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Water Depth

How is the depth of the water determined?

There are at least two reasons why it is important to know the depth of the water below the surface: to prevent running the vessel aground and to be able to relate scientific findings to the depth of the water from which samples are taken. Many water quality parameters such as temperature and dissolved oxygen vary with depth as well as with the time of day. The depth of light penetration, which is influenced by turbidity, has an effect on the productivity of plants in an aquatic ecosystem. Various depths in a lake or river host different assemblages of benthic (bottom-dwelling) organisms. Plankton and fish move from one depth to another based on changing environmental conditions.

A simple and old fashion method for finding the depth of water is to lower a weight attached to a rope over the side of the vessel. When the weight touches the bottom, the rope becomes slack. The rope is then pulled back on-board and the length of the rope needed to touch the bottom is determined. This is a slow method and is not very useful if the vessel is moving very fast. If the water is very deep, it is difficult to retrieve the rope unless a mechanical winch is used. A faster and continuous method for determining the depth of a body of water is to use sound waves. Sound travels at a very fast speed in water, about 1500 meters per second in fresh water, so there is little delay in measuring the depth of water. For example if the water is 50 meters (about 150 feet) deep, sound waves will take approximately 0.07 seconds to leave and return to the vessel.

What is SONAR?

The technique for determining the depth of water is called SONAR. This is an acronym for Sound Navigation And Range. The use of sound in water to determine the direction and distance to underwater features was developed during World War II when it became a basic method for detecting the presence of submarines when they were submerged. SONAR is still used for that purpose, but on-board the D. J. Angus and the W. G. Jackson to determine the depth of water.

The principle is very simple. On-board the vessel is a sending unit that produces a short burst of sound in the water directed toward the bottom. The sending unit then becomes a receiving unit that detects the presence of the sound reflected from the bottom. Within the sending/receiving unit is a means of measuring the time between the pulse sent out and the reflected echo from the bottom. Since the speed of sound in water is known, the simple equation "d/2 = vt" is used to find the depth "d". The letter "v" represents the speed of sound in water and the letter "t" is the total time for the sound to leave and return to the vessel. The depth "d" is divided by 2 in the equation because the time "t" is the total time from the vessel to the bottom and then from the bottom back to the vessel. This calculation is done automatically by the depth finder.

What are the types of depth finders?

The depth to the bottom can be given as a numerical value or as the position of a line on a screen or a strip of paper. The depth finders (fathometers) on the D. J. Angus and the W. G. Jackson vessels have digital depth readouts in both the pilot house and in the lab. The depth readouts are calibrated to the actual surface depth.

A continuous strip chart depth finder is located in the main cabin. A quick glance at the chart can provide information about the depth of the water at that instant or for a period of time in the past. Marks can be made on the chart paper to indicate the location of the vessel for each sampling station of the trip. For example, the depth of the water in the Grand River can be identified and the location of the vessel noted as it moves up or down the river. Sample station numbers can be noted on the paper so that an examination of the bottom contours in the vicinity of the station can be made.
Above the continuous strip chart is another depth finder. This one has a line that shows the depth. If the scale is inappropriate please ask the science instructor or deckhand to check it. In addition to depth, it also provides surface water temperature.