This river and its shifting banks represent the international border between Borogravia and Zlobenia. The river itself is the nominal border; however, due to seasonal floods, etc., it regularly changes course leaving the border constantly in dispute. It is also home to a large fortress which also regularly changes hands.
The English word kneck is so obscure that it seems to have only one definition, in the Webster's Revised Unabridged Dictionary, 1913: (UK, nautical) The twisting of a rope or cable, as it is running out. The etymology is uncertain, not to mention the reason for its existence. The author may have chosen it to describe the undulating river.