Going on a river trip, what kind of mpg or gph can I expect?

[offgrididaho]

I just got a 93 GTS in mint condition, low hours. Does 40 mph flat out on glass. If I run at half or 3/4 throttle how many hours could I run on a river going up and back?
Wonder how far I can travel before having to turn around. Going upstream first so I can always float home if something goes wrong.
Will be riding double with my wife.

It's the same river Lewis and Clark explored. Up stream of Clark fork.

 

[artr]

The shop manual lists the ski's run time at full throttle for the tank and reserve. I would be conservative and go half the time upstream and then return. If you rely on floating downstream because you run out of fuel you will not have steering.