The Breakwall

Date: May 12, 2021

In 1998 the Wisconsin Department of Natural Resources completed construction of a rock breakwall, two miles in length, enclosing 569 acres of open water at Terrell's Island for the purpose of restoring fish and wildlife habitat, and protecting thousands of feet of eroding wetland shoreline.  The breakwall acts as a barrier to wave action and turbidity from Lake Butte des Morts and the Fox River, creating a quiescent backwater effect within the enclosure, and thus ideal conditions for the re-establishment of rooted aquatic plants and the desirable fish and wildlife species who depend on them.

At the time, the breakwall was likely the largest ever constructed in North America for the specific purpose of habitat restoration.  In addition to the breakwall, six islands, one-third of an acre each, were also constructed within the enclosure.  The islands were seeded with prairie grasses to attract nesting waterfowl such as mallards and blue-winged teal.  A navigable carp barrier was also installed at the single, 66-foot wide opening along the north side of the breakwall to prevent adult carp from entering the restoration area.  Carp are non-native bottom feeders.  In high densities they can decimate rooted aquatic plants, and significantly reduce water clarity through their feeding activities.

The breakwall was immediately successful as a barrier against wave action and turbidity, as well as carp, within the enclosed restoration area.  This resulted in a dramatic improvement in water clarity throughout the restoration area, which, because of the increased sunlight penetration to the lake bed, resulted in a significant increase in submergent rooted aquatic plants during the 1999 growing season, and an attendant increase in desirable fish and wildlife species.  (Poor water clarity is the primary factor limiting significant growth and re-establishment of rooted aquatic plants on the Winnebago pool.)

By 2003 aquatic plant coverage within the 569 acre restoration area was approximately 75%.  Water clarity was pristine, with clear visibility to the lake bed throughout the entire enclosure.  Migrating waterfowl fed and rested by the thousands during Spring and Fall.  Annual waterfowl nest counts revealed ten to fifteen Canada goose nests, an equal number of mallard nests, and an occasional blue-winged teal nest.  These conditions persisted for ten years up to 2008, however, environmental conditions then began to take a turn for the worse, primarily due to a large and unexpected influx of American white pelicans and double-crested cormorants.

In 2005 thirteen American white pelican nests were discovered on one of the islands in the enclosure.  These birds were fairly common historically, but had largely been extirpated from the inland waters of Wisconsin by the end of the 19th century.  The return of a native species is usually a welcome ocurrance, but by 2009 over 1400 pairs of pelicans were nesting within the Terrell’s Island restoration area and vicinity, virtually eliminating nesting use of the islands by ducks and geese.  In addition, double-crested cormorants also began nesting by the hundreds during this period, ultimately increasing to over 1000 nesting pairs.  

The daily use by thousands of these large water birds within the enclosed restoration area from April through September caused excessive inputs of nutrients from bird guano, resulting in algal blooms that subsequently reduced water clarity, and thus sunlight penetration into the water column.  At the same time a near complete winterkill of the fishery during the winter of 2005 occurred within the enclosure.  (Winterkills occur mostly on shallow waterbodies when thick ice and snow reduce sunlight penetration to the point where aquatic vegetation and algae die, so instead of producing oxygen, the decomposing plants consume oxygen, sometimes to the point where fish and other aquatic life die because of very low oxygen levels.)  The reduction of water clarity, along with the loss of sight feeding gamefish, resulted in the few carp present within the enclosure being able to establish a permanent and ever increasing population.

By 2012 the poor water clarity had resulted in the near total collapse of rooted aquatic plants, with a resultant loss of desirable fish and wildlife.  The five nesting islands were removed in 2011 in an attempt to reduce the number of pelicans and cormorants using the restoration area, but the balance had tipped to the point where the water quality within the restoration area had become worse than that of Lake Butte des Morts.  This was a total reversal of the goal of the restoration project, however, all is not lost.  In fact, there is more to be gained than has been lost.

The Future

In 2015 the Wisconsin Department of Natural Resources formed a working group to solve the problems at Terrell’s Island; essentially to “restore the restoration”.  The group consisted of staff from DNR, the Winnebago County Land & Water Conservation Department, the Fox-Wolf Watershed Alliance, the Fox River Green Bay Natural Resource Trustee Council, and the Butte des Morts Conservation Club.  By 2018 the group had developed a plan to restore water quality within the original 569 acre restoration area, and to create a second restoration area south of Terrell’s Island in Samers Bay.

The plan consists of removing the partially exposed rock armoring surrounding the former island sites.  (Even though pelicans and cormorants are no longer able to nest there, they still rest on the rocks by the hundreds and sometimes thousands.)  The rock will be used to build part of a new breakwall in Samers Bay.  In addition, 2200 feet of the Terrell’s Island breakwall will be removed and used to construct the Samers Bay breakwall.

The 2200 foot opening in the original breakwall will allow for adequate water circulation within the restoration area, thus eliminating the high nutrient levels through flushing action.  Excessive nutrients from bird guano will be greatly reduced by eliminating roosting and resting substrate (i.e. partially submerged rocks) used by pelicans and cormorants.  The problem of recurring winter fishkills should also be eliminated because of increased water circulation from the main lake.  Fish and other aquatic life, as well as small boats, will also have easier access through the large opening in the breakwall.  Of course this includes carp, but the likelihood of them concentrating at Terrell’s Island will be reduced by the fact that they can leave the area as easily as they can access it.  Basically the Terrell’s Island breakwall enclosure will be modified to mimic large embayments such as Scotts Bay and Sunset Bay on Lake Butte des Morts, and Boom Bay on Lake Poygan, which are some of the most productive bays, in terms of fish and wildlife habitat, on the Winnebago pool.

The Samers Bay breakwall, 5520 feet in total length, will have 35-foot openings every 500 feet in addition to a main opening  of several hundred feet, creating a natural embayment of 111 acres while also protecting over a mile of eroding wetland shoreline. 

The most productive bays on the Winnebago pool, whether natural or man-made, large or small, all have one major thing in common: shallow water adequately protected from prevailing winds and/or river currents.  This one very important condition provides for improved water clarity sufficient to grow stands of aquatic vegetation to attract desirable fish and wildlife.  The only other way to provide good water clarity on a significant and long-lasting basis is to lower the summertime water level of the Winnebago pool.


In February of 2021, 458 feet of the Terrell’s Island breakwall was removed, and was used to build 1000 feet of the Samers Bay breakwall.  Construction will continue in 2022, ice conditions permitting.  The majority of the over $1 million in funding for this project comes from the Fox River Green Bay Natural Resource Trustee Council, with contributions also coming from the Winnebago County Land & Water Conservation Department, and the Butte des Morts Conservation Club.