Oso Flaco Creek drains a watershed of approximately 7,400 acres (2,995 ha) consisting mostly of agricultural land before forming Oso Flaco Lake, now part of the Oso Flaco Lake Natural Area in the Guadalupe-Nipomo Dunes, and then flows into the Pacific Ocean, and about 68 miles (109 km) northwest of Santa Barbara and 5 miles (8 km) south of Oceano, California. Guadalupe-Nipomo Dunes extends about 18 miles (29 km) along the coast of central California from southern San Luis Obispo County to northern Santa Barbara County. Coastal dunes are aeolian landforms that develop in coastal conditions where an ample supply of loose, sand-sized sediment is available to be transported inland by ambient winds. Active coastal dunes are inherently dynamic systems constantly changing in response to sand supply, wave energy, and wind velocity. Sand is deposited on the beach through wave action and blown inland by the wind. Reduced wind speeds behind objects on the beach such as plants or beach debris allow sand to collect and eventually form incipient dunes. If the nascent dunes are not removed by high tides or storm surges, the sand will continue to collect and eventually form larger foredunes. Given sufficient sand supply and space to move, foredunes will develop into a larger, more complex system of dune ridges, dune hollows or swales, deflation plains, and stabilized backdunes. Active dunes along the immediate coastline are generally sparsely vegetated and not stabilized. As dunes move inland, they generally develop a more substantial plant community that stabilizes the dunes. The majority of the sediment that formed the Guadalupe-Nipomo Dunes was carried to the ocean by the Santa Maria River about 4 miles (6.4 km) to the south and Arroyo Grande Creek about 5 miles (8 km) to the north. The dune system began building about 18,000 years ago and is now the highest dunes on the western U.S. coastline. The Chumash Native Americans were inhabitants of the Guadalupe-Nipomo Dunes for thousands of years. The prehistorical occupation of the central California coast is characterized by several cultural periods based on the archaeological record. The Paleo-Indian Period is considered older than 10,000 years ago and is the least-known human presence on the central coast. These people used large fluted projectile points for hunting big game. Between 10,000-5,500 years ago, the Millingstone Culture is represented by people that used well-made millstones and hand stones. Animal remains at Millingstone sites indicate a diverse diet, including mammals, birds, fish, and shellfish. About 5,500-2600 years ago, the Early Transition culture is represented by portable mortars and pestles and sites begin to diversify based on the use of particular resources. An increase in obsidian and exotic shell beads implies an expansion in interregional trade. About 2,600-1,000 years ago, the Middle Transition period used new fishing technology such as circular shell fishhooks, nets, and grooved stone net sinkers. About 1,000-700 years ago, the Late Transition saw a rapid shift to the use of bow and arrows, and coastal sites are used less but the use of interior sites increased.

In 1769, the Don Gaspar de Portolà Expedition traveled along the coast and represents the earliest recorded Euro-American exploration in today’s County. Many coastal features were named by Portolá and his party of soldiers and missionaries. The group camped on Oso Flaco Creek, meaning ‘skinny bear’, named after a lean bear they killed in the area. After eating the bear, two of the explorers became sick, and it was later learned that the Chumash poisoned dangerous wildlife by feeding them tainted meat. In 1772, the Mission San Luis Obispo de Tolosa was established and Spain claimed the dunes and surrounding land as part of the mission’s land holdings. Spanish soldiers, missionaries, and settlers disrupted pre-occupation traditional land-use practices by introducing an agricultural economy with the native Chumash and Salinan tribes providing much of the labor. In 1821, Mexico gained independence and the new government secularized mission lands and granted large tracts of land to private citizens. Rancho Bolsa de Chamisal was a Mexican land grant of 14,335 acres (5,801 ha) given in 1837 by Governor Juan B. Alvarado to Francisco Quijada. The grant extended along the Pacific coast from present-day Oceano in the north to Rancho Guadalupe in the south. In 1846, the rancho was sold to Lewis Burton who served under John C. Frémont. In 1856, Burton sold Rancho Bolsa de Chamisal to Francis Z. Branch who sold part of the rancho to the Steele dairy dynasty in 1866. By the 1930s, oil companies began to acquire large tracts of land in the Guadalupe dunes for oil development, with oil production starting in earnest by 1948. The Union Oil Company, a major landholder in the region from the 1890s, built its refinery on the Nipomo Mesa in 1955. Over the next forty years, Unocal leaked 18 million gallons of petroleum under the dunes. In 1994, the company publicly recognized the spill and began cleaning up 2,700 acres (1,093 ha) of the Guadalupe oil field, one of the largest oil spills in U.S. history.

The Guadalupe-Nipomo Dunes are separated into uplands and wetlands habitats. Uplands include the foredunes, back dunes, and sandy beaches. The sandy beaches are a harsh environment where no plants are able to survive. The foredunes begin at the high tide line and extend parallel to the shore and support sparse vegetation. Parabolic dunes are U-shaped and are perpendicular to the shore with vegetated ridges. Sheets of constantly moving sand make it hard for vegetation to become established. Strong winds, salt spray, and massive amounts of sand make this area uninhabitable for plants other than low-growing plants with deep root systems. Plants in the dunes adapt growth forms to survive high winds. Some grow close to the ground to avoid the wind and others adopt a Krummholz growth form. The back dunes, just behind the foredunes, are stabilized and covered with plants. The back dunes are dominated by shrub species like mock heather, dune lupine, coastal buckwheat, and dune ragwort. The wetlands include the areas that contain water such as salt marshes, fresh and brackish-water marshes, swamps, and mudflats. Plants that live there are adapted to dynamic environmental conditions including high salinity concentration and extreme temperatures. Solutions that the plants can take to adapt to high salinity include developing large tap roots to reach the perched freshwater table, thick cuticles to prevent water loss, and succulence. Oso Flaco Creek is impacted by pesticides and nutrients associated with agricultural drain water. The California Office of Environmental Health Hazard Assessment has issued an advisory for any fish caught in Oso Flaco Lake due to elevated levels of pollutants including DDT, dieldrin, and mercury. To protect the dune environment, much of the area has been set aside for conservation and has been recognized as a National Natural Landmark. In 2013, the adjacent marine habitat was nominated for a national marine sanctuary by the Northern Chumash Tribal Council, and the National Oceanic and Atmospheric Administration has placed the site on a list for future consideration. Read more here and here. Explore more of Oso Flaco Creek and the Guadalupe-Nipomo Dunes here: