I'd be really interesting what you watched and what they are talking about. Rivers, as a general rule, aren't deep enough to hide caves, particularly ones that people once lived in. Additionally, the cases where I ruins being exposed after being hidden under water for years are occurring in the American West. My guess is you were watching a documentary about the Colorado River and the caves they were talking about are those that have been exposed in the last few years, because the water level in Lake Powell (and possibly Hoover Dam) dropped alarmingly.
Assuming that is what you saw, it doesn't show what you are thinking -- those caves were well above water level prior to the dam being built. The issue is that the water has been largely dropping for the last couple of decades and, a couple of years ago, reached a critical stage. While the winter of 2022-23 helped a lot, the fact remains that the levels in those lakes (and most other western reservoirs) are still at a critical level, with serious concerns that the west getting less water due to climate change, combined with the increased population in the western states, there will be critical water shortages (which, due to loss of hydropower, assuming water levels keep dropping, will also result in power shortages) in the Western US in the next few decades.