We find that the standard deviation, hence error, of the water surface elevation data from the Shuttle Radar Topography Mission (SRTM) is 5.51 m for basin‐wide, regional and local Amazon mainstem reaches. This error implies a minimum reach length of 733km in order to calculate a reliable water‐surface slope. Resulting slopes are 1.92 ± 0.19 cm/km for Manacapuru, 2.86 ± 0.24 cm/km for Itapeua and 3.20 ± 0.34 cm/km for Tupe. Manning's equation is applied with these slopes and with channel width measurements from the Global Rain Forest Mapping project synthetic aperture radar mosaics (GRFM SAR), channel depths averaged from nautical charts, and reasonable estimates of Manning's n. Resulting discharge values are 84,800 m3/s at Manacapuru, 79,800 m3/s at Itapeua, and 62,900 m3/s at Tupe averaged over the SRTM mission period. These values are within 6.2% at Manacapuru, 7.6% at Itapeua, and 0.3% at Tupe of the in‐situ gage‐based estimates for the same or similar time period.

I analyze Shuttle Radar Topography Mission (SRTM) water surface elevation data to assess the capacity of interferometric radar for future surface water missions. Elevations from three Ohio reservoirs and several Amazon floodplain lakes have standard deviations, interpreted as errors, that are smaller in C-band compared to X-band and are smaller in Ohio than in the Amazon. These trends are also evident when comparing water surface elevations from the Muskingum River in Ohio with those of the Amazon River. Differences are attributed to increased averaging in C-band compared to X-band, greater sensitivity to surface water motion in X-band, and generally larger off-nadir look angles in X-band. Absolute water surface elevations are greater in the C-band DEM for much of the two study areas and yield expected slope values. Height and slope differences are attributed to differing usage of geoids and ellipsoids. These SRTM measurements suggest the great possibility for space-based, laterallyspatial (2D) measurements of water surface elevations.ii