A strongly polynomial algorithm is developed for finding an integer-valued feasible $st$-flow of given flow-amount which is decreasingly minimal on a specified subset $F$ of edges in the sense that the largest flow-value on $F$ is as small as possible, within this, the second largest flow-value on $F$ is as small as possible, within this, the third largest flow-value on $F$ is as small as possible, and so on. A characterization of the set of these $st$-flows gives rise to an algorithm to compute a cheapest $F$-decreasingly minimal integer-valued feasible $st$-flow of given flow-amount. Decreasing minimality is a possible formal way to capture the intuitive notion of fairness.