1000 cubic meters per second could be a LOT more than 80/sec of velocity or it could be a LOT less. They aren't the same thing.
Let's say the river is 100m wide and an average of 10 meters deep. The 2,000 cubic meters per second (the Rhine) give a velocity of 2 m/sec.
The maximum mean velocity measured with the ADCP was 3.7 m/s. The pitot-static tube, while capable of only point measurements, quantified velocity 0.39 m below the surface. The maximum mean velocity measured with the pitot tube was 5.2 m/s, with instantaneous velocities up to 6.5 m/s.