Im almost inclined to believe that the river may not have been muddy. Do we know for a fact it was? It was February and if the water was shallow in that area, its possible it was frozen. This could also be reason he took them over that exact spot as well. He knew it was frozen. Maybe he wasnt wet or muddy? If there were muddy footprints across the river I feel like the girls may have been found sooner or at least they would know boot/shoe size and make.
The more I think about it, the more Im thinking the girls didnt try to run and escape across the river. Why would they chance that and having to run up the riverbank on the other side when they could have simply run to the path on the right and had a better chance of running into another person on the path. My current theory is he intended to take them across the river and to somewhere else but possibly heard Libbys dad calling for them so he panicked and killed them and got out of there.
The creek wasn't frozen. Here is RTV6 helicopter video taken after the girls were found, it's long and shows LE at various points walking in the water. You can see them at the 6:13, 12:20 and 19:00 marks (maybe later too but my computer is acting up). Water is various depths. 20:00 mark or so they walk arm in arm from what appears to be a specific point on the south bank (bent tree) to the north bank (girls found). Possibly shoe tracks on the banks (or items found) indicated that's where they crossed? Something made them focus on that line across the creek, IMO.
https://www.youtube.com/watch?time_continue=392&v=HvhKS1eRIMM