DECOUPLING TURBIDITY AND DISCHARGE IN LANDSLIDE PRONE WATERSHEDS OF THE CASCADE RANGE, OREGON, USA

Certain water-quality parameters relate to one another in expected ways. For example, heavy rain will cause both stream discharge and turbidity to increase as runoff erodes surface materials into nearby streams. The magnitude and timing of the responses between the two parameters can be predictable. When these relations hold, it provides scientists with a sense of assurance that instream monitoring data are not spurious. However, when these expected relations do not occur, opportunities arise for investigation, insight, and interpretation to lead to discovery of something new. In the Cascade Range, Oregon, sediment contributed from landslides can produce a much different turbidity signature from that commonly observed when turbidity and discharge increase concurrently. Many examples exist of this decoupling, or separating, of turbidity from discharge, but only two streams with distinctly different landslide sources are explored here. The Evans Creek earthflow, located in the lower Western Cascades, contributes a consistent supply of sediment, regardless of streamflow. Stream incision of the landslide toe erodes sediment year-round. In addition, sediment production increases, as does the magnitude of turbidity, later in the winter, as accumulated rainfall continues to saturate the soil and raise a shallow groundwater table. In contrast, a series of debris flows between 2000 and 2006 on Mount Jefferson’s western flank continued to produce unique, isolated turbidity peaks, sometimes hours ahead of elevated discharge. Since collapsed snowfields can trigger debris flows, sometimes temperature, not rainfall, is responsible for increasing turbidity while having little effect on streamflow. These landslide-related turbidity signatures demonstrate that sediment contribution to streams in landslide-prone areas is often more complicated than assuming that an increased discharge corresponds to an increased turbidity.