One explaination for the floating logs on Steamboat Slough in 2012:  
There seems to be an inordinant amount of floating debris on Steamboat Slough this
spring.  Fish scientists use these "floating cover" to supposedly help baby salmon
hide from the stripers.  What is really happening is that hazzards to navigation are being
intentionally set onto Steamboat and Sutter Sloughs.  You do not see these floating
woody debris clumps on the Sacramento River, as the photos below show.  One CALTRANS
ferry worker mentioned that wood gathers at the bridges and the CalTrans person in charge of
making sure the bridge is kept clear simply shoves the logs and debris off of or away from the bridge supports.  If this were true, then the Sacramento River would also see woody debris.
Instead, what we've observed is that the wood is gathered at the Sutter Slough bridge and
it appears to be shoved off from there.  More photos of the pile hidden under the bridge will
be added.