From the Smith River in the north to the Tijuana River in the south, California's beaches are broken by dozens of small stream mouths that flow out through shifting sand spits to the oceans. Or at least used to. Or at least used to some of the time.
There is always competition between streams and tides trying to keep an inlet open and waves and sediment trying to close it off. Bigger rivers and large estuaries always have enough water moving in and out to stay open. But small streams, particularly when subject to low seasonal flow and emptying into estuaries with small tidal prisms, may periodically close off. This is especially likely if an energetic wave environment and abundant sediment lead to active beach transport and rapid rebuilding of the berm.
The shifting nature of the inlets, frequent flooding on the margins of the estuary, and the desire to enhance the lagoon for boats and recreation, leads to dikes, dredging, reclamation (filling) to create dry land, and structural measures to maintain the inlet. The result is a static, somewhat more predictable system. Development potential goes up and the native ecosystems decline as the messy geologic processes that maintained them are gradually brought under control.
Agua Hedionda lies just south of Carlsbad and the modern lagoon is a miracle of multi-use management (Agua Hedionda) - with railroad and highway crossings, swimming and boating, bird and wildlife habitat, aquaculture, a power station, a planned desalination plant, all at the receiving end of a heavily developed watershed. But this is probably a long way from what it originally looked like - marsh, salt pan, mud flat, and that darned shifting inlet. It is just one of a number of estuarine case studies in San Diego County - each of which has a different natural character and a different 20th century development trajectory (Batiquitos, the next one south, has also received much attention in the last two decades).
I'll admit right now that I don't know the specifics of Agua Hedionda. Its name translates loosely to "icky, foul-smelling water" and that sort of goes along with the notion of a stagnant system trapped behind a stream mouth barrier. So I suspect it occasionally closed off historically. But whether it did or not, it has been dredged and its inlet has been channeled between rock jetties, assuring that flow is maintained and the inlet doesn't shift up and down the beach.
The jetties interrupt longshore sediment transport. Drift along this coastline is north to south, so the beach is built up on the north side, and narrow on the south side (and heavily armored). The beach here is further confused by the rock jetties farther south that protect the outflow from the generating station.
At this month's estuaries conference (CERF), there were a neat series of talks about the history and ecological function of small bar-built estuaries (sometimes called intermittently open/closed inlets), with many examples from California, but also others from South Africa and elsewhere. Our historic inclination is to see periodic closure of these lagoons as a problem to be solved. Closure meant stagnant water and flooding and when the inlet reopened, it didn't always reopen where people wanted it to. But these are ecosystems that were adapted to this type of cyclical closure. Flooding when the lagoon is closed off inundates high marshes and may increase productivity and fish habitat within the lagoon, often at times when water levels would otherwise be very low. And the consequences of "managing" these systems are not without their own adverse impacts on the environment - jetties, dredging, dikes and levees, increased development of the fringing wetlands, and so forth.
Here are a couple of other examples from my brief excursion to California's central coast in July:
Sequel Creek in Capitola
The Klamath River in northern California is a nice example of a bar-built estuary, but the river is much larger and probably never closes off.