This is actually a funny saying that I heard as a child. It is a very regional saying. Years ago, dams were used much more frequently than they are now. One of the reasons was for flood control. When the water level rose too high due to storms or flooding, dams would sometimes fail and the towns that were 'downstream' or below the dam would flood, causing major damage and sometimes loss of life. 'If the creek don't rise' is another way of saying, 'as long as nothing bad happens'. If you are ever in the United States and find yourself in 'the south', or in the 'midwest' regions, people will almost always understand what you are saying. If you say it anywhere else, you may get a strange look.

They use “don’t “ in this context.... I’m guessing... because it is an informal conversation, possibly set in the early 1900s or earlier. That’s a regional way of saying “doesn’t “. A creek is like a little river. And when a river “rises” it means the level of the water goes up, as it would after a lot of rain. So “if the creek don’t rise” is a way of saying “if the level of the little river doesn’t go up” in colloquial language.