[naturalscott]

We're building an above ground 7' diameter by 30" deep water garden. It's built with 8"x8"x16" concrete block reinforced with rebar and concrete in every hole and set on a 6" reinforced concrete footing. The outside will be a facade of natural stone mortared in place.

Do I need to wash each stone before I mortar them in place?

[treedoc1]

no organic dirt should be on your stone product that will interfere with the bonding.

You will wipe the stones off sufficiently when you do your joint work.

[Fine Edge]

You need to make sure there is no heavy deposits of dirt on the back of the stones.
We usually just wipe them with our hands or keep a small brush next to us. We have found that small traces of dust doesn't interfere with the bonding process. We also will pressure wash our stonework after the mortar has set up for a few days. Sometimes if it's a small job, it may not be cost effective to go back but we try to price a good cleaning into our estimate. it makes a big difference.
You can also keep a bucket of water and sponge close by to wash off mortar that happens to spill.