There are places around the city that are high enough not to flood horribly, but even if you live in such a place you still have to deal with Houston summers (April through October) when being outdoors for any length of time is miserable.
This is a minor exaggeration. Pollen aside, April is generally fairly pleasant. May is more dicey, but it isn't necessarily miserable. (I've managed to work from a coffee shop patio at least a couple days this month without having to call it quits in the early afternoon.) We can even have pleasant if humid days as late as early June IME.
But yes, summer can be rough. I generally say summer is 6 months when I'm speaking loosely to set context for someone who hasn't experienced it.
There are other bayous collecting runoff from other watersheds, and like others, these two streams running through central Houston have apparently become miles wider and dozens of feet deeper on a routine basis for many thousands of years.
In Wikipedia, the 1873 aerial drawing of Houston shows there is only one thing in the picture; a flood plain:
https://en.wikipedia.org/wiki/File:Old_map-Houston-1873.jpg
The developing Allen brothers probably didn't know it at the time, but the purpose of Houston was to sell "false estate" as if it was real estate.