There are two basic things to bear in mind about river diving: there is no thermocline and the current is always in one direction.
The churn of water caused by currents results in good vertical mixing of water. Consequently, there is no thermocline and the water is the same temperature at the bottom as at the surface. This also means that the water temperature varies considerably with the seasonal cycle as the surface water, heated by the sun, is mixed with the deeper water.
This temperature variation makes an unexpected demand on divers' buoyancy control. At the start and end of the season, people are often in dry suits. These are dropped in favour of wet suits as much as possible as a wet suit is more flexible and creates less drag in the water. However, wet suits are not ideal: they make movement restricted, and breathing if your suit is too tight; have decreasing buoyancy with depth; and you often have to carry more weight. These last two often result in being heavy on the bottom.
So, as the season progresses, one is constantly shedding and donning thermal gear and weights; thus constantly re-adjusting ones buoyancy; and good buoyancy is essential to the enjoyment of drift diving.
A river always flows downhill, right? Well, yes, on a macroscopic scale and this is the general direction of the current. However, many a diver has surfaced in the "wrong place" because they were swimming with the current upstream. Eddies can be large and are normally found near bays and behind islands and promontories. You cannot always rely on a river current's direction for navigation; a compass is an essential piece of equipment.
The basic rule to diving in a river current is to enter the site upstream and exit downstream. This avoids working against the current. If you do a dive where you must swim against the current at some point, then do this part of the dive first; returning to the entry point at the end of the dive is easier with the current than against it.
Many dives in the St. Lawrence are done on wrecks, from boats. We usually moor onto a buoy downstream of the wreck, swim up to and along the wreck, and they often do align themselves with the current, and then return back downstream to the buoy and exit. This photograph is of a drift line we use when diving from a Zodiac. The floats keep the line on the surface and we can clip our BCDs and tanks to the line with the clips before getting out of the water. We then haul the gear in.
Drift diving is one of the great pleasures of river diving. If your buoyancy is well controlled it is a sheer joy of weightless flight that you experience; and you can travel quite a distance. Some of the best drift diving is in the Brockville Narrows. However the depths, speeds and boat traffic make these very advanced dives.
If at any time one is in a current and is fatigued, do not fight the current or try to proceed to some safer location. It is perhaps counter intuitive but the best action is to go negatively buoyant and lay flat on the river bed for a couple of minutes to regain your breath. You are out of the current here, it is negligible in the first foot or so off the bottom due to drag. Remember if the current is strong at depth, it is stronger still on the surface; surface current is the strongest.
A further note on currents and fatigue is that a moderate diver can fin at between 0.5 to 1.0 knots. That is the equivalent of 50 to 100 foot per minute. Just as divers always underestimate visibility, they always overestimate current strength. If you can hold your own position in a current by finning hard into it, it is probably about a one knot current; moderate finning will make it about half a knot.
Boat traffic and dive flags
You have already seen some of the local boat traffic we have in the St. Lawrence. There is also an immense amount of small pleasure craft Therefore one needs to take a dive flag. The exception to this are some sites with strong currents, when the flag itself can become a hazard. The surface current is faster than the current at depth and the pull of a flag makes it difficult to stop and can also represent an entanglement hazard. Diving in a group on Lock 23, off Morrisburg, several members of the group became entangled in *MY* flag's line.
The dive flag used in this area is the North American dive flag, red with white diagonal line.
Due to the unique conditions in this busy river there are a number of dos, don'ts and it's-a-good-idea-tos. First and foremost, never dive with your boat unattended. It may break free from its mooring or you lose control and drift on the surface downstream; either way you will find yourself wishing someone was in the boat to come to you. As well, it may be stolen; salvage claims can be avoided by flying the dive flag.
Do not anchor, or surface from a dive, in the main shipping channel. Those lakers and ocean going freighters are 600 foot long walls of steel; it takes them half a mile to stop so they are just going to plough straight through a small zodiac.
Frequently you will be mooring on a site's buoy along with many other boats of all sizes, from 8 foot zodiacs to 60 footers. When approaching, go very slow with someone in the bow to keep an eye out for divers. Hail the other boats to enquire if they have diver's down. Do not all try to moor directly onto the buoy; tie yourself off onto the boat that was there before you. When they go, you move up. You can still have divers in while in line like this. If a larger boat comes along, it makes good sense to allow it to moor directly and you tie off behind. You do not loose or delay a dive like this but makes it very much safer.
Only fly the dive flag when you have divers down, or it looses its meaning and relevance.