In french, we call this kind of start where you jump in the river "embarquement à l'américaine"... I wonder how you call this in english ?
"running start" would be the closest thing I could think of in American English, but it's not a perfect match. As it pertains to kayaking, a "running start" would be understood as some sort of momentum transfer while simultaneously hopping into the kayak.
But still, that definition is what a typical American would imagine if you said "kayak running start". So perhaps not totally identical to what I think you're describing.