During a bird survey on Salmon Creek Beach today, I noticed a significant amount of woody debris washed up on the beach. Recent rains must have caused a lot of material to flow down Salmon Creek and the Russian River (the Russian River is ~10 miles north of Bodega Head). Longshore currents transported, and then waves deposited branches, leaves, and seeds high on the beach. Examples of species represented — Douglas-firs, redwoods, oaks, willows, maples. In addition, do you recognize the seed in the photos below?
If you guessed California Buckeye (Aesculus californica), you're right! There were at least 12 or more buckeye seeds in the wrack line along ~2 km of the beach. Note that this one had started to sprout (look for yellow-green shoot emerging at top left).
They're impressive seeds — the diameter is listed as 2-5 cm in The Jepson Manual. I think this particular seed was even larger. With the pencil for scale (the metal portion below the eraser is 1.5 cm long), I'd estimate it was ~6.5 cm across.