raven wrote:Average sedimentation rates are determined by many individual events. At low flows little sediment is carried; at high flows stuff is knocked loose that would not be at lower flows. When a valley is stable, moderate to heavy rains will cause little sediment to be removed. When a valley is destabilized by logging or roads or fires, it does not take much.
The trail photo illustrates the destabilizing effects of trails. Roads are worse.
So yes, a point event. That's the point.
I agree and disagree. In addition to my previous post, another basic point is that the photo posted by mkrochta does not in any way, to my eyes, indicate the source within the watershed of the sediment. Mkrochta's photo shows only the total sediment level from the entire watershed.
The photo posted by mkrochta could reflect ...
a) sediment due to a single catastrophic event somewhere in the Collawash watershed. That event may or may not have been caused by or have involved a red or black road on the current version of the Bark map.
b) sediment due to general road erosion widely distributed over the watershed (due to saturated/frozen terrain due to weather conditions)
c) sediment due to naturally or unnaturally unstable stream banks in the watershed triggered by the weather that produce flood-stage river flows
d) sediment due to distributed erosion on previously logged slopes in the watershed that have not fully recovered
e) .....
It's probably due to a bit of all of #b-e and possibly one or more #a's. But the take home of #a-e is that 1) the sediment in mkrochta's photo is not necessarily due to roads, and 2) the sediment in mkrochta's photo may be due to a catastrophic event or due to distributed erosion.
Assuming the road density in the upstream watershed of the Clackamas is similar to the road density in the Collawash watershed, then mkrochta's photo shows that forest roads don't
necessarily destabilize the terrain to a critical extent. It could be that the road contractor for the Collawash goofed, or skimped. Or it could be the Collawash roads were built in a different year under different best-practices. Or ....
If the sediment was due in large part to a catastrophic, how frequent are similar events? Was it a 100-year event? 50-year? ... The week of foul weather also correlated with the rockslide that closed I84. How often does that happen? Was the rockslide a 100-year event?
Regardless of the source of the Collawash sediment, the severity of the foul weather suggests that Collawash sediment levels do not frequently reach the level seen in mkrochta's photo. Maybe only once every few decades. On all other days, and throughout most years, the difference in sediment between the Collawash and the Clackamas my be imperceptible to the eye.
The next time that a similar foul weather week occurs, the catastrophic failure may occur in the Clackamas watershed up stream of the Riverford Campground area (where mkrochta's photo was taken) ... and all the conclusions will be reversed.
Mkrochta's photo may be severely misleading, just as a photo of the I84 rockslide could lead to a severely misled generalization. I am not saying mkrochta's photo is misleading, but the coincidence with the extreme weather lead-up makes it likely, in my opinion.