Correct me if I'm wrong, but most rivers lie where the water table protrudes into open air. In other words, the water table level is exactly the same as the river water level. (Wikipedia has a good diagram.)
So now the question is, what happens if you channelize the river with impermeable walls and bed? E.g., the Los Angeles River, which has concrete walls.
What happens to the isolated water table now? It is no longer free to drain into the river, so where does it go? Does it drain away more into the ground? Does it stay more saturated than normal until evaporation? And doesn't this produce dangerous ground conditions and bad environmental effects?
Water needs to follow some cycle, or it will just fill up more and more till the whole region is a flooded lake or ocean. So I'm trying to understand the water cycle in this case.