In recent years, more and more people are paying attention to environmental problems. Their eyes are naturally turned to rivers and thus structures accessible to river waterfront have been constructed in various places. One of them has a shape of side cavity with stairs. Studies on side cavity flow have made a considerable progress in the past several years, however, time-dependent characteristics of water surface variation have not been clarified until now Through careful measurements of water surface elevation using an imaging technique, we found that water surface oscillation at a side cavity has three modes; modes dominated by channel width or cavity length and n mode depending on the main flow velocity and the length of side cavity.