Our friends at Beavers: Wetlands & Wildlife selected this day as International Beaver day, and the good news is it’s catching on. I’m told that today is a great day to give a beaver talk, write a letter to an editor about beavers or set up a display. Hmmm. I already do that on the other 364 days of the year, so I thought I’d do something truly special today. I’ll teach you a brand new thing. I learned about the concept from Michael Pollock on our Yosemite trip, and have been waiting for the right moment to share. Of course the unflattering story line is entirely my own responsibility. I figure a time when we’re waiting for beavers to build is a good time to learn.
‘Only it is so VERY lonely here!’ [without the beaver dams] Alice said in a melancholy voice; and at the thought of her loneliness two large tears came rolling down her cheeks.
‘Oh, don’t go on like that!’ cried the poor Queen, wringing her hands in despair. ‘Consider what a great girl you are. Consider what a long way you’ve come to-day. Consider what o’clock it is. Consider anything, only don’t cry!’
Alice could not help laughing at this, even in the midst of her tears. ‘Can YOU keep from crying by considering things?’ she asked. ‘That’s the way it’s done,’ the Queen said with great decision: ‘nobody can do two things at once, you know.
Consider this then:
A long time ago a tired researcher sat on his lawn chair and glared at the beaver dam in his stream and thought, I really need to blow that thing up, but first I’ll justify it. He took out the thermometer his wife had used to check his daughter’s temperature that morning and he measured the top two inches of water in the pond.
“Ah ha!” he said, comparing it to the top two inches of running water on the other side of the dam. “Beaver dams raise the water temperature and this hurts trout and salmon!” “Beavers destroy habitat for fish!” He trotted back into the house and wrote a paper which was published in the journal of wehatebeavers and soon the paper was quoted in ever other scientific paper on beaver dams known to man kind. (Then he blew up the dam which had been his goal all along, and alot of people were encouraged to blow up their dams too.) Soon every biology, hydrology and icthology student was taught that beaver dams raise the water temperature and regional agencies like Fish and Game or Department of Natural Resources wrote it into their policies and it became the great truth of the land. It was even quoted by the letter I responded to from LADWP yesterday. When a lone graduate student scratched his head and said, how do you know? He was nearly laughed off the campus and his thesis adviser had a tense conversation with his mother in the laundry room.
Hyporheic Exchange (Pronunciation: hi-poe-REE-ick)
So it turns out that when you look at a stream there’s the water you CAN see, the ground water you CAN’T see, and this layer of soggy silt & pebbles that acts as a sponge between the creek bed and the water table. This layer is constantly moving water into the ground, and pulling groundwater back into the stream. Water in the ground is naturally cooler because it gets no sunlight at all so every time it seeps into the creek bank it lowers the water temperature a bit, and when water is returned to the creek bed it is cooler.
Michael Pollock, of NOAA northwest fisheries was interested in this dynamic, and particularly what it meant to that old story about beaver dams choking out salmon and trout. He decided to set up some thermometers at different layers in the water, and also below the subsurface of the stream to find out the truth, then he repeated this at different points along a stream. Before I show you what he found, you need to know that the headwaters of any stream is cooler than the mouth. So we expect the temperature to gradually go up as the water moves down stream.
So reading the river from the headwaters on the right, the blue line is the subsurface temperatures and the lowest, which we would expect. The red line is the surface water of un-dammed areas, and the green line is the surface area in beaver pools. As you can see the red line is consistently higher than the green line, meaning that the surface area of beaver ponds is cooler than the surface area of free-flowing stream, the opposite of what our lawn chair researcher observed. Why is this?
I’m told the next graph has not yet been published so I shouldn’t post it on the web. Okay just imagine that the three measurements are combined we see this gradual sawtooth incline with a huge gap showing that the temperature suddenly falls. And guess where? There’s a huge temperature drop as tons of upwelling water seeps into the banks of the creek. It’s a beaver dam, whose deep pools increase the contact area of the water with the hyporheic zone, so there’s greater exchange and cooler temperatures. Say it with me now, “hyporheic exchange.” This is what the fish like. This is what enlivens the water and makes the creek more healthy. And this is why that researcher in his lawn chair all those years ago should be scornfully forgotten along with his entire findings.
Here’s the take home sentence for you to use in your next beaver argument, and you know you’ll have plenty. “Beaver dams cool streams by maximizing hyporheic exchange.”
Happy International Beaver Day! Celebrate by telling someone you know. Or everyone.
