mission its nickname, Mission Dolores.
Mission San Luis Obispo had a creek running close to the mission. As the city grew they filled in much of the land below the mission. Later a wonderful park that is the vibrant center of the town was built adjacent to the mission. The city created steps to the now sunken stream and this is a great vantage point to view the mission. See the photo below:
Stream Adjacent to San Luis Obispo Mission