I think we may be confounding two different things in our discussion.

Water soaking. We seem to agree that dumping a bunch of water on the ground isn't going to easily result in more water 5 km underground.

Simple weight. I think we may have a difference here. I completely agree that adding weight on top immediately adds pressure on the bottom. But, unless I am mistaken, reservoirs are quite deep. After all, look at the height of the dams used to make them. The river even in a massive flood isn't going to be that deep.

Let's use an analogy - ice on a pond. Which is more likely to crack the ice? A 200 lb person standing upright, or a 200 lb person laying down? I'm saying the reservoir is more like standing up where as the flooding is more like laying down. The weight is still there, but how it's distributed makes a difference.

In the end, we'd have to crunch the numbers to know for sure.

Brian