I finally got GS and have had canal envy for quite some time now. Now that I have it, I don't understand the rules for canals. In all my cities save one, I cannot built a canal district. The rules sound really simple, flat land next to a city or a body of water right? What am I missing? Most of my cities have lots of districts and land improvements, but on open land I should still be able to build a canal right? I am hoping that the Panama Canal wonder is NOT a requirement to build canals.