An uphill river
Jul 17, 2008
|The old 'double reverse', not an easy trick to figure out - Thursday, July 17
Today we really are going to Saint John, and high on our list of things to do there (actually the only thing on our list of things to do there) is to see the Reversing Falls.
This phenomenon is well worth the time and effort it takes to see it because it is unique in all the world. This is where the extraordinary tides of the Bay of Fundy not only make the powerful Saint John River flow backwards twice each day, they continue to make it flow backward for several hours after the tide has turned.
Anyone familiar with the oceans of North America knows they are subject to tides that ebb and flow twice each day. We also know that this action can interfere for a time with the flow of rivers, causing them to back up at the mouth as the tide comes in, and then start flowing again as the tide goes out. But only here in New Brunswick is this action so magnified by topography that changes in the ebb and flow of the tides reach far upriver and are delayed in their effect by several hours each turn.
Getting your mind around this is similar to trying to grasp how something made of many tons of metal can get off the ground and fly, or how burning gas in your refrigerator to create heat manages to freeze your ice cream.
A kind woman at the Reversing Falls Information Center explained the falls to us (several times) and I listened as she explained it to someone else several times too. The other guy was a middle aged geezer familiar with sailing, and he was having just as much trouble with it as I was.
I will try and explain it as best I can, but for a better explaination you might check out the following website which not only explains it in English but also in French.
The Saint John River drains a huge area of New Brunswick and carries that immense amount of water some 400 miles to the Bay of Fundy. At the Bay of Fundy it meets some of the highest tides in the world, and there the fun begins. Actually the real fun is several miles upstream from the bay where both river and tides are forced to flow through a very narrow gorge.
Over the centuries the action of the water has carved out subsurface features at the gorge that sometimes interfere, and sometimes magnify the flow on the surface, making this section of the river so treacherous that it can only be safely navigated for about 80 minutes out of each 24 hours. This quiet time is called 'Slack Tide' and it occurs once each turning (four in 24 hours) for about 20 minutes each time.
Slack tide is when the force of the tide coming in is equal to the force of the river flowing out, and the water at the narrows becomes calm for a few moments. The whirlpools subside, the surface gets flat and no water appears to be moving in either direction. Then the flow reverses from the direction it was flowing and begins to flow in the opposite direction. This continues for the next six hours, until the next slack tide signals another reverse and the process begins all over again.
This all sounds pretty simple, but the problem is that the slack tides are nowhere near the time of the changes in tide at the sea. Unlike places where the tides and geography are relatively 'normal' and changes in the direction of flow are immediate, here the changes at the narrows do not occur until hours after the changes at the bay. In the case of low slack tide, it follows low tide by almost four hours! In the case of high slack tide, it follows high tide by nearly three hours.
These unique features combine to make this place a source of wonder and fascination, and it is quite a thrill to stand on the bridge or one of the observation decks for half an hour and watch the flow slow down and come to a stop, and then slowly begin to move in the opposite direction. Equally as impressive are the distinct tide lines that form between salt and fresh water, and the swirling and billowing patterns the two dissimilar solutions create.
We arrived right at high tide and watched for several hours as the river continued to flow upstream, even as it was flowing in the other direction at the mouth. Then we saw it slow down and become calm, signaling the arrival of high slack tide. And moments later we watched as the water began flowing out again, something it would continue to do here until long after the tide started coming in at the mouth.
This action is seen as far as Fredericton, some 80 miles inland from the bay. At this point the tidal waters are actually 14.5 feet higher than the river.
We finally tore ourselves away and headed for our campground for the night, a private campground in Saint John where we have an appointment for tomorrow to get our engine serviced.