Having made last minute preparations, the adventurers make their way to the northwest corner of the large cavern where a broad passage exits the cavern in a northerly direction. There is a spring that merrily flows along the passageway, generally along the western edge, but sometimes meandering more toward the middle for a short distance, and then returning to the western edge. You still have various light sources, of course, but you get the sense that the water in the underground stream is producing some light as well, either from a property of the water itself, or the streambed, or even some bio-luminescence from creatures in the water, perhaps.