Most people would be able to stand upright in the pipe. It is massive, 6 foot 4.75 inches.
The control points are a valve chamber at 44th Street, along with one just south east of 43rd Street:
The one to the NE is on the other side of the Bow River:
It looks like there is a connector at home road to feed lower Montgomery, among possible others:
The area needs to remain pressurized or it is possible the entire local supply will need to be replaced (whether the 1959 pipes, the oldest I could find poking around, could survive a full depressurization I doubt can be known with 100% certainty). Here is the map where we can see marked with pink arrows pressure reducing valves surrounding what I think would be the 'danger zone':
So steps from a lay person:
find the actual break. Isolate the local system from the main. if possible do the water equivalent of a hot tap on the main (the main is concrete, so perhaps a steel jacket? even if it leaks somewhat that matters a lot less in the immediate term). Hope the system can otherwise provide service without the main feeder main to the pump station just down 16th:
Build a bypass, conduct a full repair.
This will be a multi-month affair at least. Hopefully water system to full service by early next week at worst.