Rapid adjustment of the Rio Puerco to meander cutoff: Implications for effective geomorphic processes, crossing thresholds and timing of events.

A meander loop of the upper Rio Puerco of the east experienced a neck cutoff during the late winter-early spring of 1988, temporarily changing the gradient of the stream from 0.003 (m/m) to 0.25 . Observations before, during and after the cutoff document (I) that cutoff was not a major disruption for the stream, (2) that timing, order and duration of events are important in crossing geomorphic thresholds and subsequent adjustments, (3) that the threshold for cutoff was lowered considerably by erosion and humans in preceding years, (4) that breaching was thwarted temporarily by low-flow processes, (5) that initial breaching took place during moderate flow, and (6) that subsequent large flows in 1988 caused both width and gradient to be fully adjusted with upstream and downstream values within one flow season. The steep gradient of the breach did not propagate upstream as a headwall; rather the slope increased slightly upstream and extended until the former gradient was reached.