Reading through Charlie Walbridge’s compendium of WhiteWater Rescues, he remembered the incident that started him compiling water emergencies for the American WhiteWater organization.
One respected member of the paddling community almost died while swimming after getting dumped on a normally safe paddling run. It turned out, that this person had managed to try to stand up during the swim, and his foot slipped into a hole between some boulders and got stuck. The water current was a little swifter than normal, and held the person down, so he could not surface for air.
Records over the years include several (many) accounts of people getting a foot stuck in some hole or between boulders on the bottom of a swiftly flowing river, and the flow of the river forcing the person to remain below the surface, and drowning, or almost drowning. 
I experienced that while playing around at Hidalgo Falls. Luckily, I was able to keep my head above water for the couple of minutes it took to free my foot, but it was close. 
There are not many locations near Houston that contain such dangers, but Hidalgo Falls is one of them. And any time the water is flooding, unseen entrapment can occur.
The normal advice if you tip out while going through rapids is: stay floating, with your legs near the top of the water, and in front of you going downstream. Paddle over to some shallows or an eddy where the water is still before trying to stand up. 

Stay safe out there!
See you On The Water!
Harmon

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