When entering an area of low fog,
make sure your lights are on and you are aware of other boats
that might be in the area. Never enter an area of low visibility
unless you are extremely familiar with it. A GPS is a good idea
in some cases too. In this case the fog was caused by the temperature
difference of one river where it flowed into another. The tug
boat captains were up high enough to see the river ahead, but
they could not see the fishing boats down in the thick fog.