Slip-Sliding Away
Mugu Lagoon:
A Study in Erosion

Just a darn minute! Didn’t you say (in Calleguas Creek: Regional Watershed Plan) that Calleguas Creek is dumping a surfeit of sediment, silting up Mugu Lagoon? So how can this coastal salt marsh be eroding?

It is true that this habitat of some 40 threatened and endangered species on Ventura County’s southern coastal tip is becoming smaller. While erosion of the Calleguas Creek watershed has increased over the years, filling in tidal wetlands, the ocean is carving away at the lagoon itself. The apparent dichotomy results from Nature asserting her complete disregard for human plans and sensibilities. Point Mugu, and the Navy base at water’s edge, is the site of an erosive battle of the elements—the Mugu Submarine Canyon and Calleguas Creek.

The submarine canyon, which is 350 feet deep and begins only a few yards from the shore, swallows thousands of cubic yards of soil each year—increasing dramatically during periods of heavy weather. The precipitous drop of canyon walls to such a depth, and the eleven-mile length of the canyon, cause the resultant outflow of sediment to be carried far out to sea. The Pacific’s ceaseless nibbling at the shoreline cannot be stopped or slowed by human effort.

Much of the sediment deposited by the creek at its mouth is swept away by the intruding ocean. Where sediment builds up along this front it creates a shelf of loosely aggregated rock, dirt and sand which will eventually sluff off into the canyon. When it does, its scouring action takes some of the underlying soil with it. The combination of these erosive effects creates the phenomena of a salt marsh silting up—becoming dry land—while at the same time on the opposite side of the shore margin there is deep sea.

Live With It

What can man do? First is to get out of the way. The Navy base has lost land and has had to surrender buildings and facilities to the encroaching ocean. The second is to accept that nothing can be done, that even without the watershed’s sediment buildup over decades and centuries, ocean action will continue to undercut the shoreline.

In a natural ecosystem, such as Mugu Lagoon, there will be changes affecting plants and animals that now inhabit and visit this salt marsh. According to Navy ecologist Tom Keeney, who monitors the lagoon, the plants and animals will likely have an easier time adjusting to these changes than will humans who may be emotionally invested. He cites migrating shorebirds that have learned to deal with such events. Keeney believes that while parts of the lagoon will be lost, most of the marshes will survive. The submarine canyon is currently moving northward but scientists are unsure whether it will continue or change direction.

While normal erosion advances at about two feet a year, storm surges increase that amount; the recent El Niño claimed 30 feet. Far from being a tragedy, the canyon’s progress is a natural and dynamic geologic process, creating and destroying according to Nature’s functions.

A greater concern for the lagoon involves what the sediment brings in the way of pollutants and how the ocean’s action may affect their release.

Please turn to Waterborne Pollution for more on pollution affecting Calleguas Creek and Mugu Lagoon and what’s being done about it.

Read about endangered plants and politics.

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