The Araguari pororoca is the most frightening, and dangerous, of all the Amazonian tidal bores. Feared and revered by the Tupi-Guarani Indians, who called it poroc poroc, or "great destructive noise", the tremendous Araguari pororoca is powerful enough to tear entire trees from the river bank in its fury...and captivating enough to tempt surfers from all over the world to ride its untamed waves.
The Araguari descends from the western Tumucumaque mountain range, flowing nearly 217 miles in a southeasterly direction, until it empties into the Atlantic ocean, at the north end of the mouth of the Amazon River. The Araguari pororoca is formed when the powerful Atlantic Ocean tide surges into the river basin in such a way that it creates a giant swell that flows upstream for several hundred miles at speeds of 20 miles an hour or more, and up to 30 feet high, making it one of the longest tidal bores in the world.
The Araguari pororoca forms at the mouth of the Amazon. Source: Petitcodiac
Not to be confused with a tidal wave, though equally impressive, a tidal bore is "a wave that forms at the head of the incoming tide in certain rivers or estuaries," explains tidal bore surfer and researcher Tom Wright of the Tidal Bore Research Society Web site.
Not all river's have tidal bores; tidal bores, in fact, occur in only a few places world wide. Nor are tidal bores determined by the power and grandeur of ocean waves pounding the local bay. Tidal bores require a large tidal range, usually difference of over 20 feet between low and high water. The size of the tidal bore depends on the shape of the river at its estuary. Bores form when incoming tides from a broad bay enter a shallow, narrowing river. The tighter the funnel, the sharper the increase in water levels.