a seasonal desert watercourse: the ravine that channels rushing water in the rainy season and lies dry the rest of the year, and the rushing water itself
In the dry climate of the Bible lands water rarely runs all year. A נחל is a watercourse that floods with a violent torrent when the winter rains come and stands as a dry stony gully through the long dry season. The single word holds both pictures at once, so the same term means "the rushing stream" in one verse and "the dry ravine" in another, depending on what the writer is looking at.
Because the channel is a fixed feature of the landscape even when empty, a נחל also serves as a natural landmark and border. Many of its uses are geographic: "the Wadi of Egypt" marks Israel's southern edge, and the Valley of Eshcol, the Zered, the Jabbok, and the Kishon all name particular ravines. In these the focus is the place, not the water.
The two pictures are not two different words but two sides of one thing, which is why they are kept here as senses of a single meaning rather than split apart. Writers play on the contrast: a stream that should be reliable can betray you by drying up when you need it most, while a justice that never fails is one that flows like a wadi that never runs dry.