This small mangrove island and the nearby coastline contain extensive habitats of significance for waterbirds, crocodiles and local fisheries.
Pulau Alang-besar, about 25 miles SE of Tanjung Pejudian, lies in the entrance of the Sungai Rokan.
The approach to this river is fouled by mudbanks. Off Tanjung Belanda, the E entrance point, and Tanjung Sinaboi about 16 miles ENE, the coastal bank dries out from 2.5 to 6 miles.
A shoal, with depths of less than 10m, then extends about 26 miles NW from this section of coast and about 25 miles N from Pulau Alang-besar.