Posted this recently on a similar but different question, may as well include it here because it fits:

Exploring an easy cave with a friend. Nothing tricky at all, just one way through, standing room all the way, about 1m wide, ankle deep water flowing through the whole way (walking against the flow).

As we went, the water very slowly became harder to press forwards against. The change was so gradual we were second-guessing it the entire time until it got really strong. We figured it was better to walk against it than with it - at this point it was rushing against our legs, and the thought of slipping and being swept through, bouncing off of the walls, was not great. It felt much easier to keep our footing facing the flow, and also it seemed like we were much closer to the end than the beginning (the cave had an exit at both ends, it was basically a small fork of a river that cut through a hill).

So we pressed on, until we got to a point that should have been a small scramble up a few bits of rock - except now there was a massive flow of water hitting us at chest level as we tried to climb it. We were both completely unable to push against it and get up. We were also now convinced that the cave was filling up with water so we had to get out - which now meant turning around and doing the whole length again but with the water hitting the back of our legs the whole way.

Oh and the water was freezing, coming off of some snowy mountains. So for about an hour, we held onto the sides of the cave and slowly tried to move steadily through, while by this point I had almost no feeling in my frozen feet to help with keeping my footing. It was like guesswork every step.

By the time we got out, the water had risen by almost a metre I'd say. Not much but the extra force was insane, and the feeling of a cave filling up with water behind you was not easily ignored. Anyway, turns out there was heavy rainfall way up river from us, always check the forecast and think beyond where you are when dealing with rivers and caves!