The sea sometimes becomes higher than the Amazon River, and sea water from the Atlantic Ocean flows upstream across hundreds of kilometers, mainly during spring tide (from February to April), because the vertical drop between upstream and mouth is very small. The back-flow is called "Pororoca" and can be over 6 m high and run at 15 to 20 km/h.
The area along the Amazon River is very old, going back to the Palaeozoic Era 0.2 to 0.5 billion years ago. For this reason, we can see various kinds of creatures in the tropical forest (called "Selva"(*)) and in the water. Over 2000 kinds of fish, more than in the Atlantic Ocean, have been found in this area, including ancient fish that have maintained the same shape for over 100 million years. Pirarucu, a representative ancient fish, can survive in drastically changing environments, such as flow-back of sea water and changing water levels, because it is capable of pulmonary respiration.
*Selva: Large tropical forest mainly growing along the Amazon River. Sunlight is almost blocked by trees, so it has little bottom weed. In contrast, jungles such as in Southeast Asia and Africa are penetrated by sunlight so weeds grow on the ground. |