Thursday, January 29, 2015

Pond and Debris Flow: Beavers or Snow and Ice

This has been a week of contemplating beavers and the landscape. This first post is field work related. I hope to have another on a policy aspect of beavers later.
On Monday I ventured up a steep stream drainage in effort to assess the risk of debris flows that might come down the drainage. Assessing debris flow risk entails assessing the geomorphic features of the drainage - How stable are the slopes? What types of failures should be expected? How steep is the drainage? How much material can be incorporated in a debris flow coming down the creek? How large would the volume of water and debris that can become trapped with drainage blockages? 
This particular creek had a debris flow approximately 10 years ago. So another thing to look for was the source of that debris event. What caused the debris flow 10 years ago?
I had been told via very indirect sources that it was caused by a beaver dam failure. A causation that I was highly skeptical of given the steepness of the drainage and the numerous waterfalls. I made my way up the creek to points where there was no surface water flow including side drainages. No indications of beaver, but a few shallow surface slides.
However, at the very top of the ridge hundreds of feet above the uppermost stream flow was an unexpected nearly level area with a pool of water 200 feet long and 40 feet wide.
Pool of water and outlet lip at the end of the pool
Some subtle features indicated that the pool of water had formerly been deeper and other features suggested that on occasion the pool may overtop and flow via surface water flow across the slope towards the steep drainage below. Soil had been eroded off of the forest floor below the outlet area of the pool exposing tree roots on the slope between the pool and the upper drainage below suggesting that some time in the past a strong flow of water had coursed across the slope.

Bare roots exposed along swale below the pond

Bare roots of western red cedar along swale between pond and drainage

View of subtle swale with no current water flow

Area below the swale towards steep drainage with uprooted tree
Further down there were areas of soil stripped away down to the flowing stream below
Besides this toppled tree, we noted other crater like features on the slope suggestive of other root throw divots along this area where the trees long ago had rotted away.
I am still not convinced about the beavers being the cause. Lack of beaver cut trees is one bit of evidence I would need to see. An alternative explanation would be the pond filling with snow and freezing and then having ice and snow block the over flow outlet during a heavy rain on snow event and then suddenly releasing a surge of water.
Regardless, the pond appeared to be a good candidate for a large surge of water that would flush large wood accumulated in the drainage down the creek and onto the alluvial fan below. Outside of the pond, no landslide scars were present that were large enough to have generated the scale of the event from 10 years ago that flushed so much very large wood out of this creek
The piles of wood left by the event 10 years ago are still present although a bit hidden by the new growth of red alder covering much of the fan surface.
Woody debris pile from debris flow/flood

Typical stream flow could not have transported the wood that now lines part of the lower stream on the alluvial fan. Some of the logs were 4-foot in diameter.

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