In the months of February and March, the region's dry season when the rivers are shallower than usual, the equinox and changes in the phases of the moon cause ocean waters to swell, increasing the liquid mass of the Atlantic and causing it to travel violently into the Amazon. The resulting tidal bore, known as pororoca, or great roar, constitutes tidal waves that can reach 6 meters (19.68 feet), at a speed of 30 kilometers per hour (18.64 miles) and travel 800 kilometers (almost 500 miles) inland. Continue reading →