This question has similarities to this one but concerns canal travel in more ancient times and is aimed at finding an approximate speed of flow not variables.
Assume an Earth-like world but with much less water. Desert conditions prevail over most of the surface and most of the water there is, is locked up in the polar ice caps. The human inhabitants have constructed a large canal network to move the water from the Polar Regions where there’s water but insufficient heat, to the temperate regions (3000km or more) where there’s heat but insufficient water. The original builders are long gone and the canal network is now operated by a more primitive people.
There needs to be a flow of water in the canals for irrigation on a massive scale, but the inhabitants also want to use the canals for trade. How fast can the water flow in the canals before the canals are effectively useless for two-way trade purposes? And what measures can the inhabitants take to avoid this problem?
The primitive people of this world can use any technology available to any human civilization prior to 400CE. Assume winds are similar to those on earth and would blow from different directions from time to time. The trade distances needed would range from tens to thousands of km. The canals can be up to 2km wide.
Out of scope the practicality of the canal construction and no magic.