Tidal Bore (pororoca)

The tension between the river's strong push and the Atlantic's tides causes a phenomenon called a tidal bore, known locally as the pororoca, in which the leading edge of the incoming tide forms a waves of water that travel upriver against the direction of the river's current. Tidal bores occur at the mouth of the Amazon and other nearby coastal rivers several times a year at high tide. Tidal bores also occur in other river around the world, but the Amazon's are among the world's highest and fastest, perhaps second only to those of Qiantang River in China. The pororoca flows up to 4 metres (13 ft) high and running at up to 13 miles per hour (21 km/h).

The pororoca occurs especially where depths do not exceed 7 metres (23 ft). It starts with a very loud roar, constantly increasing, and advances at the rate of 15–25 km/h (9–16 mph), with a breaking wall of water 1.5–4.0-metres (5–13 ft) high that may travel violently several kilometres up the Amazon and other rivers close to its mouth. It is particularly intense in the rivers of the coast of the state of Amapá north of the mouth of the Amazon, such as the Araguari River, but can be observed in Pará rivers as well.

The region near the Amazon's mouth also has very high tides, sometimes reaching 6 metres (20 ft) and has become a popular spot for river surfing.