Los Peñasquitos Lagoon

Los Peñasquitos Marsh Natural Preserve and Lagoon is a coastal marsh in San Diego County, California, United States situated at the northern edge of the City of San Diego, forming the natural border with Del Mar, California. The lagoon, previously called The Soledad Lagoon, divides a colony of the endangered Pinus torreyana on a narrow coastal strip. The name "Los Peñasquitos" is Spanish for "The Little Cliffs".

For many years, the Los Peñasquitos Lagoon had evolved from a tidal estuary to a lagoon that was closed to tidal action for long periods of time. Since becoming a part of the State Park System, there have been a number of changes that have increased the tidal action within the lagoon.

In 1925, the Santa Fe Railroad built a single-track roadbed causeway embankment down the center of the lagoon for its Surf Line, which still is in use today on a daily basis by the San Diego Coaster and Pacific Surfliner as well as BNSF freight trains. This embankment severely restricted the normal historical lagoon drainage for the first time, and changed the tidal flow and current pattern. The original bridges are slated to be replaced in 2015-2017, and additional plans for a double-tracking solution through the lagoon appear to be slated for the 2040-2050 timeframe.

When the Pacific Coast Highway (U.S. Route 101 in California) was expanded in the 1930s, the roadbed along the beach was heightened, and a bridge was built over the mouth the lagoon. This bridge had many wooden pilings that easily got clogged with sand and debris, impacting the water transfer between the ocean and lagoon.

Over the years, at least three different waste water treatment plants have pumped their treating effluent into the lagoon. The Callan Treatment plant pumped 50,000 US gallons (190,000 L) per day during the 1950s; the Sorrento plant produced 500,000 US gallons (1,900,000 L) per day starting in 1962; and the Pomerado Waste Water Treatment Plant pumped treated sewage into the lagoon from 1962 to 1972.

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