You often hear it said that there are only two species which deliberately alter their environment by the damming and/or diversion of streams: one, of course, is the beaver.
The other is us. One estimate suggests that there are 800,000 manmade dams worldwide. Of these, perhaps 14% are in the U.S.; nearly half are in China. Most are small; but the mere 40,000 large ones manage to have profound impacts on nearly every river system on earth.
I spend a certain amount of time-- not a large proportion, but some-- thinking about dam removals. I work in fisheries biology, partly with salmonids, and it is hard not to maintain a consciousness of dams as a constraining force on fish populations. On the other hand, this is the West, where water is scarce and precious. The stockpiling of water is not an unreasonable impulse on its face, though its effects are often far more damaging than originally anticipated.
So, I've certainly considered dam-building as an enterprise which attempts to meet human water resource needs-- whether it is successful or not, worth the cost or a form of expensive, wanton destruction.
What I had not considered, until yesterday, was the possibility that the joy of water diversion was ingrained in us instinctually as a species, becoming a source of interest and pleasure beyond its utilitarian purposes.
Many sources claim that the sound of running water, itself, is the instinctive cue for beavers to build dams; for instance, such a recorded sound will cause a beaver to initiate damming behavior.
I began to think about this as I watched the children at my six-year-old daughter's birthday party, which was held at a city park with a tiny creek running through it. The creek is shallow, with irregular grassy banks, and narrow enough to jump in many places. While it has a muddy bottom, it also contains a number of large cobbles. The kids gravitated quickly towards this creek-- it was a hot day-- and throughout the party the streambed proved a more compelling location than any of the play equipment or the shaded picnic pavilion under which we occasionally convened to eat cake or open presents.
My own daughter, in fact, was the first to ask to "be excused" from cake-eating and return to standing ankle-deep in the water. Others quickly followed. And what were they doing in there? Building a dam, of course. A fortuitous spot had been chosen, where there already existed a (very) slight drop-off. Large cobbles had at first been lined up, to form a primitive and very porous stone wall. But the children were not satisfied with this. They worked on and off all afternoon, and stayed late to finish, filling in the gaps between the big rocks with small ones, packing the spaces with mud, adding sticks at angles for strength and stability. The water behind the dam got deeper; we had to be careful of the youngest attendees. People were bathing in the "pool." The stream widened, and there was visible downstream dewatering. The grownups began to gravitate towards the stream too.
The question is, why? These children did not need to impound water for residential use, agriculture, or power generation. They made a dam because making a dam was pleasurable. It was fun to make, and fun to watch. It was neat, the way the stream got deeper, the way the course of the water changed.
Wow, said the children and the adults. That's so cool. I felt guilty for damming the stream, but it was cool. And everybody was so happy.
The larger question, of course: what is it about our species that finds environmental manipulation intrinsically cool, whether or not any rationally-developed aim exists?
Before we left (but after most of the kids had gone home), one dad partially disassembled the dam, so the water could flow freely again. As I carried bags and coolers across the park to my car a mere five minutes later, I saw a new group of children in the stream, in close to the same spot, laughing and splashing about in their excitement.
They were carrying rocks.