A modular fluid retention system and method for exemplary uses collecting and temporarily retaining fluids, for example stormwater run-off. One example of the system includes a plurality of modular retaining units which are selectively connected together to form an interior chamber volume for collecting stormwater run-off directed into the chamber volume. A plurality of modular trays are engaged with portions of the respective retention units to prevent relative movement of the retention units and eliminate, or substantially reduce, the need for porous material to be installed in and around the retention units greatly increasing the excavation void space usable for water collection and retention. In an alternate application, only a plurality of modular trays are used as the vertical support and fluid retention volume structure for the fluid retention system.