The sacredness of the URUBAMBA River.

The INCAS found a perfect way of living in one of the most inaccessible parts of the Andes, in Peru, the REGION lying BETWEEN the APURIMAC River and the URUBAMBA River.
The REGION was considered the heartland of the Inca Empire. It comprises the departments of Cuzco, Puno and Apurimac, bordered by the highlands and Amazon on the North and West; and by Bolivia and the Amazon on the South and East.
URUBAMBA rises in the Andes Mountains to the southeast of Cuzco. It originates in the Mountain Khunurana in the Puno Region, Melgar Province, near La Raya pass. The River's upstream change the name to its Aymara name VILCANOTA, which means "House of the Sun." It is called Vilcanota in the area where it comes above the ground (near Puno Region's border).
The section of the River that flows across the Sacred Valley of the Incas (exactly between PISAC and OLLANTAYTAMBO) is called WILLCAMAYU or HUILCAMAYO meaning "Sacred River."
Then it flows North- North-West for 724 kilometers when it is called URUBAMBA in Northern Peru, where it is partially navigable, before coalescing with the Tambo River to form the UCAYALI River.
The Sacred River then meets with the Apurimac and Ucayali Rivers, together with them it forms the mighty AMAZON River.
The REGION was considered the CENTER OF THE INCA UNIVERSE. Here they shared their habitat with mighty precipices, passes three miles high, granite canyons more than a mile in depth, glaciers and tropical jungles, as well as dangerous rapids.
The INCAS had excellent skills in building such stone structures all over the mighty Valley. Heavy granite stones were the primary materials. Extremely hard to cut, even with today's technology.
Why the Incas chose this complex way of building is not known. Certainly with today's eyes and minds  we can observe that the Inca buildings were done to defy time.