By signing a fixed water sharing agreement (FWSA), countries voluntarily commit to release a fixed amount of river water in exchange for an agreed compensation. We examine the vulnerability of such commitments to reduced water ows. Among all FWSAs that are acceptable to riparian countries, we find out the one which is sustainable to the most severe drought scenarios. The so-called upstream incremental FWSA assigns to each country its marginal contribution to its followers in the river. Its mirror image, the downstream incremental FWSA, is not sustainable to reduced ow at the source. We apply our analysis to the Aral Sea basin agreement.