It gets way more complicated than that. Toronto has rivers running through it so the water tends to flow towards those and collect. Then the river swells up. If the city was planned properly, there shouldn’t be anything permanent (like homes) in the flood plain. There’s also a storm drain system and a sewer system. That tends to get rid of water for a while until it overflows and backs up. Then some of the collected water can all end up in one area.
