Coronado, California, is a geographically confusing location often called an island, yet it remains firmly connected to the mainland across San Diego Bay. This popular area is not a true island but a landform created by a rare geological process. The formation story involves a former island being joined to the continent by a massive accumulation of sand. The landmass is technically a peninsula, but its unique structure requires understanding the specific term for this feature and the oceanographic forces that built it.
The Tombolo Phenomenon
Coronado is classified as a “tied island,” a landform connected to the mainland by a narrow spit of deposited sediment known as a tombolo. The original landmass was a bedrock island, a piece of older, resistant rock that stood offshore after sea levels rose following the last ice age. The Silver Strand, the long, thin strip of sand extending south from Coronado, is the tombolo itself, connecting the former island to the mainland near Imperial Beach.
A tombolo forms when an island acts as a natural barrier, disrupting the movement of ocean waves and currents. The island shelters the water immediately behind it, creating a low-energy zone where waves deposit their sediment load. This deposition gradually builds a submerged bar or spit until it breaks the surface and physically joins the island to the larger landmass.
While the terms “tied island” and “tombolo” are sometimes used interchangeably, the tombolo is accurately the sandy connection, and the landform it creates is the tied island.
Source of the Sediments
The vast volume of sand and gravel required to build the Silver Strand tombolo originated from two primary sources. Much of the sediment was carried by ancient river systems that drained the mountainous areas of Southern California and northern Mexico. These rivers, particularly the Tijuana River system, transported immense quantities of eroded material to the coastline.
The sediment then entered the coastal system, known as the Silver Strand Littoral Cell, where it became available for transport by the ocean. Additional material came from the natural erosion of coastal bluffs and cliffs further north along the coast. The Tijuana River was the principal natural source of sediment for the southern end of the tombolo, historically delivering an enormous annual load to the coast.
The Role of Ocean Currents and Waves
The engine that transported the sediment and created the tombolo is a coastal process called longshore drift. Longshore drift is the movement of sand parallel to the shoreline, driven by waves that approach the coast at an oblique angle. As waves break, they push sediment up the beach (swash) at an angle, but the backwash pulls the material straight back down, resulting in a net movement along the coast.
In the Southern California Bight, the dominant longshore drift moves sediment southward. However, the Silver Strand Littoral Cell exhibits a distinct, localized northward transport path. Waves generated by summer storms, which approach the coast from the south, are a major factor in pushing sand northward toward the Coronado landmass.
As the sand-laden currents encountered the former bedrock island, the island acted as a breakwater. This created an area of calm water and reduced wave energy on its sheltered side. The sudden drop in energy caused the suspended sediment to fall out of the water column and accumulate, forming the sandy connection.
Geological Timeline and Stabilization
The formation of the Coronado tombolo is a relatively recent event in geological history, beginning after the last glacial period. The process started roughly 14,000 to 7,000 years ago as global sea levels rose, isolating the bedrock high ground of Coronado as an island.
The Silver Strand grew steadily over thousands of years as the sediment accumulated against the former island. The landmass eventually stabilized into its current configuration, aided by natural factors like the development of coastal vegetation and salt marshes. These ecosystems helped bind the loose sand, securing the newly formed land against wave erosion.
The process was largely complete long before modern human intervention. Since the late 19th century, the area has seen significant artificial stabilization and land reclamation. Filling in Spanish Bight, the shallow water body that once separated North Island from Coronado, further solidified the peninsula.