In the nineteenth century, a sailor on a ship near the mouth of the Amazon River was
startled by a rumbling sound like distant thunder. He looked into the distance to see a
huge, white wave of water approaching the vessel “until it seemed as if the whole ocean
had risen up and was coming, charging and thundering down.” What he witnessed was the
Amazon River tidal bore, or the pororoca, a single wave that can reach fifteen feet high
as if gushes upstream against the current at fifteen miles per hour, momentarily
reversing the flow of the great river.
Tidal bores occur in about sixty wide, shallow river estuaries around the world when
tides are especially high. At the mouth of the Amazon, the topography is favorable
enough that a bore rises twice a day with the high ride, on the days of each month when
the new and full moons pull hardest on the tides. The pororoca’s wall of water is raised
when the incoming tide is pushed even higher by the shallow river bed and then surges
over the opposing stream.
Traveling upriver, the bore’s swell may not be higher than a few inches. However, the
churning action of a crested bore can be high enough to flip boats, flood riverside
dwellings, and uproot tall trees. Surfers ride upstream for miles on England’s Severn
River bore.
The strangest variation of a tidal bore occurs on the broad, sandy tidal flats that
stretch for miles between low- and high-tide marks at Mont-St.-Michel, a monastery off
the northwest coast of Normandy, France. At certain instances, the rising sea pours
across the sand in one wave faster than a horse can gallop.
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