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.