Is it absolutely sure that the connecton with the avenue was the main entrance of Stonehenge? Could it not have been used (mainly?, sometimes?) as the main exit, the start of a kind of defendable corridor for people and animals when there was an attack coming from roughly the south of Stonehenge?
Stonehenge is not really close to a river, certainly not for Neolithic standards. If people mainly lived near the Avon, the community or communities near the West bank of the Avon (roughly to the east of Stonehenge) could be attacked from the back so to speak. If they herded animals around Stonehenge and there was an attack coming from the south of Stonehenge parallel to the Avon (and possibly coming from the Avon itself but more to the south), the attack was better annihilated before the attackers reached the locations where most people lived.
There is another element that suggests a defensive aspect of Stonehenge. There seem to have been important people around at Stonehenge. The content of several graves suggests that. If highly positioned people were more or less frequently present at Stonehenge, the probability is very high that defense was important at Stonehenge itself, that this defense was well organized and may have cost really a lot of effort.
I repeat that I do not claim that Stonehenge may not have had other functions as well.
I think it is interesting also to see Stonehenge from the perspective of, e.g., the Mississippi culture. The latter culture is a proof that stones are not at all really necessary to commemorate the death (and that mounds may frequently have to do with flooding). One could say the Indians of the Mississippi culture could have tried to get huge stones from the nearest rock formations. As far as I know, they never seemed to have tried to do that despite the fact that the Missouri, Arkansas and Mississippi are quite impressive rivers which offered significant possibilities for transportation.