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Roller Mill Dam

Click this text for factual information regarding Nemahbin Roller Mill Dam issue

It is the opinion of your UNLMD board; the Nemahbin Roller Mill Dam has a history of adverse affects on the water quality of Upper Nemahbin Lake.  The lack of maintenance and repair, over the years, has resulted in a growing threat of injury and property damage to those who live downstream in the event of dam failure.  There is substantial factual evidence that appears to support this opinion.  This page of the UNLMD website will accumulate information about the Nemahbin Roller Mill Dam as time goes on.  In keeping with the Lake Management District website being an informational/educational tool the facts and perspectives of the Nemahbin Roller Mill issue will be presented here for everyone to reference in order to better understand the issue as it pertains to members of our lake management district.

Sediment, accumulated over the years in the impoundment, has been discharged into Upper Nemahbin Lake on several occasions. District members became aware of this problem shortly after the failure of the Dam in 1988.  A "delta" of sediment was noticed at the mouth of the Bark River where it flows into Upper Nemahbin Lake.  This sediment, previously contained in the dam impoundment, has continued to grow in area and volume with each addtional illegal discharge.  It has tripled from its original size, is highly weed infested due to high nutrient content and causing on-going water quality degradation. This  is exhibited in the following photos taken during one such illegal discharge.

Discharge from Upper Nemahbin Roller Mill Dam Town of Summit Wisconsin Waukesha County

The following information is presented to provide an understanding of the impact of these actions, the debauchment of Upper Nemahbin Lake water quality and the risk of injury and property damage due to the very real potential of the failure of the Roller Mill Dam.

BACKGROUND DAM OPERATION:  Margaret Zerwehk, owner of the Roller Mill Dam, has been granted a permit by to operate between an elevation of 97.50 and 99.00 feet.  Operating below 97.50 feet, without the approval of the WDNR, is an illegal discharge.  State statutes provide the mechanism to impose fines and other penalties when the dam is operated below the specified minimum elevation.

BACKGROUND DAM SAFETY:  The Wisconsin State Administrative Code NR 333 identifies the compliance requirements for the safe operation of dams in Wisconsin.  Regular inspections are conducted by the WDNR Bureau Of Water Regulation and Zoning to ensure that dams do not present at threat to public safety.  The Nemahbin Roller Mill Dam, as a result of a safety inspections beginning on or before 1994, has been designated as a Class III, "High Hazard," dam.  The Safety Inspection Report Dated April 8, 1994 indicates "As it appears in this report the Nemahbin Roller Mill Dam does not meet the requirements of NR 333.07(2) on minimum hydraulic capacity.  Upgrading the spillway is required.  The additional spillway capacity must be designed by an engineer registered in the State of Wisconsin."  Since that time Dam Safety Inspections have continued to identify this deficiency and risk of potential injury or damage to downstream property owners by non-compliance with NR 333.07(2).  Required Completion Dates were set for the resolution of this hazardous condition.  In each case the date was not met and set-back again by the WDNR.

It has been more than 15 Years since the Nemahbin Roller Mill Dam has been designated as a High Hazard.  Mrs. Zerwehk filed a request to abandon the dam in 2004.  It has been 4 years since the UNLMD held an open meeting with the WDNR to address our concerns to either repair or abandon the Nemahbin Roller Mill dam and prevent injury or property damage to our members.

BACKGROUND INCREASED RISK:  For many years the gates controlling the amount of water flow over the Fish Hatchery Dam, upstream of the Roller Mill Dam in the City of Delafield, were only partially operable limiting the amount of water that could be discharged downstream. Repairs have been recently completed that now allows these gates to be fully opened.  This will now allow more water to be discharged downstream during a storm event.  This surge condition will place a significant increase in the hydrostatic pressure on the Roller Mill Dam constructed of earth in the 1840's without the safety factors provided by steel reinforced concrete.  This problem is aggravated by the lack of adequate spillway area identified in dam inspection reports since 1994.  The increased risks, due to greater flow at the Fish Hatchery Dam, has been diminished by opening the former penstock during the June 2008 flood event. It is hoped that the impact of this increased flow and protracted delays in repairing or abandoning the Roller Mill Dam will not become contributing factors to the failure of this "High Hazard" dam.

If, as a member of the District, or a down stream neighbor you believe that these protracted delays and potential greater flood surge, as outlined above, have increased the risk injury, flood damage to your property or your use of Upper Nehmahbin Lake for fishing or recreation has been diminished due to the removal of all of the stop logs of the Roller Mill Dam, as ordered by the WDNR, and the continuing build up of muck in the Bark River, otherwise contained in the dam impoundment, please contact:

Ms. Michelle Schneider Dam Engineer at:


 Bark River Physical Characteristics - Roller Mill Dam Issues


UNLMD Restoration Plan May 20, 2013

Wisconsin DNR Letter to Commission William  Barthel UNLMD March 21, 2013

UNLMD Letter sent to WDNR Secretary Cathy Stepp February 26, 2013

Bark River Physical Characteristics & Roller Mill Dam Issues PDF September 2008  

UNLMD Letter to WDNR July 18, 2008

UNLMD Response Letter to WDNR  Order to Draw Down Dam July 7, 2008

UNLMD Letter to WDNR Requesting Hearing and Full Environmental Impact Statement June 28, 2008

WDNR Order to Draw Down Dam June 18, 2008

Letter From DNR to Mrs Zerwikh June 7th 2007

DNR Letter to Mr. Carison Roller Mill Dam November 4th 2005

Sarakins email to Barthel Roller Mill Dam January 13th 2005


Zerwehk Application to Abandon Dam 2004

Letter to Mayor Of Delafield Roller Mill Dam Inspection June 4th 1999

Letter from DNR to Mrs Zerwikh June 9th 1995

DNR Complaint against Zerwikh July 9th 1990

Wisconsin DNR Letter to Clyde Cheney February 12th 1990

Waukesha District Attorney Letter to Weidensee of Wisconsin DNR August 18th 1989  

Wisconsin DNR Letter to John Lacenski July 27th 1989



DNR Letter to Clyde Cheney July 19th 1989


Chapter NR335 Wisconsin Department of Natural Resources Dam Maintenance, Repair, Modification, Abandonment and Removal aid Program


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