The Pororoca ([poɾoˈɾɔkɐ]) is a tidal bore, with waves up to 4 metres high that travel as much as 800 km inland upstream on the Amazon River and adjacent rivers. Its name comes from the indigenous Tupi language, where it could translate into "great roar". It could be also a Portuguese version of the term poroc-poroc, which in the native indigenous' language was a way of expressing the act of destroying everything. It could be also a portmanteau of the words poroc (to take out, to tear away) and oca (house). It occurs at the mouth of the river where its waters meet the Atlantic Ocean.
During new and full moons, when the river is relatively less deep and the ocean tide is high, water flows in from the Atlantic, rather than the other way around. As river and ocean collide, the Amazon’s flow reverses and a water bulge speeds upstream with incredible force, forming a tidal bore with rumbling noise. The tidal phenomenon is best observed on biannual equinoxes in September and March. On an equinox, when the Moon and Sun fall into alignment with the Earth, their gravitational pull is combined, bringing the Pororoca to its peak.
The wave has become popular with surfers. Since 1999, an annual championship has been held in São Domingos do Capim (on the adjacent Guamá River). However, surfing the Pororoca is especially dangerous, as the water contains a significant amount of debris from the shores of the river (often entire trees), in addition to dangerous fauna. In 2003 the Brazilian Picuruta Salazar won the event with a record ride of 12.5 km lasting 37 minutes.