Andy,

Dan Wenny and I have started an online photo repository of seeds, but at the moment, the number of genera and species is quite limited, and all the seeds were collected from seed traps or harvested from plants in the upper Midwest (NE and NW Illinois, precisely). In case this may be useful, however, you can find our photos here:

https://drive.google.com/file/d/0B5OOvEeh149fMVlpNGlHYnpYYVU/edit?pli=1

A site hosted by Ohio State University that looks potentially useful is here:

http://www.oardc.ohio-state.edu/seedid/all.asp?sort=scientific

Not sure about Northamerican plants, but in Doñana Marcelo González worked on that in his PhD research (marcelo_gosan at yahoo.com.ar).

Andy, I know of no online resource but since you are working in California, there is a somewhat older publication, A Flora of the Marshes of California by Herbert Mason published in 1957 by University of California Press that really excellent detailed line drawings of marsh species including their seeds. Obviously, it does not cover all species, but it will give you a good inroad to a lot of species that waterfowl might consume.  The book is out of print, but I did a quick look at abe books (easily googable online) and their appear to be about 30 copies for sale from different booksellers that range in cost from $10.00 to $150.00 depending both the quality of the copy being sold and the bookseller it is coming from. Good luck in your efforts.  Marc

I've found a few useful sites which I will post on here. One is for weed seeds in Missouri (the seeds of many weeds are consumed and dispersed by waterfowl): http://extension.missouri.edu/p/IPM1023-37#Polygonaceae

Here is another good site from a seed ID workshop:

http://www.oardc.ohio-state.edu/seedid/all.asp?sort=scientific

This one has great photos of a much more limited number of species:

http://www.inspection.gc.ca/plants/seeds/testing-grading/fact-sheets/eng/1333136604307/1333136685768

here are some good resources for line drawings of seeds, including a classic book of W.J. Beal's

http://frontrangeseedanalysts.weebly.com/usda-plates.html

http://www.gutenberg.org/ebooks/33679

Here is another good resource

http://books.google.es/books?hl=es&lr=&id=5vd_ZE7UDV8C&oi=fnd&pg=PR9&dq=atlas+of+seeds+central+europe&ots=tmqB-2QlWx&sig=mWhjwkSXSGQ9MqiFWNJMmEoasjE#v=onepage&q=atlas%20of%20seeds%20central%20europe&f=false