Accurate bench marks and water levels, which are important for tidal studies and other hydrographic purposes such as the establishment of chart datums, can be established along river shorelines if an adequate geoid model is available. Differential Global Positioning System (GPS) offers the possibility of determining bench marks and water level profiles with a centimeter‐level accuracy using carrier phase observations, provided that carrier phase ambiguities can be resolved on‐the‐fly. The results of a differential GPS survey conducted with a survey launch along an 80‐km section of the Fraser River, British Columbia, in March 1993 to test this concept are reported. Dual frequency receivers were used to permit quasi‐instantaneous carrier phase ambiguity resolution using widelane observables. Frequent ties to bench marks were made to assess independently the accuracy of the methodology. A 2‐cm accuracy shore‐to‐launch height transfer method that did not require landing of the launch was used. The repeatability of GPS, based on revisits of selected bench marks, was calculated as 5.5 cm RMS. The agreement between GPS‐derived and leveled orthometric heights at bench marks (B.M.’s) was found to be 6.4 cm RMS. This value, which is considered excellent, includes all DGPS error sources and unmodeled geoid biases.