Hi again Kazuo
1. I have ever read an explanation about it. It says as follows.
Supposing a person who has three houses, which are a house on a river, a house by a lake and a house on a mountain.
Of these three houses, “the house on a river” refers to one of them.
I completely disagree. I would use "the river" because it's a specific river. You don't even have to know the name of the river, because it's the specific river that his house is on. That knowledge -- that his house is on this particular river -- is enough to use the definite article.
Let's say you know a very hard-working man. He has a regular-hours job in an office, and an after-hours job at a restaurant, and a weekend job at a park. You don't have to know the name of his company or where the office is, nor which restaurant he works in, nor which park to refer to "his job at the park" or "his job at the office." It is "the park that he works at" and "the office he works at."
Was the description you provided above written by a native speaker? A native speaker uses articles instinctively. It's very, very hard for a non-native -- especially one who does not have articles in his or her native language -- to understand all of the nuances.
The novel quote is very odd in many ways, isn't is? Can you picture people continuing their life uninterrupted while a river has run through their home, simply using a boat to move from room to room? This is one time that you could possible use "the house" but "a river" because the river in their home is not a real river, but a temporary one, and won't exist after the flood recedes. I'm usually good at coming on with examples where things work, but I would never have thought of this!