this phenomenon is called a tidal bore, caused when the leading edge of an incoming tide pushes against an opposing current or the direction of a river. This creates actual tidal waves. bestnya kalau dapat surf begini ...the surfers managed to surf for 5 miles..usually the ride might last for 20 seconds and covers only 50 or 60 yards..this phenomenon took place in turnagain arm, cook inlet in alaska recently.,. i can't swim anyway..hehee. credit to scott dickerson for the image.