Canada was built on the back of beavers but — much to the chagrin of stream gaugers ever since — beavers managed to outlive the fashion of top hats. After the end of the fur trade, beavers started to re-populate the north with impunity.
Anyone unlucky enough to have the responsibility of stream gauging on a beaver infested stream knows how difficult it is to produce an accurate discharge hydrograph when beavers are around.
No matter how carefully you locate your gauge initially, it will only be a matter of time before a beaver comes along and decides the velocity in that reach is a bit too fast and he will build a dam to slow the water down. Once that happens stage becomes very insensitive to change in discharge. You can move your gauge, but the beaver will eventually build a dam downstream of your new site. You can blow up the dam, but the beaver will re-build it. You can develop a new rating curve or apply a shift correction, but every time you do that the beaver will modify the dam to invalidate your effort.
The real challenge for stream hydrographers is figuring out whether the stage-discharge relation is positive or negative at any given time. Rising stage may mean increasing discharge or it may mean an industrious rodent is busy making improvements to his dam, which reduces discharge. Falling stage may mean decreasing discharge or it may mean a breach in the dam, which increases discharge.
A Swedish biologist – Lars Wilsson – discovered that beavers respond to the sound of flowing water and are hard-wired to build dams to stop the flow. The more swiftly the water flows, the faster the beaver works. Silent leaks are not repaired, but water overflowing the dam is quickly repaired. This means that beaver dams wind up being more like sieves than weirs.
This dam(n) behavior is being increasingly celebrated all over the world.
The bring back the beaver campaign is seen as a bio-engineering solution for drought in California. Beavers are credited with saving salmon. Beavers have been reintroduced to Scotland. Beavers are also proposed as being the solution for water pollution with the potential to remove 5 to 45% of nitrogen loading in southern New England.
This interest in restoring beavers to the wild has been around for a while. This “fur for the future” video from the 1950’s shows beavers being trapped and then air-dropped into the mountains of Idaho. The airborne beaver corps could be revived to wreak havoc on stream gauges everywhere!
Things are not looking too good for stream hydrographers.
We need to become better at monitoring discharge in beaver infested streams. Index velocity might help, but good luck developing a stage-area relation for a stream that is persistently over-bank with water flowing amongst the dead/dying trees of what was the riparian zone. Good luck maintaining an index-mean velocity relation when the only part of the channel with enough water to deploy an ADVM has almost zero velocity. Good luck convincing your client to pay for the needed hardware.
In the meantime we’re left with shifting a rating curve to gaugings obtained by frequent field visits.
As long as we are stuck doing that, we should endeavor to do the best job possible. McCullough et al. 2010 investigated the hydraulic characteristics of beaver dams and found that they can be modeled using the weir equation Q=CL(H-e)3/2 where C is a coefficient, L is the length of the crest of the dam, H is stage, and e is an offset value.
The coefficient can be thought of as a composite roughness coefficient for water flowing through/over/under/around the dam. Using this as the basis for managing rating curves in beaver-controlled streams constrains the solution for fitting the series of deviations from the base curve. Basically, you can assume that L and e (at least as long as the dam stays the same length and height) are constant so the only thing that is changing from gauging to gauging is the coefficient C. This is different than the more typical method of applying a single point shift correction, which is equivalent to adjusting the offset (e). I sure wish I knew this back in the day when I was still doing field work!
I am interested in hearing from anyone who is struggling with beavers at their gauging stations. Does this reflect your experience? If not, why not? How do you deal with beavers? Please share your experiences below, for the benefit of others.
McCullough, M.C., D.E. Eisenhauer, M.G. Dosskey, and D.M. Admiraal. 2006. Hydraulic characteristics of beaver dams in a Midwestern U.S. agricultural watershed. World Environment and Water Resource Congress. pp. 1-10 doi: 1061/40856(200)182
Wilsson, L. 1971. Observations and experiments on the ethology of the European beaver (Castor fiber L.): A study in the development of phylogenetically adapted behavior in a highly specialized mammal. Viltrevy. 8(3) pp.115-205.
A reliable rating curve is one that is credible, defensible, and minimizes re-work. This paper outlines 5 modern best practices used by highly effective hydrographers. Read whitepaper here.