The recent warm weather has melted the snow pack and started the “spring freshet”. The water in the river rises and cleans off all the logs and other debris from the banks, which then gets swept out to sea. Or, in this case into Canoe Pass which is one of the outlets of the delta. The current piles the debris up against the bridge, and the rise and fall of the tides does not always dislodge the longer trees. So the tugs are called. The link above takes you to a video of the Canoe Pass Renegade finishing off the remains of a large snag last weekend.