Every summer, before the dams, the river rose, spread across the valley floor and fell again, leaving a layer of silt that made the narrow strip beside it one of the most productive places on earth. Egyptian religion organised itself around that event. The civil year was divided into three seasons named for it, akhet the inundation, peret the emergence of the land, shemu the harvest, and the new year was tied to the reappearance of the star Sothis at dawn, which coincided with the flood's beginning.
The flood was not itself a god so much as a divine act. Hapi, shown as a heavy figure with pendulous breasts and a belt, personified the inundation and was thanked in hymns for filling the granaries; the water was also spoken of as the effusion of Osiris, whose death and return the annual cycle repeated, and as the tears of Isis mourning him. Richard Wilkinson's survey of the Egyptian pantheon notes how many gods carry some responsibility for the river without any one owning it.
Its measurement was a religious as much as an administrative act. Nilometers, stepped shafts cut at temple sites from Elephantine to the delta, recorded each year's height, and the tax assessment followed from it. A low flood meant famine and, in the royal ideology, a failure of the order the king was supposed to maintain, which is why the seven lean years of the Egyptian tradition preserved on the Famine Stela are told as a religious crisis rather than a meteorological one.