You can stand by a tidal river and watch something that looks wrong. For a short time, the current seems to run upstream. People notice it in places like the Petitcodiac River near Moncton, New Brunswick, where a tidal bore can push water inland. It’s not magic and it’s not the river deciding to climb a hill. It’s the ocean’s tide temporarily raising the downstream water level higher than the water upstream. Flow follows the slope of the water surface, not the slope of the riverbed, so that slope can flip for minutes to hours.
What “uphill” means on a river
When someone says a river flows uphill, they usually mean one visible thing: floating debris, foam, or the main surface current moves toward the river’s source instead of toward the sea. That can happen even if the channel still points downhill, because the direction of flow is set by the water level at each end at that moment.
The overlooked detail is that rivers don’t flow “down the land” so much as “down the water surface.” If the downstream end is temporarily higher, the surface tilts the other way. The bed can still be descending toward the ocean, but the moving water is responding to the pressure difference created by water height.
Why it’s tied to the full moon
The full moon itself isn’t a switch. It’s a calendar marker for a common tide setup. Around full moon (and new moon), the Sun, Moon, and Earth line up more closely, which produces spring tides. Those are the tides with the larger range: higher highs and lower lows than average.
A larger tide range makes it easier for the sea to “win” against the river, especially near the mouth. The moment the incoming tide rises above the river level, the gradient can reverse and the current follows. How strong it looks varies a lot with wind, offshore pressure, and the amount of freshwater coming downriver that day.
The tidal bore effect in real places
Some rivers don’t just reverse quietly. They get a tidal bore: a moving wall or strong wave front that travels upstream as the tide rushes into a narrowing, shallow channel. The Petitcodiac in New Brunswick is a well-known example, and so is the Qiantang River in China, famous for a dramatic bore that can surge upriver.
This is mostly geometry and friction. A wide estuary that narrows inland can force incoming ocean water into a smaller cross-section, which raises the water level quickly. Shallow depths add bottom friction and steepen the leading edge. The bore is the tide trying to fill the river faster than the river can carry it away.
Why it’s brief, and why it doesn’t happen every month
The reversal usually lasts only around the peak of the flood tide. Before that peak, the sea level is still rising and pushing inland. After the peak, the sea level falls again and the river’s usual seaward slope returns. The switch can be sharp in a tight estuary and more gradual in a wide one.
Even on a spring tide, heavy river discharge can prevent any visible reversal. A big rain upstream can keep the downstream water level from climbing “above” the river for long, or at all. On the other hand, during low freshwater flow, the ocean can push much farther inland, and the upstream-flowing stretch can extend for kilometers.
Small details that change what you see from the bank
What looks like upstream flow at the surface can hide complicated motion below. Near the mouth, saltwater can wedge underneath freshwater because it’s denser. That can create a two-layer system where the surface moves one way while deeper water creeps the other way. From shore, it’s easy to miss that the “uphill” part might be only the top layer.
Another easy-to-miss factor is timing relative to the tide table. The strongest push upriver often doesn’t happen exactly at high tide at the coast. It can lag as the tidal wave travels inland and interacts with the river channel. So someone might blame the full moon, but the more direct cause is a particular stretch of river meeting a particular tide height at a particular hour.
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