the phrase comes from the stereotype that lesbian women like to go camping in the woods. While in the woods they presumably engage in sexual intercourse which leads to the "flooding".
Taylor: What did you do last weekend?
Jackie: I went camping with Jessica and we flooded the woods.
The point at which during vaginal intercourse, the female has pissed, and the male has not yet pulled his penis out of the vagina; The point before being floodgated.
The act of moistening one's finger(s) and sliding them down someones pants into their ass crack. Thereby mimicking the water drenched walls of a canyon after a flood.
Man my girlfriend doesnt stop licking me, she thinks its funny or something. I guess its time i end this with a flooded canyon down her Levis