Dissolved Oxygen Sag Curve — Streeter–Phelps River Model

This simulator models what happens to dissolved oxygen in a river downstream of an organic-waste discharge, using the Streeter–Phelps equation that underpins real river water-quality permitting. Bacteria breaking down the biochemical oxygen demand (BOD) pull dissolved oxygen down while natural reaeration works to replace it, producing a characteristic sag-and-recovery curve. Four physically meaningful controls — the initial BOD load, the bacterial deoxygenation rate, and the stream's velocity and depth (which set the reaeration rate via the O'Connor–Dobbins correlation) — let you explore how a river's own hydraulics determine whether a discharge causes a mild dip or a fish-killing oxygen crash, with the critical (lowest-DO) point and its downstream distance computed live.