Chris,

It does seem that Sirius was at least associated with the New Year and the height of the Nile (hence inundation) rather early, since by the 6th dynasty the seasons were already out of whack. Breasted writes in Ancient Records of Egypt (vol I):
In the Sixth Dynasty, Uni, a nobleman who had
been sent to Assuan to procure granite from the quarries
there, narrates that he succeeded in landing his cargo at
the king's pyramid, although it was in the eleventh month,
when, he adds (as everyone knew), there was no water for
such transportation. The time of advancing low water,
terminating heavy transportation of this sort, normally in
the eighth to the ninth month of the calendar, thus fell two
months later in Uni's time.

Ritva