Reservoirs and Cisterns

Some info about Reservoirs

Valley-dam reservoirs are created in valleys between mountains. Usually, there is an existing lake or body of water. The mountain sides are used as the walls of the reservoir to hold the water. A dam, or artificial wall in the reservoir, is built at the narrowest point to hold in the water. Most of San Diego's large reservoirs are behind dams.

Service reservoirs are entirely man-made. The water is stored in concrete basins either above or below ground. 

Cisterns are underground service reservoirs. Cisterns must be at a high elevation to allow the water to flow to customers by gravity. Here's one in Pt. Loma:

Here's another sunken reservoir, in Vista.

Grassmont Reservoir is also underground but has sod on top. It was "buried" a couple decades ago. Here's how it appears now.

Here's Meadowlark Reservoir, one of the most recent ones I've located. 

This is Cimarron Canyon, in the 4S Ranch area.

The large water towers you see on hills or stilts are also considered service reservoirs.  There are hundreds of these cistern reservoirs in San Diego County. I am not searching for them, I just find them. Here are two on a hill in North County.

This is the iconic 1.2 million gallon water tower in University Heights near El Cajon Blvd and Texas Street: photos from 1925 and 2015. It replaced an in-ground reservoir as the city grew.

Typically, water towers are already at a higher location than the surrounding land. In both cases the supply of water may have to be pumped uphill into a service reservoir, which adds to the water's cost.

Here is a map showing nine outdoor, above-ground Reservoir Lakes operated by the City of San Diego water district.