Splain this to me

Without getting into the weeds about the reservoirs north of LA, the largest is empty it normally contains several million ga.lons of water which could have been used to fight the fires.
Its been dry for several months because of a leak.
Well that is a lie. It has a cover which had a tare. Supposedly it was allowed to run dry to fix the tare.
This is my question. The cover is there to prevent contamination from bird shit.
Chicago and surrounding cities in Michigan and Wisconsin
get their water water from Lake Michigan.
Pretty certain there is bird shit, fish shit and people shit.
It gets filtered and treated. Why cant the California water be treated and filtered?