In this paper, we describe and study a phenomenon unique to bufferless optical burst switched (OBS) networks called the streamline effect. This is the phenomenon wherein bursts within an input stream to a core node only contend with those from other input streams but not among themselves. It causes the burst loss probability at a link to depend strongly on the number of input streams to the link and their relative burst rates. We analyse the streamline effect and provide a burst loss probability formula that is more accurate than the traditionally used Erlang B formula. The formula is then used in a load balancing scheme for reservation-based quality of service (QoS) traffic to give a better link cost function. Through extensive simulation experiments for different traffic scenarios, we show that the proposed load balancing scheme utilising the streamline effect performs better than the shortest path routing scheme and the load balancing scheme without considering the streamline effect.