How did this fleshless femur roll along in a river, being damaged as it went, but part of the lower leg and foot (tibia, tarsals, metatarsals, and phalanges) remained protected and attached to it?
One word: podotheca.
The taphonomy of the Aniksosaurus darwini holotype, including an articulated right leg complete with femur, tibia, metatarsals and phalanges, has been examined in a new paper by Casal et al. (2013). Unfortunately I can't read Spanish, so I can't read the rest of the paper beyond the translated English abstract. And the paper doesn't appear to be available online.
But from what I can glean from the abstract, the authors found the fossil in an overbank deposit of a fluvial system - essentially, buried in an ancient river bank. Weathering patterns and crocodile tooth marks were found on the femur, that were not present on the rest of the lower leg bones. The authors suggest that the lower leg of the theropod was protected from weathering by a soft tissue resembling the podotheca: the scaly covering of the foot of a bird or reptile.
The femur (top most bone in image) has been stripped of flesh, as has the knee and part of the tibia/fibula (second bone from top of image). The podotheca remains below the ankle along the length of the foot. There would be little to no damage on the bones beneath the podotheca. And everything has remained articulated! It's an almost perfect analogue for the situation described for the theropod remains by Casal et al. (2013), sans river.
So would the presence of podotheca mean this rhea leg could remain articulated while transported downriver? As I can't read the rest of the paper beyond the abstract, I'm not sure what Casal et al. might have to say about that. But as for myself? ...Possibly, if the leg was only floating for a day or two before burial.
More importantly, the inference by Casal et al. (2013) that A. darwini had a podotheca-like covering on the lower leg could suggest that this feature first appeared in basal coelurosaurians.
Casal, G. A., Martinez, R. D., Ibiricu, L. M., Riga, B. G., Foix, N. 2013. Taphonomy of the theropod dinosaur Aniksosaurus darwini, Bajo Barreal Formation, Late Cretaceous of Patagonia (Argentina) [In Spanish, English abstract]. Ameghiniana, 50: 6.