I spent some time walking along 18th century British canals today, and at one point read that, via barges and canals, a single horse could transport as much as 40 to 60 tons. Obviously, this is a ridiculous amount for an equine-based system.
The question that semi-randomly came to me was, why did the Romans not have industrial scale canals? Certainly, they had the capability for vast quantities of labour, as evidenced by their roads, aqueducts, and other important infrastructure. But a little very cursory research tells me that the Romans largely used them for drainage rather than transport, and built only a fairly limited array of canals suitable for navigational use.
Why did the Romans not use canals more extensively? Did they not need them? Did they lack sufficient canal-digging manpower? Did they lack sufficient trade to justify the initial outlay? Did they simply not think of it? Or, perhaps, is there some other factor that I've misunderstood?
But, the Romans did use and build canals extensively, also for transport! A number of them were build for transportation of troops, such as the Canal of Drusus that linked Rhine delta with Lake Flevo, or the Fossa Corbulonis, that connected the rivers Meuse and Rhine.
But, there are probably a couple of things in play as for why the Romans did not build more big canals that were big enough for boats (this is purely my own speculation). Firstly, digging canals was much more time-consuming and difficult than e.g. building aqueducts. Imagine what it was like to dig and carve through earth and rock and hills before the invention of explosives! Canal projects that are useful as short-cuts for transport purposes usually need to be considerably long, so, building them in Antiquity could easily take decades even with an army of slaves. Megalomaniac canals did not therefore really suit the Roman political scene, local and Empire-wide, where euergetism and impressive building projects were an important part of creating an image and courting support. Ten years was simply too long to wait for a canal to finish when an emperor or local magistrate could reap the almost immediate political benefits of, say, construction of an aqueduct. The biggest aqueduct in Rome, Aqua Marcia, took only two years to complete (Sextus Julius Frontinus, The Aqueducts of Rome, 6–20). Also, Roman emperors were not often long enough in power to complete canals. For example, a canal through the isthums of Corinth was a favourite project on a planning level of a few Caesars, as it would have avoided the dangerous circumnavigation around the Peloponnese peninsula, but, Nero was the only one who actually started digging the canal but afer he died no later emperors felt like picking up his vanity project to finish it.
Also, the idea "let us build a canal to facilitate trade and boost economy" is quite a modern one. The Roman empire simply never was an unified entity comparable to modern nations where decisions for the common benefit where made from the center. Economic thinking wasn't very sophisticated and there's quite a lot of academic debate over whether any of the emperors ever had any sort of economic "grand plan". Big Roman building projects were usually dictated by immediate necessity (e.g. transportation of military troops, water reserves) or motivated by the desire for good old glory and prestige. A finished canal certainly did collect immense amount of respect points - the Romans quite enjoyed all projects that made a mark on the landscape and showed the 'triumph of man over Nature' - but building them was a risky and long business, and there were faster and easier ways to make your name immortal than financing a canal.
