A method of making a molded plastic arch shape cross section stormwater chamber having a corrugated wall comprises molding half chambers which are connected by hinges at a joint at the top of the chamber. The chambers may be compactly stored and transported in splayed out configuration. Later, upward force is applied to the hinge joint location of each chamber, so the half chambers are rotated by force of gravity, whereby a faying surface on each half chamber is urged toward the like surface of the other half chamber. The faying surfaces are then fused to each other, preferably as a result of prior heating or by melting of a fusion weld element that is captured in the space between the faying surfaces.