Synthesis of Geomorphic Analyses


The geomorphic analyses made in this study show that, in contrast to early hypotheses suggesting widespread sedimentation, it is unlikely the reach of the Molalla River in GR3 has been aggrading in the past ten years. Observations of the channel bedrock just upstream and downstream of GR3 (fig. 11) suggest the convexity in the long profile observed in GR3 is not the result of a wedge of alluvium. Although the total depth of alluvium above bedrock is unknown, the water-surface profile suggests that it is likely the Molalla River here has adjusted its gradient to transport sediment through this reach. The supply of bedload entering GR3 from upstream is probably small. Moreover, gaging-station data dating back to 1928 show that at the location of the Canby gaging station, the river has incised about 0.5 m since the 1960s (fig. 14). Aerial imagery acquired from 2005 to 2009 (fig. 20) also shows that the likely source of sediment that has affected some sections of the river in GR3 was derived locally from meander cut offs and not from upstream. An example of such a gravel deposit from GR5 is shown in photograph 3.


This study also showed qualitatively that channel-migration rates in GR3, which can be an indicator of increased sedimentation, were not significantly greater in the past 10 years than those rates observed during the 20th century. The avulsions and stranded channels observed recently have occurred repeatedly in the past. It is also important to note that although high flow events from 1996 to 2009 were greater than those in the previous two decades (1975–95), these events were not unusually large if evaluated in the larger hydrologic context of the Molalla River. For example, the recurrence interval for the February 1996 high flow was between 25 and 50 years (table 4). Similarly, the January 2009 high flow was about a 10-year event. Both of these events were large, but neither was exceptionally so.


There is little evidence that the river in GR6, GR5, or GR4 is aggrading or otherwise filling with sediment. Channel bedrock is present through the river corridor in these reaches (fig. 11), and channel migration rates between 2005 and 2009 (fig. 17) were small compared to migration rates observed in GR3. Although there were indications that a pulse of sediment accumulated in GR4 by 1948, no contributing effects of this sediment are evident in the channel dynamics of the last few decades. Upon closer inspection, the gravel and cobble bars upstream of the Highway 211 bridge in GR5 and GR6 that appeared elevated when viewed from the flood plain were found to be deposited in the river corridor at an elevation controlled by underlying bedrock, not by excess sediment. Although not directly evaluated or calculated, the sediment transport capacity of the Molalla River appeared to exceed the available sediment load. Moreover, there is little evidence to support the hypothesis that large volumes of sediment are being transported into the study reach from the upper part of the catchment.


First posted February 29, 2012