The introduction will have the camera pan from the destination to the characters, showing the path to take. The camera stops at the end of the stream, where the four survivors get to share one sub machine gun, one magnum, and other melee weapons. Once you are ready, advance down the river flow. Note that the horde is more aggressive at this point, since they can easily approach from the cliff walls and foilage.
The stream will end at a rock wall. Walk around it by the left, and climb down the bridge.
The stream turns away, allowing you to proceed by land to the small shack, where you can find a few items. Climb to the second floor, and jump out the window.
Walk across the fallen tree, and enter the drain pipe. The safehouse is at the end of the pipe.