Columbia subject headings:
Harvard subject headings:
Europe--Historical geography--Congresses.
Human ecology--Europe--History--Congresses.
Rivers--Europe--History--Congresses.
Princeton subject headings:
Canals--Europe--History--Congresses.
Europe--Historical geography--Congresses.
Human ecology--Europe--Congresses.
Inland navigation--Europe--Congresses.
Rivers--Europe--History--Congresses.