Costa Rica - Arenal Observatory Lodge
We stopped for lunch in the valley between Arenal's two mountains.
As it was the middle of the rainy season, a huge thunderstorm soon drifted over us. The heavy showers were quite peaceful at the start, but conditions soon worsened. Our guide was the first to notice that the small stream we had crossed at the bottom of the valley was changing dramatically.
When we first crossed the river, the water was halfway up to our knees and crystal clear. When I made my second crossing back - a couple minutes after the start of the rainfall - the water was dark brown, close to my waist level and nearly impossible to cross.
I later found out that we were lucky to have our guide with us; the area we were in was notorious for flash flooding, and if we had stuck around too much later there is a chance we could have been swept away. Flash flooding was a foreign concept to me until this trip, but seeing the danger in person was quite sobering. It's hard to imagine, but rivers can turn to deathtraps within minutes if you are in the wrong place.