Im a huge fan of surfing, here in FL, we actually do a lot of river surfing.
Cool I've never heard of river surfing.  Does that involve a sail?
No basically the water flows over a rock in high volumes causing a wave behind it.
Interesting.  What rivers in Florida have the best river surfing conditions?
River surfing
River surfing is the sport of surfing either standing waves, tidal bores or upstream waves in rivers. Claims for its origins include a 1955 ride of along the tidal bore of the River Severn.
River surfing on standing waves has been documented as far back as the early-1970s in Munich, Germany, today offering the world's largest urban surfing spot.
Standing waves.
In this type of river surfing, the wave is stationary on the river, caused by a high volume of water constricted by flowing
over a rock and creating a wave behind. It is a form of hydraulic jump. A river surfer can face up-stream and catch this wave and have the feeling of traveling fast over water while not actually moving.
River surfing conditions are created by a combination of underlying rock formations and specific water levels, i.e. not too much or too little water. Water flow is usually is measured in the SI-unit cubic meters per second (m/s) (alternatively in liters per second or cubic feet per