While standing on the dock at a friend’s house on the Androscoggin at low tide, she asked a good question, “Are all rivers tidal?” It makes sense that the Androscoggin River is, as it flows into Merrymeeting Bay — a vast connector of several rivers and the sea. In thinking logically about it, it seems that at some point the tide would push its way up the river and cause its impact well after high tide in the ocean. But the Androscoggin has some pretty strong current in parts, and it seems surprising that the tide could overpower a current strong enough to generate power. I wondered whether the damming of the river might have dampened the overall flow of the current or segmented the river such that the tide had an easier time pushing its way up through the bay and into the river.

There are so many factors involved here that all led to the simple fact that there, way up the river, it was low tide. After doing a bit of digging, I was reminded of the fact that Merrymeeting Bay is a particularly unusual confluence of various bodies of water. In addition to the Androscoggin, five other rivers flow into the bay: The Cathance, Kennebec, Eastern, Muddy and Abagadasset all converge to create this inland tidal swirl of water where fresh and salty combine in various combinations depending on the many factors mentioned above. Apparently, there are only three other places like this in the world. The only other one in the United States is the Sacramento-San Joaquin delta in California. The other two are Iraq’s Tigris-Euphrates delta and Bangladesh’s Ganges-Brahmaputra delta. There’s an incredible book by local author, Frank Burroughs, “Confluence: Merrymeeting Bay,” that describes the rarity of what we have here in Maine.

Getting back to the question of whether all rivers are tidal, there are apparently a number of degrees of what it means to be a tidal river. For some, the tide only impacts a portion of the river known as the “tidal reach.” This describes how far the salty water, also known as a “tidal bore,” travels up the river. For others, the tide may impact the entirety of the river, sometimes even overpowering the outgoing water flow and thus temporarily reversing the direction of the river. One of the clearest examples of this flow reversal is in the form of “reversing” fall. Maine is the only state on the East Coast that has reversing falls, owing to its unique geology. There are eight sets of reversing falls here spanning much of our coast and several different rivers. A good list of these can be found at seagrant.umaine.edu/2016/09/01/experience-the-rare-wonder-of-maines-reversing-falls. In a third category of flow situations, the incoming tide pools rush in as the river water tries to rush out and results in a blockade of sorts that creates an estuary. This, in part, describes the formation of Merrymeeting Bay.

On the flip side, as water rushes out through the bay, it goes back into the ocean. Along with it goes a lot of fresh water, which contains sediment and any other inputs, physical or chemical, made at the many points along the way. Merrymeeting Bay drains an impressive amount of fresh water — somewhere close to 40% of the freshwater that Maine contributes to the Gulf of Maine.

The simple answer to my friend’s question is, “No. Not all rivers are tidal.” But it turns out that there is a much bigger gray area in between what is and what is not considered a tidal river than I’d previously understood. It is questions like these that are important not just in appreciating the powerful forces at work in our local waters but also the incredibly unique set of waterways and watersheds that connect Brunswick to many surrounding communities. From there, it is easy to make the case for collective stewardship of our resources, and the first step is to take a little time to learn more about the landscape where we live.

Susan Olcott is the director of strategic partnerships at Maine Coast Fishermen’s Association.