First, you should know that you can't use "sb" to mean "somebody." This abbreviation is only used in dictionaries (to make the entries shorter). It isn't used anywhere else.
Most of your answers are pretty close. Here are my thoughts:
1) My guess is "although." In other words, the deserts have valuable minerals, BUT no one can live in them. (The word "deserts" should be plural here, by the way, since the verb "were" is plural.)
2) There are a few options here, but I think the simplest answer is simply "the." We're talking about the specific flooding that happens annually in October, as mentioned in the previous sentence, so the article "the" is appropriate. ("Then" sounds a bit strange, because it seems like you mean "next" or "after that." If you were allowed to use more than one word, you could say "at that time...")
3) The answer here is "even." The downfall of a dynasty would be an extreme example of a consequence if the rivers were too low.
4) "Like" seems correct for the second gap. For the first gap, "by" or "beside" would be okay. "On" is also possible, and would fit with the "beads on a thread" simile. Personally, I would prefer "along," because this shows that the villages are spread out along the side of the river (not all in one place), and this creates an image that is more like "beads on a thread." But the "correct" answer is probably "on."