The hydraulic transition between nonorifice and orifice flow regimes at a rectangular sluice gate was analyzed to determine the value of a coefficient (Co) used to define the threshold between the two regimes. The transition coefficient was defined as the ratio of vertical gate opening to upstream water depth. Several dozen data sets were collected in a hydraulic laboratory, each including the measurement of upstream and downstream water depth for five different vertical gate openings, and 17 different steady-state discharges from 0.02to0.166 m3/s. Various approaches were tested to define the limits of the nonorifice-to-orifice regime transition, but the one presented herein uses the specific-energy equation for open-channel flow. After the transition limits were defined, an estimation of the nonorifice-to-orifice transition coefficient, Co, was made. The experimental results indicate that orifice flow always exists when Co is less than 0.83, and nonorifice flow always exists when Co is greater than 1.00. A procedure was developed to determine the flow regime and the discharge at a rectangular gate in the range 0.83<Co<1.00. © 2009 ASCE.