Diving in actual fresh water is generally straightforward. There are no tides and virtually no currents in most lakes (although some large lakes and inland seas there are). Lakes present an enclosed body of water and surface marker buoys are not usually needed except for training purposes or if other water users are present.
Fresh water is less dense than salt water. Consequently buoyancy will be reduced , typically by 2 to 3 kg. It is necessary therefore to carry less weight if you have been diving in salt water previously via the weight-belt.
The visibility of fresh water is largely controlled by the catchment area for the water in the system. Rivers very often carry large amounts of sediment (and always do after large rain) and, on occasions, this reduces the visibility to zero. It also reduces visibility in the lakes into which the rivers run and causes the bottoms to be covered in a layer of silt. If the river or lake is fed by water that has drained from peaty areas or healthland, then the water will often be yellow, or even brown, in colour. This will quickly reduce the visibility. Fresh water visibility can vary from zero, through 2 - 3 meters to over 30 meters on occasions.
Fresh water is often gloomy and the lack of life contributes to this feeling, as do the substantial dark depths. However, the shallows of most lakes are often clear, bright and pleasant.
Be careful not to stir the bottom unnecessarily with your fins. A small mud trail can be useful for finding your way back to your place of entry but heavy trails of silt will reduce the visibility to zero. This can easily jeopardize the safety of students when they attempt to increase their depth experience.
The bottom of all large areas of fresh water is at a temperature of 4 degrees Celsius. Most divers consider this to be cold. In general terms, the temperature of fresh water will be cooler than the sea (partly due to the water originating at altitude), and this coldness adds to the seriousness of the diving in fresh water.
Some lakes lie at altitude, so the appropiate corrections should be made to the measured depth for calculating decompression or no-stop dives.
Lakes and manmade areas of fresh water are frequently used for training, with drills such as underwater search methods being popular. The greater degree of control and the less changeable conditions make fresh-water sites particularly useful for practising such techniques.
Sometimes during the wintertime, divers are tempted to carry out dives under the ice of frozen-over lakes and rivers. There are several dangers to diving under ice, not least becoming lost and running out of air. It is straightforward to mention that ice diving is for trained divers only.
Moving Water Diving Techniques
Lake diving is relatively easy, but this is not always the case with river diving. Slow-flowing rivers and large river pools are usually quite safe, but fast-flowing rivers present a number of problems.
Shallow, rocky areas of the river only present dangers if there is substantial water flow and if the the diver cannot avoid being swept over the boulders. In these difficulties regulator mouthpieces may be ripped away from the diver and limbs damaged. If in doubt keep clear.
In fast-flowing rivers it might not be possible to reach the side immediately adjacent to the dive site. This is not normally a great problem since usually it is possible to reach the side and exit farther downriver when the rapid-flowing section has been passed.
There can be a good deal of turbulence in certain parts of very fast rivers. In the UK this is not a problem for the properly equipped and trained diver. There are much bigger rivers overseas, however, and it is certainly possible to get into very serious difficulties in some of the big tubulent eddies and 'stopper' waves.
When water is sufficiently agitated to form foam there is another danger. The amount of foam displaced by the diver is not enough to provide adequate buoyancy and the diver will consequently sink. This is not a real danger in streams and small rivers, but requires attention in larger rivers.
Fresh Water Diving Overview | Fresh Water Diving Techniques