Shoals are also known as sandbanks, sandbars, or gravelbars. Bligh turned south-west, and steered through a maze of shoals, reefs, sandbanks, and small islands. In particular, waves shoal as they pass over submerged sandbanks or reefs. The coastline includes shifting sandbanks and sandy beaches. Canford Cliffs is south of Parkstone and north of Sandbanks. Armoured beaches and submerged offshore sandbanks may also protect parts of a coastline from erosion. The Rother estuary was always difficult to navigate, with many shallow channels and sandbanks. Beds of eelgrass occur on the more sheltered mud- and sandbanks. The most popular beaches are Wasaga Beach, Grand Bend, Sauble Beach, and Sandbanks. They roost and loaf communally on beaches, sandbanks, and in shallow water. It is particularly adept at colonising new riverside sandbanks formed after floods. The region of Barra da Tijuca was originally a large beach, with typical undergrowth sandbanks. The sand in these sandbanks comes mainly from the Waikato River. It contains environmentally significant habitats and large areas of sandbanks. African skimmers nest in loose colonies on large sandbanks.