Scuba Diving by Dennis K. Graver

Author:Dennis K. Graver [Dennis K. Graver]
Language: eng
Format: epub, pdf
ISBN: 9781492525769
Publisher: Human Kinetics
Published: 2017-08-14T16:00:00+00:00

General Freshwater Diving Conditions

Freshwater is 2.5 percent less dense than saltwater, so you are less buoyant in freshwater than in saltwater. The density of water varies with temperature. Freshwater is most dense at a temperature of 39.2 °F (4 °C)—the usual temperature and density of the water at depths greater than 60 feet (18 m) in freshwater lakes and quarries. The water becomes lighter when it is either warmer or cooler than this. Freshwater often forms layers: a layer of warmer, lighter water on top of a layer of colder, denser water. The change from the warm water to the colder water, called a thermocline, is abrupt. When the water is calm, the thermocline appears to have wisps of smoke on top of it when viewed from above. The refraction of light is different when the density of the water is different, so there is a slight visual blurring at the interface of a thermocline. When you dive in freshwater, you must insulate yourself for the water temperature below the thermocline. Although the surface of a lake may be warm and sunny, the water at the bottom may be close to freezing.

The best season for diving in a freshwater lake or quarry depends on the body of water. Spring and fall are often good times because the water temperature is the same from surface to bottom, there is oxygen for fish at all depths, populations of plankton are low, and visibility is usually good.

In later spring, the sun warms the surface water, and the lack of wind keeps the surface water from mixing with the colder water at depth. The water stratifies, and a thermocline forms that remains until fall. The layer of water below the thermocline stagnates in most lakes during the summer. Decaying matter depletes oxygen and creates toxins. Fish seek refuge above the thermocline. Sunlight and warm water often lead to plankton blooms, which are aggravated by pollution.

In the fall, the surface water in lakes and quarries cools until it equals the temperature of the water at depth. Colder water and reduced sunlight stifle plankton growth, and visibility improves. Winds cause water circulation. The movement of water from the top to a depth of about 60 feet, called an overturn, carries oxygenated water to the bottom and leads to what is called spring diving conditions. Visibility tends to be poor during an overturn.

In the winter, the surface water in a lake or quarry is colder than the water at the bottom. A reverse thermocline exists and remains until the temperature of the surface water warms to that of the water at the bottom. This allows the spring overturn to begin. Water becomes 10 percent lighter when it freezes. If ice sank when it froze, bodies of water could become solid ice from top to bottom. Ice that forms over water insulates the water beneath it. Figure 5.7 shows the annual cycle of freshwater lakes and the concept of thermoclines.

Figure 5.7 Thermocline, reverse thermocline, and the annual cycle of overturn in lakes.

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