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Look, it's known that reservoir was empty, but it's a covered reservoir. You're looking at the the _cover_. That image tells you nothing about the state of the reservoir at the time.

> primary uphill source of water for those fire hydrants

was 3x 1M gallon water tanks. Hydrants were gravity fed until the tanks ran out (8-15 hours into the firefight), at which point water tankers supplied responding companies.