Three points come to mind. First, the economic externalities and path dependent effects of the efforts to control commerce on the Rhine today generate a significant economic benefit in the form of tourism as riverboats ply the waters. The long hand of economic history has created a stunning visual panorama of castles that now capture tourists in hotels and museums. Along with major riverboat cruise lines of Croisieurope and Viking, there are numerous smaller operators, which provide excursions and longer voyages along the Rhine and through the lock system to the rest of Europe via the Danube or the Rhone. I have not been able to find a specific estimate of the economic impact of Rhine River tourism but it must be substantial given the day visits provided into surrounding towns.
Second, there is the microeconomics of entry, exit, and pricing as applied to the rise and fall of Rhine towns and castles. Numerous castles at key choke points – often about every five kilometers – were erected for defensive and tolling purposes. An interesting unpublished paper by Gardner, Gaston and Masson points out that between the years 800 and 1800, 79 different locations served as tolling stations along this commercial thoroughfare. While the rights to toll were awarded by the Holy Roman Emperor, from time to time there were outbursts of non-officially sanctioned tolling activity. As the authors write:
“…an Emperor faced a classic complementary monopoly problem: how many toll stations to have, where to site them, and what toll to charge at each. As a basic part of the answer to this problem, Emperors tended to keep the number of stations low. For instance, in 1250 an important date in our analysis--there were 12 stations on the Rhine between Mainz and Cologne [Pfeiffer, p.332] . Siting was a complicated decision, whose components included the local power structure (powerful ecclesiastical or noble interests were likely recipients), spacing (a 5 kilometer minimum seems to have been observed), and defensibility (some of the castles which acted as toll booths survived as military structures until the French invasion of 1689).”
While decision-making was quite decentralized, the authors argue that in the end the parties in engaged in collusive practices that resulted in joint profit maximizing Nash Equilibria.
Third, the Rhine also offers a comparison between the relationship between transportation routes and political development. In Canadian economic history, the Great Lakes-St. Lawrence system facilitated penetration of the continent and centralized control from the East first by the French, then the English and finally the Dominion government. In the case of the Rhine, it appears to have facilitated decentralized political development and control that remained so well into the 20th century. The wealth of the surrounding agricultural hinterland must have played a role in generating the numerous nodes of population and economic activity. The Great Lakes-St. Lawrence on the other hand was much longer and the fur trade activity did not generate the wealth and density of activity that invited numerous forts and tolls. On the other hand, it can be argued that the Iroquois made an effort to capture benefits from the fur trade by eliminating other middlemen along the routes. By the time agricultural settlement and industrialization began to generate greater wealth, centralized control had already been established. No castles on the Great Lakes in the long run unfortunately.