Recently I have carried out a reservoir bathymetry using GIS (with 30m aster global dem v2) and echo sounding survey. The reservoir volume derived from aster gdem compares well with echo sounding data. The reservoir is small (1.5 sq.km) and was constructed 30 years back.
Is it possible the ASTER DEM was constructed from imagery acquired when the reservoir was dry? That would be my first guess, especially if the reservoir is small and filled only seasonally for flood control or irrigation purposes.
The other possibility, especially if the reservoir is shallow and flat bottomed with not a whole lot of turbidity, is that there is enough reflectance from near-shore bathymetry to provide "good enough" slope and depth detail to give you reasonable volume estimate.
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