We study the inviscid multilayer Saint-Venant (or shallow-water) system in the limit of small density contrast. We show that, under reasonable hyperbolicity conditions on the flow and a smallness assumption on the initial surface deformation, the system is well-posed on a large time interval, despite the singular limit. By studying the asymptotic limit, we provide a rigorous justification of the widely used rigid-lid and Boussinesq approximations for multilayered shallow water flows. The asymptotic behaviour is similar to that of the incompressible limit for Euler equations, in the sense that there exists a small initial layer in time for ill-prepared initial data, accounting for rapidly propagating "acoustic" waves (here, the so-called barotropic mode) which interact only weakly with the "incompressible" component (here, baroclinic).1 Because we assume that the layers are all of comparable depth and the vertical stratification is balanced, in the sense that we fix m > 0 such that sup i∈{1,...,N } {δ i , δ −1 i , r i , r −1 i } ≤ m, then c 0 measures the typical velocity of propagation of the baroclinic modes; see Appendix B.