Choclab,
Spencer is pretty low on the river, it's brackish water, and fully influenced by the tide.
Be careful out there, the mud is soft and deep, the tide rushes in and out of "Spencer Lake", and there's places that end up 12 feet underwater.
The south half of the "lake" is off limits to hunting (from land), but some guys float in off the river at high tide.
On a good day, you can still bag a limit of birds there, and I have several times myself.
I'd be glad to run through the "tour" of the place with you.
The fishing can be good off the mouth, but there's not much that goes on in the lower river.
We fish for Silvers downstream of Snohomish (hwy 9 Bridge).
I do a little fishing just below the confluence (Snoq and Sky),for Summer Steel, but all we catch are a few small trout, a few whitefish, and a bunch of big suckers.
Around the Marysville/Everett area the best fresh water fishing is in the dead end sloughs and ponds, there used to be some good bass fishing in some (one in particular, on Ebey Island, was well stocked).
Access isn't as easy to get as when I was a kid (30 or 40 yrs ago), so the fish might be doing very well?
Dude, you live where I grew up, now... anywhere you go, to play, around here... I probably already been there.

We should get together.
Krusty
