view counter

Deciphering Chesapeake Tides

So many variables are at play it can sometimes be baffling

We arrived at our fishing spot at 9am, two hours after the predicted low tide. Consultations with tide and current charts told us that at our location about a quarter-mile below the Bay Bridge, the incoming tide would just be starting. It ­wasn’t; the current was still going out.
    Anchoring and expecting the change at any moment, we set out our chum bag and flipped our baits over the side. After an hour with no tidal change and no action, we headed farther south, reasoning that the outgoing tide would be starting earlier there. Again we were wrong.
    We debated going down the Bay farther still but decided to stick it out. Our fish finder was showing a substantial population in the waters around us. Logically, we concluded that all that we needed was a tidal change and an increase in current to get the stripers feeding. After all, the tide sooner or later would have to change, right?
    Undoubtedly that was true. Yet four hours later it became clear that it was not going to change while we were there. With the tide still inching out and our baits going untouched, we headed home.
    Tides are the result of the gravitational pull of the moon as it orbits the earth. Ocean tides are regular and predictable. It seemed inconceivable that in the Bay an outgoing tide could continue for over 12 hours.
    I decided to renew my acquaintance with how the tidal functions in our great estuary can behave so erratically. The Chesapeake, I was reminded, has a unique and vastly more complex tidal operation than the ocean.
    The moon sets up the basic tidal rhythm of two high tides and two low tides during a typical 24-hour period. But those tidal surges have to travel the length of the Bay, 200 miles. Much can happen in that distance, and many variables can impact the flow of tidal water.
    One of the more important variables is caused by density differences between heavier saltwater coming up from the ocean colliding with lighter freshwater from the Bay’s tributaries. Because of the Coriolis Effect, generated by the turning of the earth on its axis, the incoming tide is always stronger (and saltier) on the eastern side. The fresher water exits the Bay on the western side’s stronger outgoing tides.
    This difference between salt and fresh creates a stratification of Bay waters and generates a secondary circulatory current with the heavier saltwater tending to sink to the bottom as it moves up the Bay and the lighter freshwater tending to float on top and moving south to exit the estuary.
    There are also secondary currents and eddies created as the water moves over different depths. More than 25 percent of the Bay is less than six feet deep, but the channels coursing down its length often average 50 to 60 feet deep.
    Wind is another factor. Sustained high winds can delay, accelerate or even cancel tidal phases. Northwest winds associated with high-pressure areas can push water away from the Atlantic Coast, resulting in very low tides. Northeast winds and high pressure can create exceptionally high tides.
    The interactions of these many variables can also generate seemingly impossible effects. Occasionally currents flow in one direction on the bottom of the Bay and the opposite direction on the top. An outgoing tide that seems to continue for 12 hours can be caused by conditions some distance away and invisible to those experiencing the phenomenon.
    Considering all these forces, the overall accuracy achieved by our tide and current charts is remarkable. It wouldn’t surprise me if the old saying Just go with the flow was coined on the Chesapeake.