I adopted Olaf Itoff's caches along this trail, and decided that it would be fun to fill in some of the gaps with a few more caches to reward the intrepid cachers who pass this way. If you look uphill from GZ, you will see EBMUD's Pearl Reservoir. In this context, reservoir seems to mean a large water tank.
At the time the cache was placed, there were signs indicating that Pearl Reservoir was being rehabilitated. Bidding documents available on the internet suggest that the tank holds between 0.5 million gallons and 1.5 million gallons, and that rehabilitation required demolishing an existing wooden roof, constructing an aluminum dome roof, abrasive blasting and recoating of the tank's interior and repainting the outside of the tank. During the rehabilitation, a 0.16 million gallon "bolted steel temporary tank" was assembled nearby and seems to be still in use.
Congratulations onebadpenny for beating out the others for FTF!!!