I was there again this weekend. (I LOVE Lock 21)
First dive, vis was 10 feet (which is good for this site) and current was fast (3-4 knot range). Had a great dive.
Came up, did a nice sit, basking in the sun for a while chatting with the campers, and watching the hundreds of Canadain Gease swim around.
Second dive, we wen't in as these nasty black clouds were comming down the river. We didn't see the clouds, all we noticed was the chop on the water picking up a bit.
Vis was probably down to 3 feet toward the end of the dive. But the current. Hot damn, thats the fastest I have ever seen it. I could just make headway swining against it.. and I am a very good swimmer. I am guessing 6+ knots. Hot damn, the 2 drifts we did were a rush.
When we get there is a mamoth storm, wich you may have heard about.
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As an aside, there are ropes and lots of stuff to pull your self along around the lock, so you typicaly pull your self up and drift down the lock, repeating if you have air. My normal route is to go in down stream, where there are some ropes along the bottom. Pull along the ropes to the lock, pull along the bottom to the frount of the lock, drift down the lock, pull back up to the frount. Play along the slush gates, then drift back to the rope we came in on.
When your in behind the slush gates, the current actualy reverses and pushes you toward them. So its not that hard to get back to the top of the locks.
You are also quite far from the shipping channel, there is a rather large island between the lock and it. You do see a fiar number of smaller personal boats dirve passed, so use a flag.
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If you had planed on diveing on Sunday, you would probably have had to turn the dive anyway. Sunday was not a good day to learn the lock. Most of the other people there swam like mad for 15 minutes to get to the "start" bouy, exausting them selves against the current. Some didn't even bother decending, just swam back. Most of the ones who went down were up 10 minutes later, drifting passed the lock system and fighting to get back to shore.