A lot of the sail ships used on inland waters here have flat bottoms, like for instance the skûtsje. So they can sail in quite shallow waters.
[edit] I’ve looked it up and for instance a tjalk (historic freight ship) can have a mast height of around 20-25 meters, but has a sailing depth of only 80-200 cm. Due to their flat bottoms, they can easily move around the inland lakes and canals, which are often only a few meters deep.
A lot of the larger boats on inland waters here have flat bottoms, and the smaller boats are just not that deep. So it’s sufficient for the typical boat traffic.
Die of beaties