I think it is the 20th that she dumps her, the day her texts says she ran out of gas, this is the same area Tony picked her up at (I don't believe the 23rd date) I think she had the body in the trunk when she ran out of gas and had to dump the body, and did exactly what it appears, she walked into the woods and chucked her down the slope. I am not sure the body moved much in the water, if it got stuck on that tree they pulled out it easily could have remained about where it landed. The 20th is closer to the forensic data too.
I think the brush with George prompted her to dump the vehicle and start talking about the smell and squirrels to Amy.