Have a look at the fourth photo at the link below. Notice the concrete base once again. Allsion was found just below that concrete base. Also notice how the bank just drops off. Allison was placed there on 19-20 April. She was found on 30 April and there had been significant flooding about that time. It's possible that the flooding continued to damage the creek bank where she was found for some time after, hence the erosion of the creek bank that we see in Mani's photos.
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The first picture, showing how dense the undergrowth is on the southern side, makes me think he is unlikely to have made his way through that in the dark of night, and then down a bank towards water as well, especially with a heavy awkward body to carry, pull, drag etc.