My Watermark is the Don River, Ontario.
During the massive Southern Ontario flooding in August 2009, I walked down to the local ravine, which followed the Don River. From a bridge above the ravine, I could see how high the river had flooded, all the way up to the tips of the trees.
Against the protests of my friends, I decided I wanted to go get a better look, and so walked around to a different (drier) access point, and began walking my way into the forest and ravine. Pretty soon, I was wading through shallow water, then deeper, until I finally took off my shoes and socks, and headed further down. It never got very high, nor was there a powerful current, but I still remember the sensation of wading through an out-of-control river, totally dominating the landscape and washing over the paths I normally biked in, took my dog for walks, went for Sunday runs in the park. It was impressive.