Monday, November 3, 2014

The Blair Witch Beavers of the Santa Ynez River

All right today I want to go over my idea on what the hell those intrepid beavers of the Santa Ynez River were doing making all those strange rock piles across the river when the river was essentially drying up. They were not making the beginning stages of a dam but were instead actively hydro engineering the river itself to best suit their needs!!! This is my working hypothesis and let me explain my reasoning.

As I have mentioned in several past posts the Santa Ynez is a human modified river in terms of flow. It largely dries up starting in spring depending on how much rain we get. Water is usually released late summer/early fall to satisfy downstream senior water rights. Beaver on this river have adapted to this unusual water regime for at least several decades since reintroduction in the middle of the last century (but remnant endemic populations can not be ruled out).

Above is a pic of the unique rock constructions I am talking about. This pic was photographed in late April 2014 and the river was already well on its way to drying up. Upstream is a section of small, rocky riffles and above that is what I believe to be an old disused beaver pond. Immediately downstream is a slower section of river and a new dam under construction and a potential beaver bank burrow I have located. (New readers should be aware that humans constructing these rock piles is unlikely as there is no path here and only a steep bank and marshy area on the right side - a residential area is nearby to the left however)

This is a picture of the beginning of a new dam taken in late April 2014 a little downstream of the above rock piles.

Now above is a picture of the river with substantial flow from a late season water release taken in October at the location of the strange rock piles pictured above. What you can see is that when water hits the first pile of rocks it forces it slow down and spread out - a speed bump. The water also has substantially eroded the bank on the other side creating a nice deep pool where there was not one before. In a sense accentuating a nice thalweg.

In this pic above you are looking at the second pile of rocks. Water, after it has flowed over the first pile of rocks and starts to erode and enlarge the hole is held back a bit by the second pile of rocks to further increase the "thalweg" effect.

So what in a sense the beavers are doing is modifying a section of the river that runs relatively fast - a riffle section, not great habitat for a beaver due to the lack of swimmable sections and sparse vegetation - and using the power of the moving water to carve out a deeper pool and create more amenable habitat and slower flow which the beaver prefer. What is not pictured is that a few meters away from this thalweg is a marshy creek area. Separating the river and this marshy area is more rocks and debris. Given enough time and erosive potential of the river I think that the two area can merge over time. I will monitor this.

What should not go understated is that the bulk of this activity was underway before there was any substantial water flow at all. The beaver were not only actively managing where and how the water flowed but they were predicting the return of the water flow itself.

Pretty cool no? These beaver never cease to amaze me making a go at it in the ephemeral Santa Ynez River.

And a bearded bad ass in front of a beaver pond.

And a big dump of youtube clips from the same excursion in October.

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