Šumava web site calls them. I would translate it as "flume" since these canals were built to transport logs, not ships. An intricate system of water flumes was built by Joseph Rosenauer in 1791 and some of the flumes were used as recently as 1961.
It is hard to believe today that these roads used to divide the world into West and East. On "our" side, there were watch towers, strips of smooth sand to see footprints, electric and barbed wires, and also land mines. The mines were eventually removed by the communists after many border patrol soldiers blew themselves up.
One good thing resulted from this nonsense: very well preserved nature and miles of excellent bike paths.
I rode over Knížecí Pláně, a high plateau with huge mountain meadows and tall solitary spruces, before starting a long and steep climb over an unnamed mountain below which the Vltava river source is. Vltava (Moldau) is called the mother of Czech rivers and it becomes Labe (Elbe) at the confluence in northern Bohemia, before traversing Germany on its way to North Sea.
This trip left many places still unexplored, as well as enough room for planning a longer bike tour across the whole Šumava mountain range. It is amazing that in the center of densely populated Europe, there are still places largely untouched by civilization, but with enough infrastructure to provide for safe and comfortable bike travel. Renting good quality bikes is also easy and cheap ($100 for an 8-day Specialized 29er bike rental). I can easily see myself bike touring here after I won't be able to ride at Tahoe anymore.