Tidal Currents

Computer Model Reveals Secrets of Tides
(Published in the Quoddy Tides, February 11, 2000)

Anyone who is familiar with Cobscook Bay knows about its tides. With an average height of 18-20 feet, the spring tides can push 30 feet in height during extreme weather. The strong tides and cold waters support an environment that allows an extraordinary number of different marine species to thrive in its waters and along its shore. Understanding these tides and how th e water that forms them moves about in the Bay can help us make decisions that will keep the Bay a productive economic resource.

David Brooks grew up in Eastport and is now an oceanography professor at Texas A &M University. He has been studying how tides flow in and out of Cobscook Bay for over ten years. His work began with studies of the possible impacts of tidal power projects proposed in the 1950's. His current research focuses on circulation patterns in the bay, that is, where the water goes and how long it stays. He gathers information from various parts of the Bay and enters it into computer models to produce moving pictures of how particles or nutrients dissolved in the water move with the tidal currents.

Each pulse of the tide into the bay carries with it the cold and nutrient rich ocean waters of the Gulf of Maine. As the tide recedes, it draws out excess nutrients, waste products, and pollutants deposited in the bay from feeding marine organisms and human activity. The twice daily flushing of the bay enhances the habitat for shellfish and other marine invertebrates, such as starfish, clams, or bloodworms, found here. It is also one important factor that influences the success of aquaculture within the bay. Brooks' computer models show us where the tides quickly flush out excess materials and where they are more likely to deposit nutrients or pollutants.

Some water stays in certain places in Cobscook Bay for a long time, while other water is flushed out on the next tide. Whether the water, and whatever plant or animal material it carries, lingers for a while or turns around and leaves depends on where the tides have carried it. Brooks' models show a pair of 'counter-rotating back eddies' in the central portion of the bay, near the mouth of the Pennamaquan River and at the head of South Bay. These eddies form when the huge volume of water on the incoming tide hits the narrow openings on each side of Falls Island. Not all of it can push through at once. The water forced backward creates these eddies, which influence how long water stays in various parts of the Bay.

The waters found in the innermost reaches of Whiting Bay and South Bay are flushed out at a much slower rate than the waters flowing between Gove Point and Birch Point. This flushing action creates different conditions that may benefit one type of marine animal over another. The abundance of scallops found in South Bay are probably due to slower cycling of the waters there, which allows more food and scallop larvae, or spat, to settle in this part of the Bay.

Another important source of information about Cobscook Bay and its tides is found in the knowledge of people like George Harris, who live around the bay and work its waters. Harris, a long-time Eastport fisherman and the city's harbormaster for many years, drew a series of maps for the Quoddy Spill Prevention Group showing how the waters move around the bay. They are remarkably consistent with Brooks' modeling results in many important ways.

Brooks explains, "Both show that the tidal flood seems to be felt first off Estes Head, where the current reverses and flows strongly inward while the ebb continues between Gove and Birch Points farther west." He believes that first-hand, local knowledge is indispensable and serves as a valuable resource in any study of the region.

This combination of local knowledge and scientific research is continuing and contributing to a larger understanding of how the Bay works. Currently, Chris Bartlett of the Sea Grant Program, Scott Fraser of the Shead High School and Heidi Leighton of the Cobscook Bay Resource Center are leading a joint effort to test the results of Dave Brooks research. Drifting devices are released at various points in the Bay and their movements mapped to show where the tides carry objects placed in the water.

This research has real importance to anyone who uses the bay. It can help the finfish aquaculture industry determine where to best site pens as well as determine the Bay's overall capacity for aquaculture activity. Brooks notes that most of the existing pen sites in the eastern and central arms of Cobscook Bay appear to be well chosen, in areas where the tidal currents are strong and excess nutrients are flushed out within a few days. It can also provide key information to local fishermen working on improving the bay's scallop fishery through collection and distribution of scallop spat.

As Brooks puts it, "Improved understanding of how the bay works will help local managers make sound decisions about economic development in the bay while maintaining its natural beauty and clean waters." A copy of his report is available at the Cobscook Bay Resource Center in Eastport. His computer models can be viewed at the following internet website: http://cobscook.tamu.edu

This column was prepared by Cheryl Daigle and Jim Dow. Cobscook Soundings was a monthly column produced by the Maine Chapter of The Nature Conservancy. Its purpose was to share what is known about the workings of the Cobscook Bay marine environment, so that all who make decisions about the use or care of the bay have the best available information. 

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