Still point in time

According to Dorium Maldovar, still points in time made it easier to create fixed points in time. The Kovarian Chapter of the Silence used one such point in space-time — 5:02 PM on 22 April 2011 by Lake Silencio — to make the Eleventh Doctor's supposed death there a fixed point.

Instead, the Silence merely assumed the Doctor died at Lake Silencio without realising he could and would find a way to fake his death thanks to the time-travelling vehicle the Teselecta, allowing him to survive. (TV: The Wedding of River Song)

With little information beyond this, theories exist on what creates a still point in time. One such theory is they form in places that are isolated and natural with little change having occurred over a long period of time. Lake Silencio was an incredibly empty, unaltered space with nothing being around for miles, so it fits this theory well.