UPDATE: Dam fears spark warning downstream

Saturated ground on the north side of the Phantom Lake Dam has stirred concerns about the structural integrity of the dam that creates an impoundment for Upper and Lower Phantom Lakes, Mukwonago Fire Dept. Lt. Todd Beschta said this afternoon.

If the dam were to burst, it would not likely cause any danger to residents along either lake, Beschta said, but Mukwonago officials have notified officials in Big Bend and Vernon in Waukesha County and Tichigan and Waterford in Racine County about the threat of the dam bursting and the potential for it to send massive amounts of water into the Fox River toward those communities.

A branch of the Mukwonago River flows out of Lower Phantom Lake, and drains to the Fox River, east of Mukwonago and southwestern Waukesha County.

The trouble began about 9 a.m. when a hunk of grassy bog material, about 20 feet wide, broke loose and drifted across Lower Phantom Lake into the outlet of the Mukwonago River and became jammed in the dam, forcing water to cascade around the dam instead of over it, Beschta said.

That flowing water saturated the surrounding soil, weakening the stability of the structure, said Tonya Meyer, water management specialist with the DNR.

The Mukwonago Fire Department and Department of Public Works removed the bog material and discovered that the cascading water had saturated the surrounding ground.

All boating traffic on Lower Phantom Lake has been closed. Police have warned residents living on Lower and Upper Phantoms to not use the lake.