An intriguing mystery in Pescadero Marsh
by Lennie Roberts

Pescadero Marsh, like most coastal estuaries, changes from saltwater to fresh or brackish conditions during the summer and fall months, due to the buildup of a sand bar at the mouth. The sand bar closes off the marsh from the ocean, creating a large shallow basin that is slowly filled by the reduced summer flows in Butano and Pescadero Creeks. Without a connection to the ocean that lets salt water enter the marsh at high tide, the water chemistry gradually changes to a nearly freshwater lagoon, with stagnant or brackish water in some locations.

After the first significant winter rains in November or December, the sand bar opens, and tidal action is restored to the marsh. The summer’s backed-up water drains into the ocean in one dramatic rush, as the first big tide goes out. It’s this “first flush” draining action that appears to be stirring up some muddy trouble for resident steelhead and other fish.

For the last few winters, within a few hours of the opening of the sand bar, a significant number of fish have been killed in the portion of Butano Marsh that lies between Pescadero Creek Road and Butano Creek. Scientists are just beginning to suspect the reasons for these fish kills.

Local fishermen, with careful field observations and water quality sampling, theorize that as the sand bar opens, the outgoing rush of water causes turbulence such as that you’d see after removing a plug in a bathtub. In the case of the marsh, fine sediments, mud and decayed vegetation on the bottom are stirred up and mix with the cleaner layers of water above. A distinctive smelly plume of “muck” is mobilized, releasing oxygen-deficient water and hydrogen sulfide, suffocating fish and other gill species. This year, observers counted some 350 dead fish — mostly juvenile steelhead. It’s likely that many more than this number were killed.

The Pescadero Marsh is managed today as a Natural Preserve by the State Department of Parks and Recreation. However, the marsh’s natural conditions have been greatly altered by human activities over the past 150 years. A persistent and provocative question is, “what is the natural condition?”

Historically, farmers have diked off and drained extensive areas of the marsh for farming. Old photographs from the 1920’s clearly show large fields of hay and other crops that have now reverted back to wetland vegetation, as some of the old levees were opened up to restore the area to a more “natural” condition. A legacy of clear-cut logging in the upper watershed, with no regard to stream and slope protection, has created excessive doses of sediment in the creeks that have reduced the capacity of Butano Creek to carry flood waters. Even today, there is a large quantity of sediment waiting in the upper watershed to be transported downstream.

Efforts a decade ago to restore the marsh’s hydrology to a more “natural condition” by opening levees may only have encouraged more sediment to be deposited in the area where the fish kills are occurring. As winter waters spread out onto the marsh’s flood plain, the accompanying sediment may have raised the elevation of the wetlands to encourage such plants as tules and cattails. When the water level rises in late summer, the older leaves die and begin to quietly decay on the bottom, thus setting up the unintended surprise for the steelhead. The “natural” condition of this complex ecosystem is not functioning in a “normal” way, or at least the way that is best for some of the fish.

The challenge now is to see whether the area can be managed to improve conditions for the fish without having negative consequences on other marsh inhabitants. The marsh is home to multiple species of concern, including steelhead trout, tidewater goby (a small fish), California red-legged frog, San Francisco garter snake, and the brackish water snail. Some have conflicting habitat requirements, so any alterations to the marsh will need to consider these species, and be approached from an ecological perspective.

Recently a group of scientists, agency representatives, fishermen, and interested citizens gathered in Pescadero to discuss what is known, and not known, about the fish kills. Efforts are now underway to monitor several key water quality indicators and use this information to develop appropriate strategies for short-term and long-term management of the area or at least to know what should not be changed by humans.

It’s been encouraging to imagine the possibility of a new consensus forming over restoring the marsh. Actions such as removing dams built by the non-native beavers have already helped increase the stream flows in Butano Creek. But as with everything else in this complex ecosystem, it will take a concerted effort on the part of many people with diverse viewpoints to craft solutions that can restore the area to a more functional hydrological and ecological condition, and ensure the persistence of the marsh as a natural resource.
Published March 2004 in Green Footnotes.