Why is it ok to use tap water/toilets on Shabbos? Considering the source of the water pressure.
The water pressure that enables the water to flow out of the tap has to come from somewhere. It is almost always mechanically enhanced.
New York City in some cases might be an exception, since almost all the water pressure is entirely due to gravity. The water from the reservoirs in the mountains north of the city, literally goes downhill to the city, and is still benefiting from a gradient as it hits the city. This is apparently sufficient for even reasonably tall buildings! 10 stories or so before mechanical assistance is needed. In which case it is pumped to a tank on the roof, which then uses gravity again when needed.
So whenever you turn on the tap, flush the toilet, use the water, you deplete the current source, and if it is mechanically assisted, that needs to be replaced. Be that into a water tower, or via a pumping station.