The present disclosure describes systems and apparatus for detecting an impending sewer backup by detecting fluid undulations of predetermined magnitude inside a main body of a backwater valve. When at least a predetermined number of fluid undulations of predetermined magnitude are detected within a predetermined time period, an alarm can be triggered. Such undulations can be detected by monitoring a buoyant gate in a normally-open backwater valve to detecting instances of movement of the gate from a lowered position in which fluid flow from an inlet through the backwater valve is unobstructed by the gate toward but stopping short of a raised position in which the gate closes the inlet to obstruct fluid flow through the valve. For example, a sensor can be carried on the gate to detect movement of the gate. Alternatively, fluid levels inside the main body can be monitored directly.