It seems like your round glacial-type rocks would be perfect for a French drain-style cistern: they'll hold lots of open space between them.
To elaborate a little bit: The idea that came to mind is basically what Robert van de Walle reported building on The Sustainability Blog:
http://homeofthefuture.blogspot.com/search?q=french+drain If you want to bury a water system (to prevent evaporation, say, or to change its thermal properties), two common options are building an overhang or filling it with gravel. People sometimes take measures to prevent sediment or biomass from filling in between the stones, but not always.
A French drain is basically a gravel-filled drainage ditch. What Robert built in the above link has a storage function as well as a re-direction function...if I understand correctly, the idea was to restore the seasonal creek that had run through his property before development, and add a less-seasonal pond to irrigate with, but build (part of) his garden on top of all that.
I would be tempted to use it as a thermal reservoir, too, bringing water and/or air from the gravel bed to a heat exchanger in the house if inside temperatures became uncomfortable. Using air as a working fluid appeals to me, because the whole system could be driven by a solar chimney, which drives air more forcefully when the sun is hot or when the air is particularly dense, i.e. when the house is more likely to need climate control.
"the qualities of these bacteria, like the heat of the sun, electricity, or the qualities of metals, are part of the storehouse of knowledge of all men. They are manifestations of the laws of nature, free to all men and reserved exclusively to none." SCOTUS, Funk Bros. Seed Co. v. Kale Inoculant Co.