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The San Luis Rey River Basin is located in San Diego County, 38 miles north of San Diego, CA and encompasses approximately 560 square miles (358,400 acres) of which 342 square miles (218, 880 acres) are located below Lake Henshaw Dam. Of the nine watersheds within the San Diego region, the San Luis Rey Basin is the third largest. The elevations in the watershed range from sea level in Oceanside to over 4,300 feet at the headwaters. A few of the tributaries’ headwater areas reach over 5,000 feet. Lake Henshaw Dam is located 50 miles from the ocean at approximately 2,700 feet in elevation. Principle tributaries located below the dam include: Paradise Creek, Pauma Creek, Frey Creek, Agua Tibia Creek, Gomez Creek, Keys Creek, Moosa Canyon Creek, and Pilgrim Creek. These tributaries flows have all been altered by anthropogenic uses.

The San Luis Rey River Basin has a moderate climate with average annual high and low temperatures ranging between 53°F and 69°F. Average annual precipitation ranges from slightly above 10 inches along the coastal region to 45 inches in the mountainous areas. A majority of the precipitation falls during the months from November to February with snow occurring in the higher elevations.

The San Luis Rey River is located in an area where anthropogenic actions have greatly altered the hydrology of the river. These anthropogenic influences include:
  • The building of the Henshaw Dam - limiting flows in the upper river to surfacing groundwater and precipitation for extended portions of the year;
  • The Escondido Canal Diversion Dam - located ten miles below the dam, which diverts practically all flows out of the river into the canal, usually leaving the river dry below the diversion;
  • Imported Colorado River water, which, in addition to supplying water for agricultural production, allowed groundwater aquifers to recharge from years of overpumping and thus returned perennial flows to the lower river in the late 1960s; and
  • Increased salt loads entering the groundwater from storm water and agricultural irrigation runoff.
The San Luis Rey Basin has many different land uses, but approximately half of the watershed remains undeveloped or vacant. The majority of vacant land occurs in the middle to upper watershed as land use and development generally increases as one moves west through the watershed. The upper and middle portions of the watershed are mostly composed of small communities and seven Native American Tribes. Land uses in the upper watershed consist primarily of grazing, while citrus crops and ornamental nurseries comprise a majority of the agricultural production in the middle basin. Diverse agricultural crop production occurs throughout the lower watershed in conjunction with expanding residential housing and commercial development in the Cities of Oceanside, Fallbrook, and Vista. These land uses, along with urban runoff from developed areas, have introduced many pollutants into the river resulting in degraded water quality. According to the State Water Resources Control Board, the river shows high levels of chloride and total dissolved solids. Elevated levels of bacteria have also been observed at the mouth of the river, near the Pacific Ocean.

Historically, steelhead trout (Oncorhynchus mykiss), a salmonid species native to western North America and the Pacific coast of Asia, runs were present in the SLR River until the 1940’s. Prior runs were reportedly sufficient enough to provide a major food supply for the Luiseño Indians as late as the 1890’s and early 1900’s (USFWS 1998). The influx of changes that have occurred in the watershed has disrupted the natural hydrologic conditions in the watershed, thus nearly extirpating steelhead from this system.

Current efforts to restore the San Luis Rey River focus on the implementation of watershed management guidelines developed through the San Luis Rey Watershed Management Project and the Watershed Urban Runoff Management Plan (WURMP) for the San Luis Rey Watershed. The WURMP identifies tasks related to urban runoff that all jurisdictions in the SLR Watershed are committed to implementing in order to improve water quality. The San Luis Rey Watershed Council (2000) also identified twelve priority issues with consideration to long term planning within the watershed. Some of these issues are as follows: water quality and quantity, heavy industrial uses, invasive plant species management, flood plain management and flood plain warning, and wetlands protection and restoration (see Restoration Programs, pp. 73-74 for the entire list of issues). Furthermore, the Mission Resource Conservation District (RCD) has taken an active role in invasive plant species management along the SLR River and its tributaries and in conjunction with the Natural Resource Conservation Service (NRCS) has worked with local landowners on Best Management Practices (BMPs). Steelhead recovery in the SLR River is consistent with many of these and other ongoing activities intended to protect and/or restore ecosystem functions within the watershed.

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