Natural soils and underlying bedrock typical of channel banks near a river and floodplain system are utilized to improve the economics and efficiency of constructing an underground porosity storage reservoir. A man-made barrier, typically a slurry wall, is keyed into these banks and forms a first portion of a closed boundary for the reservoir. The channel banks between the two ends of the slurry wall form a substantially impermeable natural barrier defining a second portion of the closed boundary for the reservoir, thereby reducing the construction costs on sites appropriate for such a design. Locating the bank-sided porosity storage reservoir over naturally occurring scour regions in the bedrock can greatly increase the storage capacity. By building an embankment and extending the slurry walls higher, an open water storage area can be created on top of the reservoir that is bounded by the elevated slurry wall and the channel bank.