There is no ferry from Rijeka to Zadar unfortunately. You have to drive from Rijeka to Zadar. Its about 140 km between Rijeka and Zadar. There are bus connections from Rijeka to Zadar. Buses to Zadar are very frequent. Rijeka bus
information
Since mid summer of 2007 between
Pula to Zadar there is a fast catamaran connections. This only passengers ferry runs only in high season.