View File - Development Services - City of Oxnard

WATER RESOURCES TECHNICAL REPORTand increased pumping. Because these longer-term climatic patterns are not wellunderstood, the prediction of future groundwater levels also results in uncertainty.2.4 Surface WaterSurface water in the Santa Clara-Calleguas Hydrologic Unit consists of stream flow andremnant wetlands from the extensive estuarine wetlands that existed on the Oxnard Plainin the mid-1800s before development. Runoff from precipitation in the upland areas is thepredominant source of natural stream flow. Discharge of treated wastewater effluent,which began in the late 1930s, provides an additional source of water to stream flow.Stream flow has historically been diverted for agriculture for enhanced artificial rechargeof the groundwater system. Surface water has been imported from outside theSanta Clara-Calleguas Unit since the 1950s as a source of potable surface water toVentura County and the Oxnard Plain.2.4.1 Stream FlowAs described above, the Santa Clara River and Calleguas Creek are the main watershedsthat drain Ventura County (Figures 2-1 and 2-2). The Santa Clara River drains the northernand eastern portions of the Santa Clara-Calleguas Hydrologic Unit, and Calleguas Creekdrains and southern portions of this unit. The Santa Clara River, which drains theheadwaters of the mountainous areas of northern Ventura County and northwesternLos Angeles County, is the largest river system in Southern California that remains in arelatively natural state. Its major tributaries are Piru, Hopper, Pole, Sespe, Santa Paula, andEllsworth Creeks. Calleguas Creek drains a predominantly agricultural area on the OxnardPlain and empties into Mugu Lagoon, one of the Southern California few remaining largewetlands of Southern California. Its major tributaries are Conejo Creek and ArroyoSimi/Arroyo Las Posas. Revolon Slough is a smaller watershed that drains coastal areas onthe Oxnard Plain between the Santa Clara and Calleguas Creek watersheds. Its majortributaries are Arroyo Hondo and Beardsley Wash.Hydrographs for the Santa Clara River, Arroyo Simi, and Conejo Creek are provided inFigure 2-6 at the following locations:• Santa Clara River at County Line and Santa Clara River near Piru• Arroyo Simi near Simi and Arroyo Simi at Madera Road Bridge• Conejo Creek above Highway 101Stream flow gauging stations were first established on Arroyo Simi in 1934 and on ConejoCreek in the 1970s. Continuous gauging of stream flow at downstream sites began atMontalvo on the Santa Clara River in 1955, on Calleguas Creek above U.S. Highway 101 in1971, and at Camarillo in 1968. Most natural stream flow occurs from December throughApril in the winter and spring as flood flow. Construction of reservoirs and discharge ofshallow groundwater (for dewatering) and treated wastewater effluent have contributed toregulated flow and modification of river systems in the basin. The Santa Clara River, PiruCreek, Arroyo Simi, and Conejo Creek all have components of regulated flow, whichincrease the mean flow and decrease the number of days with no flow.W112003002SCO LW1458.DOC/ 033390002 10