This is a fantastic documentary that explains exactly why, how, and where the floods happen in Sao Paulo, long story short, there was a big debate in the 1920's, some smart politicians wanted to keep a 1 kilometer area around all the rivers free of "urban" development, so the rivers could be "RIVERS" and flow as nature intended, with vegetation and areas that could flood without causing any harm to anyone.
The city tried to mirror itself in large American cities such as Chicago with large Avenues and giving total preference for an "INDIVIDUAL" mode of transportation ( CAR ) versus trains, subways or buses.
The idea was to built roads and avenues so they could SELL, SELL, SELL more and more cars and fuel the beginning of the Brazilian industrialization process.
The bad guys won, Mr. Prestes Maia ( Sao Paulo's mayor in the 1930's) was one of them, he thought the river basins should be used to built the city's many avenues, because it was a cheaper area to built the avenues, they didn't have to buy too many properties, just follow the valley and built the avenues along them.
Sao Paulo became a city of underground rivers, there are hundreds of small, medium and large rivers that have been paved over and sealed deep underground, but they still exist and are still running under the city.
So, next time you are driving in and around Sao Paulo and it starts to rain, STAY AWAY FROM THE RIVERS, avoid MAJOR AVENUES AND ROADS, its the time to literally run for the hills.
Like I have written before in another post, I usually go into a Mall, catch a movie or have dinner, by the time you are out of the Mall, the floods are usually over and you can go home.
Now, this video is a great Geography class if you are interested in learning about Sao Paulo and understanding the city, why it is the way it is, and you will understand why it floods where it floods.
The documentary is in PORTUGUESE, however, the narrator has a very clear Portuguese and it might be easy for you to understand if you have a basic Portuguese, not to mention it is very visual and fun to watch.
"ENTRE RIOS" - a urbanização de São Paulo