Navigable canals are an exception in the 1st millennium AD in central Europe – but not as rare as often thought. Although they form key elements of the European harbour network, only a few prominent examples have so far been subject of considerable (geo-) archaeological research, first and foremost the Kanhave Canal (Denmark) , the Fossa Carolina (Germany) , the Glastonbury Canal (Great Britain) , the harbour canals in Ostia/Portus (Italy) , the Fossa Corbulonis (Netherlands) , a canal near Spangereid (Norway) , in Avenche (Swiss) , in Tendu (France) and in Ephesus (Turkey) . Many more are poorly documented/published or only proofed in written sources, some of them only as planned projects which have never been realised. Furthermore, in many cases the navigability is not confirmed and the original purpose and de facto use is unknown . By means of canals, transition zones of the transportation network have been artificially modified. The construction of canals, especially their water depths and fairway width, reflects the specific requirements regarding the usability of inland harbours and waterways. These requirements result from the interplay between local to regional site conditions and vessel characteristics. In our paper, we present three case studies to discuss this topic for different types of canals, different transport zones and different periods. The pivotal point of our interdisciplinary team in Priority Program 1630 is the Fossa Carolina or fossatum magnum. The other case studies have been researched and published by colleagues from Denmark and the Netherlands, whom we are grateful for their excellent work.