Strwiąż is a river flowing through Ustrzyki Dolne, which is situated in the Black Sea basin. It has its sources under Mały Król peak, west of Ustrzyki Dolne. Its length is 94 km and the catchment area is 955 km². Only the upper part flows through Poland, it is 13 km long and has catchment area of 193 km². It continues to flow north-east, where in the proximity of Sambora receives its largest tributary of the Błażiwka River, then turns south-east and in the area of Chłopczyce village gets to the Dniestr River.
In the Strwiąż River we can find protected species of fish, among others, Ukrainian lamprey. In the Polish Carpathians its presence was noted only in the Strwiąż River and its tributaries – Stebnik and Jasieńka. In the Strwiąż River (in the Polish part of the river) there are 5 protected species, in addition to the Ukrainian lamprey there are also alburnoides bipunctatus, sabanejewia aurata, Alpine bullhead and ground gudgeon.
The Black Sea catchment in Poland covers 0.2% of the area of the entire catchment area. Among them is the Dunaj and Dniestr basin.