what's the "free canals" issue in Th. Nast Cartoons

In the Thomas Nast cartoon below, and some others from the Machine Boss era of urban politics, there’s a reference to “Free Canals”. What is it there to represent?
http://www.thomasnast.com/TheFeatures/NastFeatures.htm

The link doesn’t go to the cartoon, but here’s a little light reading on the controversy.