Don’t forget you have none of the benefits of wetland plants (cooling, regulated evaporation, food and habitat for wildlife), when the canals go stagnant they breed tons more mosquitoes because they don’t support the amphibians that eat them, it’s no longer an accessible water source for bigger wildlife, storm water is faster and more dangerous in a straight channel than in a natural stream, the concrete and the speed prevent water from infiltrating into the soil and maintaining the aquifer… I could go on.
