The ability of light to pass through water (and our ability to see clearly) is also influenced by its turbidity. Turbidity is influenced by many factors but they can be grouped into two large classes, things suspended in the water and things dissolved in the water. Suspended things include silt, plankton, etc. while dissolved things include salt and chemicals. When a diver is swimming along near the bottom, the water movement caused by their fins often stirs up the silt and for a while the water behind them has high turbidity this often results in death when open water divers enter caves. The water is crystal clear in front of them and they do not realize until they turn around that they have been kicking up the silt and they can no longer see the way out of the cave.

If the water is still, the silt will eventually settle back down to the bottom and the water will become clear once again. The time this process takes depends upon the size of the silt particles. Grains of sand will settle in a minute or two while the ultra fine grains of rock flour produced by a glacier will take several weeks to completely settle out. During a cave diving trip under the Columbia Ice fields in 1987 the water was ‘air clear’ when we arrived at the dive site. After we had made the area safe and several hundred (or thousand) kilograms (pounds) of mud and rocks had been dropped into the water, the visibility was less than 2.5 cm (I inch)! We went back to the surface and returned to the dive site two days later. The silt was so fine (glacial flour) that even though the water was absolutely still, the visibility had only improved to 30 cm (12 inches).

Salt water has large quantities of salt and other chemicals dissolved in it and for this reason, can never he as clear as fresh water. The best clarity to be found in the ocean would be around 60 meters (200 feet) while in some fresh water caves the water is so clear it cannot be seen, visibility is over 300 meters (1,000 feet). These ideal conditions are rarely encountered and we usually have much less visibility. For example, the Ottawa River contains water with many dissolved chemicals (from soil, trees, etc.) and visibility is seldom more than 6 meters (20 feet). The St. Lawrence River used to be similar but the infestation of billions of zebra mussels has changed this situation drastically. Zebra mussels are filter feeders and each mussel filters the silt and chemicals out of a large amount of water every day. The result is that places where 6 meters (20 feet) visibility used to be considered good now frequently have 30 meters (100 feet) visibility or snore. The problem is, now that the water is clear enough for divers to see well, the wrecks still can’t be seen because they are now covered in many layers of zebra mussels!