For
Cravath and Trippe Lakes opening the dams and allowing the water level slowly
lower until the water level is down to a stream bead have performed a drawdown.
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.
The Cravath and Trippe Lakes drawdown started on July 8, 2019.
City
Manager, Cameron Clapper and Parks & Recreation Director Eric Boettcher
worked with city staff David Himself and Andy Ascher to open the dam at Cravath
Lake allowing water to flow both under and over it.
Eric
Boettcher, Parks and Recreation Director mentioned,
“Some other cities will drawdown during the winter seasons only and Whitewater is doing a two season drawdown for a couple reasons, being able to freeze the lakes twice so the evasive species die and on top that Whitewater is going to dredge the lake and having it last longer will dry it out more.”
Monday,
July 8th was the first day to release water from the lake. A marker was painted
on a nearby rock in order to monitor the amount of water dropping each day.
Cameron
released more water Wednesday, July 10th. There is a drain located in the Millpond
near the 5 American Flags Memorial. The water will drain here and at the bottom
of the dam allowing additional water to flow under Main St. toward the creek.
This
is the first step of the drawdown. Staff will monitor the amount of water that
is removed from Cravath Lake and make alterations as needed to stay on track.
Stay tuned for more updates as the drawdown progresses.
Why
does the City want a lake drawdown you might ask?
Whitewater
is trying to freeze out and control invasive aquatic plants, such as Starry
Stonewort and Eurasian Milfoil. There is already a weed harvest that happens
twice a season to reduce the number of weeds in the 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.
Some
worries that the community has are about what will happen to the fish in the
lakes. The drawdown must happen gradually so all fish and wildlife has 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 die if they don’t move up or down stream quickly enough, but this is not expected to be significant,” said Boettcher.
After
the drawdown process is complete there will be a plan in place to restock to
allow for a healthier fish population to return.
Another
worry from the community and the people that share the shoreline is that they
are losing a couple seasons of the use of the lakes and is wondering if the
drawdown will really work.
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.
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.
Boettcher was able to add that,
“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.
The
lake drawdown will continue through the spring of 2021.