An underground water detention and release chamber including two concrete galleys and a plastic or concrete cap. Each galley has a substantially horizontally disposed top deck and two substantially vertically disposed side walls. The top deck and side walls connecting at respective ends to form first and second side edges. The two galleys positioned parallel to and a distance apart each other such that adjacent side walls of the galleys define a longitudinal channel between the galleys. The cap spanning the longitudinal channel and includes an arch-shaped body having two side base portions. Each side base portion has a horizontal strut and a vertical strut extending away from the body such that each base conforms to a side edge of a respective galley to position the cap on the galleys. The cap has a plurality of reinforcing members.