The functional relationship between benefit level and resource availability, which is demonstrated in a marginal benefit (MB) function but not in a total benefit (TB) function, is mostly absent in environmental valuation studies. This paper aims to establish the total and MB functions for instream water direct uses. Incomes of the beneficiaries considered as the water-use benefit and its seasonal variation within a year obtained from a semi-structured primary survey are used to establish a quadratic TB and subsequently a linear MB function. The Teesta River from Bangladesh is taken as a case study where capture fishery and small-scale navigation are the instream water direct uses on which many riparian poor rely for their livelihood. Despite several challenges including data paucity, the study carries importance to persuade instream flow provision and safeguard the livelihoods of the poor riparian communities in a developing country perspective.