Spring (summer?) floods are on, remember what happened to SUP'ers Marcus Steininger and Nils Hornischer at a low head dam in 2010. Had no idea there were so many hydraulic hazards beyond that in fast flowing inland waters, more at
http://www.paddling.net/guidelines/showArticle.html?566
Surface signs of underwater hydraulic hazards
Other things to understand and be aware of on rivers
Sections of some underwater hydraulic issues.
The above images are just to prompt your curiosity and get you thinking. You need to read the article they came from to get the full point at
http://www.paddling.net/guidelines/showArticle.html?566
.