Tuesday, September 18, 2007

Padilla Bay

Too many meetings; not enough beaches. Yesterday's meeting was at the Padilla Bay Reserve, however, so with sandwiches in hand a few of us walked down to the shore at lunch. Padilla Bay is an inactive portion of the much larger Skagit Delta - the Skagit River currently dumps all of its sediment into Skagit Bay between La Conner and Stanwood. Maybe because of the lack of fluvial sedimentation, the tidal flats of Padilla Bay are infested (maybe I should just say thick) with eelgrass (mainly zostera marina, some z. japonica). The bay is the largest single expanse of eelgrass in the Sound, although it is a bit unrepresentative of the narrow, fringing beds that characterize the steep, narrow platforms typical most everywhere else.

The gravel beaches along the edge of the bay are narrow and pretty low energy. As is typical of high-sediment areas around the margins of deltas, the transition from the steeper, gravelly foreshore to the fine-grained flats is pretty high - probably about mid-tide. On exposed beaches without big sediment loadings, the break is typically near MLLW.
I suspect the original gravel beach probably extends down below the fine grained sediment and the eelgrass, though I haven’t actually checked. Presumably as the delta moved into the neighborhood, it basically buried the existing low tide terrace and lower intertidal gravel beach. If the river had stayed, or if it were to come back, the edge of the bay might all eventually turn to salt marsh as the delta front marched north.

This time of year, eelgrass can wash up in thick windrows along the beach – two distinct deposits were evident today, both hopping with amphipods and small flies.