Along the coastline, near Cabo do Norte, there are many islands partially covered with water. There are also sandbanks. The tides of the Atlantic generate a wave that reaches into the Amazon river. This wave goes along the coast for about . The phenomenon of this wave generated by the tides is called a tidal bore. Locally it is known as "pororoca". The pororoca occurs where the water is less than deep. It starts with a loud noise, and advances at a speed of . The bore is the reason the Amazon does not have a delta. The ocean rapidly carries away the large amount of silt brought by the Amazon. This makes it impossible for a delta to grow past the shoreline. It also has a very large tide, that can reach . The place has become popular for river surfing.