Tijuana River Reserve, California

Geology

As continental drift shifted North America toward the west, a steep coastline and narrow continental shelf developed. Marine terraces were gradually carved along the shores. Then, in the late Cenozoic, tectonic uplift raised alluvial terraces to several hundred feet above modern sea levels. What is now Tijuana River presumably cut through these terraces, although the narrow floodplain suggests that flows were not consistently large.

Then, in the Holocene, a rising sea began to reclaim the exposed margins of the coastal shelf. Rivers were drowned and lagoons formed as longshore drift created sandy barriers along the coast. With flooding, most of the coastal embayments filled with sediment. Without continuous river flow and scouring, their mouths closed between flood seasons.

Recent geologic factors that have shaped the estuary are the competing forces of rising sea level, which promotes inland migration of the estuary, and tectonic uplift, which reverses that trend. The location of the shore and the configuration of the mouth are additional variables that influence the size and condition of the estuary.

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