Naples Creek rises in the foothills of the Santa Cruz Mountains at an approximate headwaters elevation of over 400 feet and runs southwest through agricultural land and urban developed areas in the lower reach to the Pacific Ocean. Regionally, both the surface water and the groundwater flow with the slope of the terrain, that is, from the northeast to the southwest. Groundwater in the lower reach has been found at 20 to 80 feet below the surface in unconsolidated Quaternary marine terrace deposits and in unconsolidated Quaternary alluvium. There are few groundwater barriers in the vicinity of the mouth of Naples Creek, a circumstance giving rise to local salt water intrusion. Chlorides exceeding 100 parts per million in groundwater were found in the early 1970s along this part of Half Moon Bay State Beach. Near the mouth of Naples Creek the banktop elevation varies between an elevation ranging from 30 to 40 feet above mean sea datum.