Al-Hasa Lagoons near the towns of Hofuf and Abqaiq is the only large body of fresh water in the Eastern Province. It's caused by the over-flow from the Al-Hasa oasis. There is a long, narrow river which flows from Hofuf and Abqaiq towards al-'Uqair. From time to time the river goes underground beneath sand dunes for several kilometers before resurfacing. There are many pools connected to the river, ranging in size up to 250 hectares. The countryside around the river and pools is a mixture of sand dunes, salt pans and beautiful, wind formed sand fields. The larger pools are surrounded by reed-beds, with some pools protected naturally by their inaccessibility, as they lie amidst sabkha and sand dunes. The lagoons have been identified as an "Important Bird Area" by BirdLife International. Bird life includes the protected Houbara Bustards which pass through the area during the winter migration. The ponds are a popular stop-over for birds on their annual migration. In winter you can see significant numbers of Black-winged Stilt and Great Bittern and in the summer are breeding ground for Ferruginous Duck, Little Bittern, Ruddy Shelduck, Cream-colored Courser, Moustached Warbler (the only known breeding site in Saudi Arabia), and probably Savi's Warbler. During a number of aerial surveys of the area, large concentrations of wintering and migrating waterbirds have been recorded, notably Grey Heron, Little Egret, ducks including Ruddy Shelduck, Common Crane and small flocks of Ruff, and Black-tailed Godwit.