We've done a lot of miles on the Clarence (one of our favorite areal in Oz) including during the floods of 2008. Never had any noticeable problems from silt, but I'll admit that the situation is specific to each engine and its installation
On the other hand, if the flooding is still active, floating debris and weed can be a real hazard. A huge accumulation of Hyacinth on the rode
and the bow caused us to drag slowly downriver in the middle of a pitchy black night... an uncomfortable situation when you're not keen to go motoring about with all the floating crap threatening your prop. Should this happen to you, we found that anchoring
across from the pontoon at McLean, right near the shore, got us out of the main stream of junk.
Cheers, and enjoy a great river trip
Jim and Ann s/v Insatiable II lying Gladstone Qld Oz