A modular liquid dam having a plurality of individual elongated members that can be assembled in an end-to-end relationship to form a temporary liquid retainer or diverter on a substrate surface. The dam members are provided with one or more longitudinal bores or passages that reduce the amount of material in each member. The bores may extend completely through the dam member, may be internal bores closed off at each end, or may be bore segments intermittently spaced within the dam member. Preferably the bore members are open on at least one end of the dam member such that a stiffener rod or rods of varying rigidity may be inserted to increase and vary the rigidity of the dam member.