This dataset includes 5 sets of velocity measurements and water-surface elevation measurements collected at the Lazy Day (LD) reach on the Big Piney River near St. Robert, Missouri between November 19, 2020 and August 25, 2021. Velocity data were collected by USGS staff from the Columbia Environmental Research Center at different locations in the study reach using an acoustic Doppler current profiler (ADCP) and Real Time Kinematic Global Navigation Satellite Systems (RTK GNSS) positioning. Data were used to evaluate the calibration performance of a one-dimensional hydraulic model of the LD reach. Velocity data are provided as mean velocity at distances along a stream centerline that corresponds to the one-dimensional hydraulic modeling reach in HEC-RAS. Water-surface elevation data were collected by USGS staff from the Columbia Environmental Research Center using RTK GPS for the length of the study reach. Data were used to calibrate a one-dimensional hydraulic model of the LD reach. Water surface elevations are provided at distances along a stream centerline that corresponds to the one-dimensional hydraulic modeling reach in HEC-RAS.