Pororoca

The Pororoca (Portuguese pronunciation: [pɔɾɔˈɾɔkɐ], [poɾoˈɾɔkɐ]) is a tidal bore, with waves up to 4 m (13 ft) high that travel as much as 800 km (500 mi) inland upstream on the Amazon River and adjacent rivers. It occurs at the mouth of the river where its waters meet the Atlantic Ocean. It can be observed from the islands Marajó and Caviana, and in various rivers of the state Pará.