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We are trying to freeze out and control invasive aquatic plants, including Starry Stonewort & Eurasian Milfoil. We currently perform a weed harvest twice a season to reduce the number of weeds in our lakes. An extended drawdown has many other benefits to the lake including sediment desiccation, which means the silty or mucky bottom can compress up to 1/3 of its depth when fully dried out. This would result in deeper water in our shallow shore areas. This along with a dry dredging while the lake is drawn down would allow for deeper lake and a navigable channel for recreational use. An extended drawdown also would also allow other invasive species to be controlled, while some beneficial native plants, that provide excellent fish and wildlife habitat, are expected to rebound. The extended drawdown would also give the city and/or DNR an ideal chance to inspect the dam while it’s dry.
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For Cravath and Trippe Lakes a drawdown would be performed by opening the dams and allowing the water level slowly lower until the water level is down to a stream bead. A Lake drawdown is one tool that can be used to manage aquatic weed problems. Lake level drawdowns often start in the fall and continue through the winter when water recreation uses are at their lowest. Most aquatic weeds are found near the shallow shoreline. Our drawdown will start on July 8, 2019 and be an extended drawdown that will last until the spring of 2021.
The drawdown would begin the July 8, 2019 and would be part of a two-year process ending in the Spring of 2021. The lake would be drawn down slowly allowing for fish and wildlife to move up and down stream. Dredging some of the lake area would take place in the winter of 2020/21. The lake would then be allowed to refill in the spring of 2021.
If we get average rainfall in the spring, the lake should be back to normal levels by June of 2021. If we get a lot of snow or early season rain it could be full even quicker.
The drawdown must happen gradually so all fish and wildlife have enough time to locate to deeper water. The fish are expected to move with the water as the lake level goes down. It is possible that some fish may perish if they don’t move up or down stream quickly enough, but this is not expected to be significant. After the drawdown process is complete there will be a plan in place to restock to allow for a healthier fish population to return.
The dredging exemption is for manual dredging, which means dredging by hand or using a hand-held device without the aid of external or auxiliary power. View more information on the manual dredging exemption.
DNR also has a New Small Scale Dredging Permit, which allows for the removal of 25 cubic yards over 5 years. Landowners would have to comply with the conditions of the checklist.
Larger scale dredging permits need to follow the process outline.
The DNR does not generally recommend contractors.
If the removal of the material is by hand, the manual dredging exemption would apply if less than two cubic yards. If the removal is not by hand, it would fall under the conditions of a dredging permit.
Riparian owners can place pea gravel above the ordinary high-water mark; below the ordinary high water mark a permit is required. More information on pea gravel.
Yes, if done by hand. If any machinery is needed, it would fall under the conditions of a dredging permit. If less than 25 cubic yards, it may qualify for a small-scale dredging permit.
DNR has a general rip rap repair permit.
If a riparian wants to repair rip rap on their property and they don’t meet the conditions necessary to get the general permit, they can apply for an individual permit. View for more waterway information.
If a riparian wants to replace their rip rap entirely, they may qualify for an exemption from permitting. For more information on that, see the exemption checklist below. If the riparian does not qualify for the exemption, they can apply for a permit.
DNR has a general permit for the placement of new rip rap. Riparians should see the checklist to see if they meet the conditions. Otherwise, biological shore erosion control is a good option for shore stabilization. These activities are exempt from permitting, provided they meet the conditions of the exemption checklist.
The starry stonewort bulbils and stems are heavier than water and typically sink to the bottom, so it is unlikely that many will be transported downstream by water currents. This species also hasn’t been found near the outlet dam. It could be possible for some pieces to move downstream in the outflow, but the risk would be higher for species with floating fragments that are found close to the dam.
Boaters are the number one-way aquatic invasive species are moved between lakes. Remember to inspect your boat and trailer, remove aquatic plants and animals from your equipment, and drain your boat, live well and equipment before moving to a new lake, and put your catch on ice to help prevent the spread of aquatic invasive species.
The exposed lake bed should freeze over the winter although there may be soft spots in areas where groundwater enters the lake. The severity of freezing will depend on the weather conditions this winter. Snow cover will insulate the exposed lake bottom and warm weather during the winter will allow water to run under the snow/ice layer. A cold and relatively dry winter will provide the best conditions for solid and deeper freezing. The DNR will measure the exposed lake bottom surface this winter to determine the depth of sediment freezing.
The boat launches will be open, but it will likely be difficult to get boats and trailers to the water edge after the drawdown this fall.
Removal of living plants is only allowed in a single 30-foot-wide path adjacent to a landowner’s property, and only if the area is not located in a sensitive area as determined by the DNR, and the removal doesn’t interfere with the rights of other riparian property owners.
Aquatic plants that reproduce mainly by seed are known to be most successful following drawdown. Muskgrasses (Chara), Naiads, and thin-leaf pondweed species have been the most successful following other lake drawdowns. Plants that reproduce mainly by rhizomes and vegetative reproductive structures (winter buds, turions, bulbils) such as milfoil, coontail, and certain pondweed species are expected to decrease following drawdown. It is expected that emergent plants (cattail and bulrush) will recolonize shallow water areas to some extent the year after the drawdown. These plants have evolved to adapt with changing water levels and may benefit from the extended drawdown.
This is likely a filamentous alga called Spirogyra or a similar species. Spirogyra is a native green alga that grows on the lake bottom in early summer. Air bubbles get caught in the algae and it eventually floats to the water surface and decomposes. Aquatic plants cause some shading of the lake bottom and prevent the growth of this type of algae to some degree. Excessive removal of aquatic plants means that there will likely be more Spirogyra. The amount of aquatic plants that return following the drawdown will likely dictate the amount of filamentous algae that is floating in the lake.
There are numerous examples in Wisconsin and elsewhere that show that Eurasian water milfoil can be substantially reduced for multiple years following an overwinter drawdown if the exposed lake bottom freezes. Preliminary laboratory testing with starry stonewort has shown that freezing, even for short periods of time, will kill the star shaped bulbils that allow for plant regrowth.
In addition, the exposure of lake bottom sediments to dry and freezing conditions can cause the organic sediment in the exposed lakebed to compact and oxidize; increasing the water depth following the drawdown. This oxidation can lead to increased release of phosphorus from exposed sediments initially after the lake is refilled, but less phosphorus release after the initial flush from refill.
The extent of control of EWM and SSW and compaction of lake sediments will depend on the severity of the two winters and the amount of drawdown that is possible. Colder and dryer fall and winter weather will create conditions for better control of these invasive plants and organic sediments. Cracking sediment on the exposed lakebed is a sign that the lake bottom has dried enough to allow compaction of organic sediments and plant seed germination. The exposed sediments will be checked in winter to determine the depth of frost and freezing conditions.