Hello Glen -
Yes, 500yds is plenty close as a water source for bees. However, in my experience, bees usually prefer a source other than the one we think will be the best for them.
You say the bees will be on a farm. More than likely, they will find standing water on the ground around the farm or in containers like water troughs to be more appealing than a large reservoir of fresh water. Either way, with the river and reservoir near by, you need not worry about adequite water supply.
I have wetlands, rivers and lakes within 500yds of my hives and I still see them by the road ditch getting run off ground water and I'm sure, salt, as water softners discharge into this also. Have a wet spot over the septic or livestock mucking around in mud with manure in it, you'll see the bees there too!
-Barry