I think they go deeper as a result of the season, more than anything else. Notice I didn't say water temp.
How do they know what season it is? Length of day. The more I fish year round, the more I notice fish can be found (or not found) in certain areas based on the length of day, rather than water temp.
This time of year, the days are at their shortest. So methinks they'd be deepest.
That said, I also feel big environmental changes can and will affect the fish no matter what time of year.
This weekend, for instance, was marked by a huge warm front followed by a high-pressure system. I thought the warm water near where streams entered the lake would produce best. Even though the fish were "deep", I felt they would move in to feed, being as opportunistic as bass are.
But that wasn't the case on Sammamish on Sunday. After flogging the flooded shallows (the docks were under 1 1/2 - 2 feet of water!) for most the morning, Jim Shaffer and I switched to the 50'-70' zone. I got a strike as result (oh goody), but no fish.
Moral? Well... there isn't one. I just thought I'd share.
Tangent airlines has now departed.
No, actually, the moral is the fish don't always do what the book says they'll do. In this case, something else kept them deep - be it high pressure, the muddy water, a big change in pH from the flooding, all of the above or none I don't know. But apparently the deeper water was more stable so that's where they went (or stayed).
Judging from Chuck's post, this was also true for Washington.
Maybe that "length of day" thing posseses a stronger power than I give it credit.