The Millenary Canal is featured in The Brigmore WitchesDLC. Located in Drapers Ward, it serves as the dividing line between the Hatters' and the Dead Eels' territories, and the majority of the conflicts between the two gangs, as they fight over the district, take place here.
Due to the water being rerouted by the Brigmore Witches, the canal is dried up, some of its bridges replaced with makeshift walkways after they collapsed into it. Oxrush Flowers can be spotted growing in the canal, and with some searching, a few valuables may be found as well.
The spelling "Millenary" meaning "[n.] a group of 1000 units or things" or "[adj.] suggesting a millenium"  is most likely a mistake, and in the mission notes the correct spelling of "Millinery" or "the business of making or selling women's hats" can be found.