The Raša (Latin: Arsia, Italian: Arsa) in Croatian Istria is a major river of Croatia's Istria County.It is 23 kilometres (14 mi) long, and its basin covers an area of 279 km2 (108 sq mi).Its mouth is in the long ria of Raški zaljev/Porto d'Arsia, which is a drowned river valley scoured out when world sea levels were lowered, then drowned by the rising waters of the post glacial era. The Raša rises in springs near Pićan and flows south through a steep-sided valley before opening into the head of the Adriatic Sea. The river, although short in length, has an ancient history as a border.