Complex sentence: The river initially marks the Franco-Belgian border as it flows north, Then turns west and flows for to its confluence north of Noyelles-sur-Sambre, as a southern tributary of the Sambre.
Split into simpler sentences: The river initially marks the Franco-Belgian border as it flows north. Then it turns west and flows for to its confluence north of Noyelles-sur-Sambre, as a southern tributary of the Sambre.