Morro Dunes, Los Osos

Morro Dunes, Los Osos

by | Apr 27, 2022

Morro Dunes form a barrier spit separating Morro Bay to the east from the Pacific Ocean to the west, about 6 miles (10 km) south-southwest of the community of Morro Bay and 2 miles (3.2 km) west of Los Osos, California. The dunes are named after the bay, which in turn was named after the distinctive rock at the bay entrance, and is a Spanish geographical term for a crown-shaped rock or hill. The northern end of the spit is a protected area called Morro Dunes Natural Preserve and is a northern extension of Montaña de Oro State Park. Montaña de Oro means ‘mountain of gold’ in Spanish and refers to the golden wildflowers found in the park. Coastal dunes are located landward of the beach and develop where there is a prevailing onshore wind and an abundant and consistent supply of sand. The duration and velocity of the onshore winds, as well as the size of the sand grains, are fundamental properties that govern the size and shape of coastal dunes. Sand accumulates to create a dune system when the wind carrying the sand encounters an obstacle. Pieces of driftwood, trash, or piles of seaweed can all provide such an obstacle, causing the velocity of the wind to locally decrease and the sand is deposited. Most often, the obstacle that creates large continuous sand dunes is salt-water tolerant vegetation, either beach grasses or shrubs, or trees depending upon the climate of the region. Therefore, the establishment of vegetation promotes the deposition of sand and acts to stabilize the dune system. During fair weather, the base of the dunes is generally not affected by wave erosion since wave energy is dissipated on the beach face. During storm surges when large waves are produced and the sea level is elevated, a dune system may be subjected to erosion from breaking waves. In extreme situations, dunes can be completely washed over by storm waves creating blowouts or an overwash. The eroded sediment can be transported farther inland and deposited in the adjacent estuary creating extensive tidal flats. The presence of well-established dunes will act as a barrier against storm waves and help to protect infrastructure that is located landward of the dune systems. Dunes are therefore often protected to control erosion and damage to dune vegetation.

When Europeans first arrived in Alta California in the 1500s, the Chumash people inhabited this area in many small coastal villages between Morro Bay and Malibu. The closest prehistorical Chumash settlement to Morro Bay is represented by a large archaeological site called Back Bay which is situated on a stabilized sand dune in Los Osos and dates to between 800-1200 AD. In 1769, Don Gaspar de Portola marched his troops north from San Diego to establish new territory for the king of Spain and this began the California Mission period. The coastal people were moved inland to work at the missions and many subsequently died from European diseases. Following the Mexican War of Independence in 1821, the mission lands were secularized. In 1842, Rancho Cañada de Los Osos, meaning the valley of the bears, was granted by Governor Juan B. Alvarado to Victor Linares. Linares had lived at Mission San Luis Obispo since 1839 when he had been a majordomo of the mission and a militia commander. In 1844, John D. Wilson and James G. Scott bought Rancho Canada de Los Osos from Linares and the adjacent Rancho Pecho y Islay to the south which was originally granted in 1843 by Governor Manuel Micheltorena to Francisco Badillo. In 1845, the two ranchos were combined into one land grant called Rancho Cañada de los Osos y Pecho y Islay with 32,431 acres (13,124 ha) and included 7 miles (11 km) of the Pacific coast between the mouth of Islay Creek to the south and the mouth of Morro Bay to the north. In 1846, a group of American settlers in and around Sonoma rebelled against Mexican rule during the Bear Flag Revolt that escalated into the Mexican-American War. In 1848, the Treaty of Guadalupe Hidalgo ended the conflict and the United States annexed the westernmost portion of Alta California which became the State of California in 1850. In 1861, the rancho was inherited by Wilson’s wife, Maria Ramona Carrillo Wilson, and their daughter Ramona Hilliard, who used the land for grazing sheep. In 1891, Ramona Hilliard sold the southern portion of the Pecho y Islay ranch to Luigi Marre, and in 1892, leased 6,500 acres (2,630 ha) of the northern portion to Alden B. Spooner who raised livestock and began a dairy farm. In 1902, Spooner purchased the leased property from the family of Henry Cowell of San Francisco who had acquired the debt on the Hilliard property. In 1963, approximately half of Pecho Ranch was purchased by the State of California and became known as Montaña de Oro State Park, the other half is owned by Pacific Gas and Electic. The Morro Dunes are part of a private land purchase converted to a protected natural area and annexed to Montaña de Oro State Park to preserve the threatened western snowy plover and the Morro blue butterfly. Morro Dunes Natural Preserve is within the Los Osos coastal dune complex that includes habitat for several endangered species including plants, mammals, reptiles, birds, amphibians, and butterflies. Significant plant resources include coastal sage scrub, dune scrub, Morro manzanita, and maritime chaparral.

Morro Bay is an estuary of 2,300 acres (931 ha) fed by Chorro Creek and Los Osos Creek that drain a combined watershed of 48,000 acres (19,425 ha). The bay is part of the National Estuary Program and includes a state park, a state marine reserve, and a state marine recreational management area. The National Estuary Program was established in 1987 by amendments to the Clean Water Act, with the intent to protect and restore nationally significant estuaries. The program focuses on water quality as well as the integrity of the entire estuarine system, including its physical, biological, economic, and recreational values. Morro Bay State Park has two parts, one borders a lagoon at the mouth of Chorro Creek where there are saltwater and brackish marshes that support thriving bird populations, and the other includes the coastal dunes on the sand spit as part of the Morro Dunes Natural Preserve. The Morro Bay State Marine Reserve is situated in the estuary at the mouth of Chorro Creek and Los Osos Creek and protects all marine life within its boundaries, and fishing and taking any living marine resources is prohibited below mean high tide. The Morro Bay State Marine Recreational Management Area protects the marine life inside the bay entrance and west of the marine reserve. Fishing and taking any living marine resources is prohibited in about the southern one-third of the bay, and the recreational taking of finfish and aquaculture of oysters is conditionally permitted in the northern two-thirds of the bay. The priority issues facing the Morro Bay National Estuary include accelerated sedimentation, bacterial contamination, elevated nutrient levels, toxic pollutants, drought or the lack of freshwater, preserving biodiversity, and balancing the many uses of the bay with these environmental issues. Read more here and here. Explore more of the Morro Dunes here:

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About the background graphic

This ‘warming stripe’ graphic is a visual representation of the change in global temperature from 1850 (top) to 2021 (bottom). Each stripe represents the average global temperature for one year. The average temperature from 1971-2000 is set as the boundary between blue and red. The color scale goes from -0.7°C to +0.7°C. The data are from the UK Met Office HadCRUT4.6 dataset. 

Credit: Professor Ed Hawkins (University of Reading). Click here for more information about the #warmingstripes.

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