The current view is that migrating bars are the result of morphodynamic instability in straight or mildly-sinuous alluvial channels and are therefore an inevitable feature of alluvial river beds. Steady bars, instead, require some external forcing or specific morphodynamic conditions to develop. Yet, recent numerical tests showed that steady bars may develop as a result of spontaneous morphodynamic instability, just like migrating bars, without meeting the specific conditions. We investigated this possibility in the laboratory, following the temporal evolution of alternate bars in a straight flume with mobile bed. The experiment was run with a constant discharge for about 10 weeks. Initially, the bed topography was dominated by the presence of fast growing migrating bars. After three weeks, however, slowly growing, larger, steady bars emerged. These bars had the same wavelength as the ones that formed in another experimental test in which the flow was perturbed by the presence of a transverse plate. The experiment confirms the recent numerical results. Considering that the presence of steady alternate bars is a prerequisite for initiation of meandering, this is now shown to be an inherent feature of alluvial rivers.