Quick primer on Canals, Historical Background, Ships, for Use Of:
1900 BCE (Ancient Era) The Egyptians built a canal to haul boats around one of the cataracts on the Nile, increasing the useful length of the river, basically.
1380 BCE (Ancient Era) first attempt at a Nile-Red Sea Canal
approx. 600 BCE (Classical Era) Corinth in Greece had a 'shipway', a smoothed passage so trade and warships (up to Trireme, at least) could be hauled across the Isthmus of Corinth. Not sure of the date of the original shipway, but by the 7th century BCE they had carved out a genuine sea-level canal across the Isthmus.
approx. 600 CE (early Medieval Era): Grand Canal completed in China. This included some primitive locks, again was primarily to lengthen the usable stretch of the river system and improve internal communication and trade.
1600 CE (Renaissance Era) Rhone and Seine Rivers connected by a canal in France
Between approximately 1600 and 1800 CE the British Isles, France, and the Low Countries were covered with canals for barge traffic, which included locks to cross raised ground and even tunnels to take canals from one side of a mountain to the other
1681 CE (late Renaissance Era) Lanqueduc Canal completed linking the Atlantic Ocean and Mediterranean Sea: a genuine sea-linking Civ VI-type canal.
1869 CE (Industrial Era) Suez Canal completed
1914 CE (Modern Era) Panama Canal cmpleted
1914 CE: Houston Ship Canal opens, a 50 mile long waterway between the city and Galveston Bay that turned an inland city into a Deep Water Port.
1933 CE Baltic Sea - White Sea Canal opens in the USSR: another sea-to-sea canal, this one built almost entirely using convict labor.
To Summarize, sea-level canals across flat land (Civ Terrain: plains, desert, grasslands) were built starting in the Ancient Era, but they were horribly expensive (the Red Sea Canal silted up constantly, required constant, expensive maintenance) and so few and far between.
In the Medieval Era in China, Renaissance Era in Europe canal locks were invented, allowing canals to be built across hills.
The earliest use of canals was to 'improve' rivers to get around obstacles (rapids, cataracts) and improve Trade and Transport/movement along rivers, or even build 'artificial' rivers (Grand Canal in China, canals in Renaissance/Industrial Era Europe).
This feature seems to be still entirely missing in Civ VI.
Canals could be enlarged when they got too small for shipping. The Kiel ("Kaiser Wilhelm") Canal, for instance, was built across the 'neck' of the Danish peninsula to allow Germany's High Seas Fleet to get from the Baltic to the North Sea without going through Danish waters. First completed in 1895 CE, by 1907 it had to be enlarged to accommodate the new Dreadnaught-type battleships. It was finished in 1914, just in time for World War One. Note that even though it was a High Priority Project, it still took 7 years to enlarge, or about 50% longer than it took to build a Battleship from scratch.
In short, we got about half the canal features possible in Civ VI Gathering Storm. Better than Nothing that we got in all previous Civs, but still incomplete.
IF they had a realistic Trade system that reflected the importance of rivers to trade before the railroad, then a 'river improving or expanding' canal system would make sense in Game Terms, and a Grand Canal or Erie Canal Wonder could also be implemented.
Maybe by Civ VII . . .
