Revisiting the serial discontinuity concept

We revisit the serial discontinuity concept (SDC), which predicts river ecosystem responses to stream regulation in the context of recovery with distance downstream from the dam (discontinuity distance). Many studies have described pervasive interruptions of natural biophysical gradients of dams by comparing conditions in tailwaters to reference or pre-impoundment conditions. But only a few studies provide data or interpretations that explicitly test the SDC within entire stream corridors or along specifically defined reaches where recovery was expected in view of the predictions of the SDC. We present discontinuity distance measures for nine rivers around the world where the predictions of the SDC were substantiated. In two cases, recovery trajectories were overwhelmed by other human sources of disturbance. In one case, the SDC did not hold up, but only biotic measures were made. We conclude that, in general, the SDC is a sound construct that in most cases can be used to predict, or at least clearly articulate, the consequences of new regulation. The next step is to develop better empirical models of the SDC and to validate them experimentally through re-regulation of entire river corridors.