Wednesday, September 24, 2008

Foulweather Preserve

The northern tip of the Kitsap Peninsula has two great coastal marshes. One is a large salt marsh behind a recurved spit just west of Foulweather Bluff, but unfortunately it's private, or at least access to it is. The other is the brackish marsh at the Preserve - the Nature Conservancy's signature holding on Puget Sound.

I call this a closed marsh or lagoon, meaning that most of the time the barrier is continuous and there is no surface connection to the Sound. Occasionally a big storm may wash over the berm and if the water behind wants to get out badly enough, it may develop a shallow channel for a short period of time. But it always closes up. This one is large for a closed marsh, since as the potential tidal prism increases, the ability to maintain a channel becomes easier.

Closed marshes occur behind barriers and generally lack significant stream input (otherwise the stream would maintain a channel). In theory, it's easier to maintain a closed marsh behind a coarse gravel berm than behind a sandy one, since drainage can occur more readily through the permeable berm.

Closed marshes haven't gotten as much attention in recent years as tidal wetlands, since they are usually small, and without an open inlet, they have limited value as refuge or forage for passing fish. But they're still significant coastal features. Many may have originated as open systems. And maybe with sea level rise, some will become open systems again! Generally, they seem more common as you move north in Puget Sound, for which there are several potential explanations. More wave exposure leads to greater sediment transport and the North Sound has more barriers behind which estuaries and lagoons can form. Smaller tidal range in the north may reduce the hydraulic viability (smaller tidal prism) of these systems on the brink of being open or closed. And because precipitation in less as you go north, their are fewer streams intersecting the coastline and a larger proportion of small barriers occur along shorelines with no significant upland drainage.

This feature occurs in a low glacial trough - the till dives below beach level on each side creating a shallow bowl. Sea level eventually flooded the trough, but there was sufficient sediment to form a barrier across the mouth. This is a pretty sandy beach, although the high berm is gravel. It is swash-aligned - almost a pocket beach, held in place by the resistant promontory of till at the west end. For years, there was a beautiful madrone desperately hanging out over the bank, but in the mid-1990s, it finally gave up.

No comments: