In Blood Gorge, the intrepid party takes to the water and, heading downstream, is at the mercy of the tides and the fast-flowing river. Though the locations in this encounter have been named as the places along the Blood River in northern Yex, feel free to change the names as appropriate or move the whole river to a new location in your campaign world. As there is very little in the way of plot to this Critical Hits encounter (the river is the major NPC), it is easy to place into an ongoing campaign, whenever the players have reason for rapid travel. Blood Gorge makes for a perfect backdrop in any adventure.
The most important thing is that the party needs to get somewhere quickly and the overland route is slower and more dangerous than traveling by river. If this is to be a one-off encounter, not part of a larger on-going campaign, you will have to invent a motivation to drive the party onwards (perhaps an urgent message for a head of state or a valuable but perishable spell component for an impatient and powerful mage). Otherwise simply intersperse the various stages of the river down to Blood Gorge with the events of the other storyline, so that not every event drives the main plot, thus giving the story an added depth.