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, about 68 miles (109 km) northwest of Santa Barbara and 5 miles (8 km) south of Oceano, California. The Guadalupe-Nipomo Dunes stretch 18 miles (29 km) along central California’s coast, from southern San Luis Obispo County to northern Santa Barbara County. Coastal dunes form when abundant, loose sand is carried inland by prevailing winds. These dynamic systems change continuously in response to sand supply, wave energy and wind velocity. Waves deposit sand on the beach, which is then blown inland; reduced wind speeds behind vegetation allow sand to accumulate into nascent dunes. Absent the removal of high tides or storm surges, these dunes grow into foredunes that eventually develop into complex systems of ridges, hollows, deflation plains, and stabilized backdunes. Active dunes on the immediate coast are sparsely vegetated, while inland dunes support denser plant communities that help stabilize them. Most of the sediment that formed the dunes was delivered by the Santa Maria River, about 4 miles (6.4 km) south, and Arroyo Grande Creek, about 5 miles (8 km) north. The dunes began forming around 18,000 years ago and now rank among the highest on the western US coast. Chumash people inhabited the area for thousands of years. Prehistoric occupation is divided into several cultural periods. The Paleo-Indian period, over 10,000 years ago, is known from large fluted projectile points. The Millingstone Culture, from 10,000 to 5,500 years ago, left behind millstones, handstones and animal remains. Later cultures introduced portable mortars, pestles, obsidian, shell beads and advanced fishing tools, culminating in a shift to bow and arrow around 1,000 to 700 years ago as coastal settlements declined.

In 1769 the Don Gaspar de Portolà Expedition traveled along the coast, marking the earliest recorded Euro‐American exploration in today’s county. Many coastal features were named by Portolà’s party of soldiers and missionaries. The group camped on Oso Flaco Creek—’skinny bear’—so called after a lean bear they killed. After eating the bear, two explorers fell ill; it was later discovered that the Chumash poisoned dangerous wildlife by feeding them tainted meat. In 1772 Mission San Luis Obispo de Tolosa was established, and Spain claimed the dunes and surrounding lands as part of its mission holdings. Spanish soldiers, missionaries and settlers disrupted precolonial land-use practices by introducing an agricultural economy that relied on native Chumash and Salinan labor. In 1821 Mexico gained independence, and the new government secularized mission lands and granted large tracts to private citizens. Rancho Bolsa de Chamisal—a Mexican land grant of 14,335 acres (5,801 ha)—was awarded 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, and in 1856 he sold it to Francis Z. Branch, who in 1866 sold part of it to the Steele dairy dynasty. By the 1930s oil companies had begun acquiring large tracts in the Guadalupe dunes, with production taking off by 1948. The Union Oil Company—a major landholder since the 1890s—built its refinery on Nipomo Mesa in 1955. Over the next 40 years Unocal leaked 18 million gallons of petroleum beneath the dunes. In 1994 the company acknowledged 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 divide into upland and wetland habitats. The uplands comprise foredunes, back dunes and barren sandy beaches. Foredunes, which begin at the high-tide line, run parallel to the shore and support sparse vegetation. Parabolic dunes are U‑shaped, oriented perpendicular to the shore and feature vegetated ridges. A constant flux of sand, strong winds and salt spray render much of the area inhospitable, allowing only low‑growing species with deep roots to survive. Some plants grow close to the ground to avoid the wind while others adopt a Krummholz form. The back dunes, located behind the foredunes, are stabilized by vegetation—chiefly shrub species such as mock heather, dune lupine, coastal buckwheat and dune ragwort. Wetlands encompass salt marshes, freshwater and brackish marshes, swamps and mudflats. Plants in these areas adapt to high salinity and extreme temperatures by developing large taproots to access perched freshwater, thick cuticles to reduce water loss and succulence. Oso Flaco Creek is affected by pesticides and nutrient runoff from agriculture, and the California Office of Environmental Health Hazard Assessment advises against eating fish from Oso Flaco Lake because of elevated pollutants such as DDT, dieldrin, and mercury. Much of the area has been set aside for conservation and recognised as a National Natural Landmark. In 2013 the adjacent marine habitat was nominated for national marine sanctuary status by the Northern Chumash Tribal Council and the National Oceanic and Atmospheric Administration. In 2024 the Chumash Heritage National Marine Sanctuary was designated as the third-largest national marine sanctuary and the first Indigenous-nominated sanctuary in the United States. Read more here and here. Explore more of Oso Flaco Creek and the Guadalupe-Nipomo Dunes here: