In 2008 the French government launched its Quantitative Water Management policy initiative, as part of which the Rhone-Mediterranee Corse Water Agency undertook studies to ascertain monthly environmental flows at key sub-basin control points and the corresponding 'allowable [water] withdrawals'. Despite simplification, uncertainties, and insufficient data, the studies produced environmental-flow (e-flow) targets endowed with the power of allocating water between humans and nature. We analyse the fluctuation of the e-flow target at Point T6 in the Tet river basin, in the South of France, and show that, rather than an objective, quantitative ecological threshold dictated by science, it can be seen as a boundary number that embodies imaginaries, values, ideologies and interests. Science, but also law, appear to be selectively mobilized, massaged or contested. As a 'slider', the e-flow target reflects the state's political will and/or capacity to impose change and a reduction in water abstraction. Although the e-flow at Point T6 so far failed to play its role as a 'boundary number' and to achieve a settlement, it both exposed the limits of the pre-existing status quo and reshuffled the cards, legitimizing in particular the entry of environmental actors.