@hanswolters
You mean probably South Holland, not Germany. https://en.m.wikipedia.org/wiki/Leidse_Rijn

Yes that story fascinates me as an example of big **voluntary and cooperative** infrastructure project in medieval times. Unlike elsewhere, where big projects were undertaken by an authoritative ruler, in South and North Holland villages and cities cooperated on joint solving of problems of downstream silting of the Rhine river. Downstream towns had a problem with river navigation and it could have been only solved father upstream. Without presence of a united region government control, that is not an easy problem to solve even nowadays - look at e.g., the problems in upper Nile. I've read about this in a book on history of the Poldermodel. Thanks for the reminder.