"The message tonight is that we are still expecting that the worst has not yet come in terms of the flow," Nenshi told the Canadian Broadcasting Corp.
"The dams will crest on both the Bow and Elbow rivers over the course of the next little while and the downstream impacts will be significant. If you live in any of the neighborhoods that have now been affected by the mandatory evacuation it is time to leave. Gather your valuables and go," he said.
Evacuation orders covered portions of at least 25 Calgary neighborhoods and affected up to 100,000 people, the CBC reported.
All public and Catholic schools in the city were closed Friday, and Nenshi told residents to avoid all non-essential travel.
The Bow and Elbow rivers were expected to crest between midnight and 2 a.m. local time Saturday, the CBC said.
In Canmore and High River in Alberta, thousands of people were ordered to evacuate their homes Thursday.