Stonehenge alignment

Environmental archaeologist Mike Allen and Charley French noticed the ditch and bank of the Avenue were dug alongside a pair of natural chalk ridges whose orientation was on the midsummer sunrise in one direction and the midwinter sunset in the other – see British Archaeology 110 (Jan/Feb 2010). This point is in Mike Parker-Pearson's recent book Stonehenge, Simon and Schuster:2012. In 2009 GT Frontline, a ground penetrating radar team confirmed the presence of the two parallel ridges (a natural feature) and a third ridge further to the east (as seen on BBC TV). It seems the stone circle is located on to the end of a natural phenomenon where he alignment of the Sun is marked out by the contours of the land. This is why the Avenue starts towards midsummer sunrise, for some 500m, before changing direction towards the river, another 2300m. Hence, it was more important for Stoenhenge to be linked to the river than the Sun – why?