Tapping into history’s depths

The key difference between kettle lakes and “normal’’ lakes, as Thorson puts it, is that “unlike normal lakes, which usually have significant inlet or outlet streams, kettle lakes are natural wells tapping the groundwater table.’’

And in addition to being sensitive to pollution - and to development, commercial and recreational - they are “individually quirky.’’

Thorson, a geologist at the University of Connecticut, has previously explored New England’s stone walls. Growing up in North Dakota and Minnesota, at the western extent of the kettle lake region, his family summered at Union Lake in northern Minnesota.

Kettle lakes, he recalls, “were ideal for young families. The waves were non-threatening, the shorelines were sandy, a surface layer of warm water developed fairly quickly, and there were neither sharks nor jellyfish for mothers to fret about.’’

But observing Union Lake “as the summers rolled by,’’ Thorson saw “a shift from a leaner and cleaner lake to a murkier [one]’’ with rising counts of fecal bacteria and levels of toxins such as DDT. “At the time,’’ he writes, “I didn’t make the connection between changes within the water and the rising number of cottages on the shore.’’

One of Thorson’s first excursions after arriving at UConn was to Walden Pond where similar changes were occurring but where “because of its international fame, [its] degradation . . . has been unusually well documented.’’

It began in Thoreau’s time, he writes, under the impact of summer visitors and, more recently, with the building of a town dump which “will probably leak forever.’’ And even now under state protection, its fish “remain inedible’’ and “the input of tainted dust from the nearby urban population and urine from its many swimmers remain threats.’’