Tidal Marsh: Extent and Patch Size

Around 1800, there were approximately 555,000 acres of tidal marsh in the Estuary (19 times the size of San Francisco).

Over the next 150 years, the vast majority of marsh was diked, filled, or otherwise disconnected from the tides. By 2009, there were only 53,000 acres remaining—a decrease of 90%.

Of those 53,000 acres, 85% are in the Bay and 15% are in the Delta.

This is very different from the historical situation, when 34% of the tidal marsh was in the Bay and 66% in the Delta. These changes amount to a reversal in the distribution of tidal marsh in the Estuary.

Also important are the sizes of individual marsh patches. While >80% of tidal marsh in the Bay is part of a patch large enough (>500 acres, as highlighted in salmon) to support maximum densities of marsh birds like the Ridgway’s Rail and Black Rail, less than 30% of Delta marshland meets this size threshold.

photo credit: Aaron Maizlish/Flickr

We are making progress on restoring tidal marsh in the Bay. Just prior to completion of the Baylands Ecosystem Habitat Goals in 1999, the Bay had about 40,000 acres; by 2009 there were about 45,000 acres. Since 2009, an additional 6,300 acres of former tidal marsh have been restored to tidal action. Most of this area is expected to transition into tidal marsh over time.

In January 2015, with the breach at the North Bay’s Cullinan Ranch, the region reached a major milestone in its tidal marsh restoration goal: as Cullinan and other restored areas evolve and mature, the regional extent of Bay tidal marsh is expected to surpass the halfway point of the 100,000-acre target set in the 1999 Baylands Ecosystem Habitat Goals Report. The pace of restoration has been much slower in the Delta, where only 300 acres have been restored since 2002.