Don't try to cross these areas of rushing water. You don't know how deep the water is, and you don't see the hidden obstacles lurking under the water. Maybe the water is shallow enough to cross, but you ride and then -- whoops! -- your front wheel hits a deep pothole and down you go into the water.
Outdoor writer David Whiting has his tips on safe stream crossings that focuses on hiking, but for urban cycling, I suggest taking an alternate route. The paths are all covered in fallen branches anyway.
If you do fall in the river and find yourself tumbling headlong in the current, you absolutely want to float on your back and position yourself with the feet pointing downstream. Head first, and you hit your noggin on a rock or tree and die. Go sideways, and the hydraulic force of the water will pin you against a rock or tree, and you drown. Feet first, and when you hit an obstruction you have a chance of flipping your head up and out of the water.