Fluctuations in water surface elevation (WSE) along rivers have important implications for water resources, flood hazards, and biogeochemical cycling. However, current in situ and remote sensing methods exhibit key limitations in characterizing spatiotemporal hydraulics of many of the world's river systems. Here we analyze new measurements of river WSE and slope from AirSWOT, an airborne analogue to the Surface Water and Ocean Topography (SWOT) mission aimed at addressing limitations in current remotely sensed observations of surface water. To evaluate its capabilities, we compare AirSWOT WSEs and slopes to in situ measurements along the Tanana River, Alaska. Root-mean-square error is 9.0cm for WSEs averaged over 1km(2) areas and 1.0cm/km for slopes along 10km reaches. Results indicate that AirSWOT can accurately reproduce the spatial variations in slope critical for characterizing reach-scale hydraulics. AirSWOT's high-precision measurements are valuable for hydrologic analysis, flood modeling studies, and for validating future SWOT measurements.