The Lake Koshkonong sea serpent: A. I. Sherman and Charles Bartlett's rowboat chase (1887)

Lake Koshkonong lies in southern Wisconsin. It begins about 5.5 miles downriver from Fort Atkinson on the Rock River and spans Jefferson, Rock and Dane counties. Today it is broad and shallow, averaging just 6 feet deep.

In the 1880s and 1890s, Wisconsin lakes produced a run of sea serpent reports. Rock Lake had its Terror from 1882. Red Cedar Lake, only a few miles from Koshkonong, already had a big snake history of its own.

Early November 1887. A. I. Sherman of Fort Atkinson and his cousin, Charles Bartlett of Milwaukee, were out duck hunting in a rowboat. They were rowing down the south edge of the northeast bay.

Then both men saw it. A "huge snake-like object," about fifteen rods off, swimming toward the center of the lake. It swam with its head raised about two feet above the water.

About ten feet of its trunk showed. It looked eight inches thick. The water was calm. The wake was easy to see. Judging by that tremendous long wake, the animal appeared to be about thirty or forty feet long.

The hunters rowed rapidly toward it, hoping to kill it at close range. The newspaper told the ending plainly: "as soon as the saurian saw them it slipped under the water and they were forced to give up the chase." A later local account adds that the men stoutly maintained they were in no way under the influence of the ardent spirits.

The account appeared in the Watertown Republican on 16 November 1887, reprinted from a Fort Atkinson paper. It called the sighting "about the sixth or seventh appearance of perhaps the same animal," counting the five or six appearances at Red Cedar Lake. Fifteen rods is about 248 feet, though later accounts give the distance as about 150 feet. By the time an Oshkosh newspaper reported the story, the creature had grown to 60 feet. Modern retellings added a body 8 to 12 inches thick and a wake strong enough to rock the boat, none of which is in the 1887 text.

The lake had a spirit long before the hunters. In Potawatomi tradition, recorded by Charles E. Brown and Dorothy Moulding Brown, a water demon lived there. It had to be given tobacco, or it would tip canoes and drown people. A Ho-Chunk story tells of two brothers who denied the spirit. They drowned in a sudden storm, with white clay found in their nostrils and ears.

No search or inquiry followed. No hoax was ever found or confessed. No one has published an explanation aimed at this sighting itself. The Pine Barrens Institute suggested that separate, undated tales of torn fishing nets might point to a large muskellunge. The size of the animal rested entirely on a wake seen from a distance, and that leaves the debate open.

Charles E. Brown included Koshkonong in his 1942 pamphlet Sea Serpents, the first folklore collection to cover the lake. In 2016 Chad Lewis gathered the state's stories in Lake Monsters of Wisconsin. When Watertown heard of Loch Ness in 1933, it had already been talking about its own lake monster for at least 46 years.