What is "aiviekste"

The Aiviekste is a river in Latvia. It begins at Lake Lubāns (the largest lake in Latvia) and drains into the Daugava (the largest river in Latvia). The Aiviekste is the largest tributary of the Daugava in Latvia. The Aiviekste, along with the Pededze, forms the border between Vidzeme and Latgale.