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Oso Flaco Lake Watershed

Oso Flaco Creek drains approximately x square miles of agricultural land and connects with Oso Flaco Lake and the ocean.

Featured Project

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Oso Flaco Lake and Little Oso Flaco Lake Non-Point Source Pollution Assessment

Oso Flaco and Little Oso Flaco Lakes are located in the western portion of the Santa Maria Valley in southern San Luis Obispo County, California. The Oso Flaco Creek watershed contains about 7,400 acres, nearly all of which consist of prime agricultural land. On average, 2½ crops per year are produced on these intensively cultivated fields. The primary crops are strawberries, lettuce, broccoli, cabbage, celery, cauliflower. The average annual rainfall is about 12 inches, most of which occurs from early November through mid-April. Groundwater is pumped for irrigation and applied to row crops through combinations of furrow, overhead sprinklers and drip irrigation.

The western terminus of the watershed is Oso Flaco Lake, the largest of the freshwater lakes associated with the 18-mile-long Guadalupe-Nipomo Dune Complex. These freshwater lakes occupy a surface area of 82 acres and are classified as palustrine emergent wetlands. The lakes and surrounding area are owned and managed by the California Department of Parks and Recreation, Oceano Dunes District. This area provides habitat to several threatened or endangered animals and California State species of concerns including the horned lark, least bell’s vireo, California red-legged frog, the western pond turtle, the California least tern, Gambell’s watercress, marsh sandwort, and beach spectacle pod.

Water quality in the Oso Flaco watershed has been found by the RWQCB to be impaired by several pollutants, including pesticides, nitrate and excessive sediment.  In order to better inform management measures for Oso Flaco area, State Parks contracted with the Coastal San Luis RCD to conduct water quality monitoring and streamflow measurements in Oso Flaco. 

The CSLRCD conducted monitoring from 2010 to 2012. Monitoring included collecting data from continuous sampling devices installed in Oso Flaco Creek and Oso Flaco Lake, and in addition collecting grab samples on a regular basis for laboratory analysis.In October 2012, results from the study showed high sediment and nitrates loads in Oso Flaco Creek. The study recommended biofiltration methods appropriate for farming activities and soil types. Filter strips, vegetated waterways, compost filters and vegetated retention ponds/basins are a few possible practices recommended to address specific issues. Additionally, further study is needed to understand the natural water treatment patterns discovered that created reduced nutrient and sediment loads between two monitor stations (one upstream, other in between Little Oso Flaco & Oso Flaco Lakes). A two to three year survey is also proposed, focusing on fish habitat, following habitat quality in conjunction with seasonal changes. 


Mills Family Honored for Conservation Efforts in 2015

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Mike Mills (front center) and the Mills family were presented with the Conservationist of the Year Award from the Coastal San Luis Resource Conservation District (RCD) for making outstanding conservation-related improvements to their agricultural operations. The presentation took place Tuesday, February 3, 2015, at the Board of Supervisors' chambers.

The Mills family grows broccoli near Oso Flaco Lake in an environmentally sensitive area between Nipomo and Guadalupe that is home to several threatened species. Going to great efforts to improve their agricultural operations, the Mills recently installed an efficient drip irrigation system and vegetated buffer strips to reduce runoff and improve the water quality. Other improvements include vegetable treatment ditches, sediment basins and a denitrifying bioreactor to improve water quality in Oso Flaco Lake.



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