By Dr. Robert Thorson

`A River Runs Through It.” This 1976 novella by Norman Maclean describes a family bound to itself and to the land by a river.

A dam runs across it. This was my first thought when I learned last week that the Connecticut River population of blueback herring migrating past the Holyoke Dam has plummeted from more than half a million in 1985 to less than 200 total today.

As if by magic, the next day an article on the tragic ecological effects of large dams appeared in my mailbox. Published in the April 15 issue of Science, the article was written by scientists from Sweden’s Umea University and the Global Priorities Group of the Nature Conservancy. Their conclusion? The sum total effects of large dams around the world are enough to change the world’s oceans — which is why scientists suspect the blueback herring is having its problems.

Damming large rivers can be a good idea for flood control, production of hydroelectricity, and irrigation. But the Science article demonstrated what everyone who loves rivers already knows: Each dam on a river clogs the flow of life, as if the dam were arteriosclerosis in a life-giving artery.

The number of dams regulating flows on the world’s rivers is truly staggering. There are nearly 50,000 dams nearly 50 feet high that together hold back enough water to flood Connecticut more than a quarter-mile deep. More than 300 of these dams are truly colossal, the tallest being the new Three Gorges Dam on the Chang Jiang (Yangtze) River in China, which reaches nearly 600 feet high.

The upstream and downstream effects of such dams on river systems are well documented. Most obvious is submergence of terrestrial habitats and the elimination of fast-flowing reaches. But reservoirs create new problems as well: They trap standing water behind each dam, thereby stimulating the release of nutrient pollution and fostering the growth of green slime in calm reaches downstream. They create biotic dead zones devoid of oxygen. They trap silt, sand and gravel needed to maintain downstream channels. Finally, they contribute to rising greenhouse gas concentrations by pre-empting the growth of forests and fields.

Under natural conditions, rivers flow vigorously during floods, then decline to lazy flows. Under dammed conditions, however, floodwaters are held back, then released more evenly to generate hydroelectric power or to irrigate land. Such manipulation of river flows shrinks and unnaturally stabilizes downstream channels. It eliminates or drains floodplains, which are otherwise productive lands. By reducing the influx of mineral-laden water, it stunts the growth of deltas and marshes. This, in turn, dramatically alters the biological productivity of estuarine ecosystems.

The Science journal study examined nearly 300 large river systems on all six inhabited continents. Combined, these rivers drain more than half of Earth’s land area; carry about 60 percent of “virgin mean annual discharge” (the average flow before impoundment or regulation began); and cross all 16 of Earth’s non-marine biomes. A slim majority of watersheds were relatively unaffected by large dams, whereas a slim minority were strongly affected. Earth’s two largest river systems (the Amazon-Orinoco and the Congo) were moderately affected, whereas the third largest (Chang Jiang) is strongly affected.

Nature’s river systems developed to their “virgin” state during thousands of years of postglacial adjustment. Over time, all of the subsystems — biological, hydrologic, geologic — adjusted to each other and to the flux and flow of matter, energy and nutrients. Massive river development projects, even when done with the best of intentions, fragment these larger, flexible, integrated natural systems into ecological bits and pieces, making it harder for their species to adjust during global change.

These projects also change the nearby oceans into which they drain. With as many as 49 new large dams being planned for each of 46 large river systems in the world, I am beginning to wonder how much more strain Earth’s watersheds can manage before failing.

I also wonder if the Holyoke herring might be some sort of canary in the coal mine for New England’s little corner of the Atlantic Ocean.