Effects of earthflows on stream channel and valley floor morphology, western Cascade Range, Oregon

Slow moving earthflows (0.1-15 m/yr.) may constrict valley floors and directly impinge on stream channels. Earthflows that move laterally into channels deliver organic and inorganic material to the stream from the earthflow toe. If the amount and particle size of this material is too large to be removed by streamflow, aggradation and subsequent steepening of the channel gradient occur. However, if the rate of material input is too slow or the size of material is too small, the material can be removed as bedload or suspended sediment load and there will be no change in gradient. Where earthflow encroachment causes channel aggradation, the valley floor and channel upstream of the zone of direct earthflow constriction experiences widening and decrease in the gradient of the valley floor and channel. This increase in width of the channel is due to the gradient change in the earthflow-constricted zone and to hydraulic backwater effects at stream flows which carry bedload.

Effects of the earthflow constriction at five sites in the western Cascade Range of Oregon are examined at two scales 1) that of stream reaches (10² to 10³ meters of channel length in areas having similar valley floor characteristics) and 2) that of channel units (features which are 1 to 10 channel widths in length, e.g. pools, riffles and cascades). In earthflow-constricted reaches (defined by length of the earthflow toe entering the channel) where channel gradient is steepest, there is a greater percentage of cascades per unit of reach length. The reach upstream of these constrictions contain the lowest percentage of cascades per unit of reach length, but the highest percentage of riffles.

Three of the five earthflows studied followed the pattern of steeper gradients in the earthflow-constricted reaches. This pattern was not evident in the other two sites, apparently because of the size and rate of the material entering the channel from the earthflow, as well as the over all gradient of the channel which may limit other changes in gradient.