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USGS Water-Year Summary 2024
11046000 Santa Margarita River at Ysidora, CA
LOCATION - Lat 33°18'40", long 117°20'47" referenced to North American Datum of 1927, in NW 1/4 NW 1/4 sec.18, T.10 S., R.4 W., San Diego County, CA, Hydrologic Unit 18070302, on Camp Joseph H. Pendleton Naval Reservation, at Basilone Road Bridge, 5.2 mi upstream from Ysidora, and 7.9 mi upstream from mouth.
DRAINAGE AREA - 723 mi².
REVISIONS HISTORY - WDR CA-87-1: Drainage area.
SURFACE-WATER RECORDS
PERIOD OF RECORD - February 1923 to February 1999, September 2001 to current year (see GAGE paragraph). Low-flow records not equivalent prior to Dec. 10, 1980, due to installation of conservation ponds above downstream site.
CHEMICAL DATA: Water years 1980-81.
WATER TEMPERATURE: Water years 1969-81.
SEDIMENT DATA: Water years 1969-78, 1982-83.
GAGE - Water-stage recorder. Auxiliary gage 2.3 mi upstream with crest-stage gage and steel drop structure (diversion dam). Primary gage temporarily out of operation from Feb. 26, 1999, to Sept. 27, 2001, due to channel work and replacement of Basilone Road Bridge. During this period, the auxiliary gage (station 11045050) was operated as a temporary replacement. Datum of gage is 70.40 ft above NAVD of 1988. February 1923 to Feb. 16, 1927, at site 4.4 mi downstream at different datum (destroyed by flood). Feb. 17, 1927, to Feb. 1, 1931, no gage in operation; records based on discharge measurements. Feb. 2, 1931, to Feb. 24, 1970, at site 5.4 mi downstream at different datum. Feb. 25, 1970, to Dec. 10, 1980, at site 6.2 mi downstream at different datum.
REMARKS - Flow partly regulated by Vail Lake (station 11042510) since November 1948 and by Skinner Reservoir since 1974. Flow in Warm Springs Creek, a tributary to Murrieta Creek, slightly regulated beginning in water year 1999 by Diamond Valley Lake, capacity, 800,000 acre-ft (see station 11042800). Diversions to O'Neill Lake and to ground-water recharge basins are made at point 2.3 mi upstream by Camp Pendleton personnel. Regulated return flows from O'Neill Lake can occur at times, as can unregulated spills. See schematic diagram of Santa Margarita River Basin available from the California Water Science Center.
EXTREMES FOR PERIOD OF RECORD - Maximum discharge, 44,000 ft³/s, estimated, based on regression equation and flood routing of upstream flows, Jan. 16, 1993, gage height, 20.47 ft; no flow for all or part of most years.
