Lake Cornelia and Lake Edina are shallow lakes with poor water quality that are on the state’s impaired waters list for excess nutrients. The Nine Mile Creek Watershed District aims to control nutrient levels (specifically phosphorous) in Lake Cornelia and Lake Edina.
The goals of the Lake Cornelia and Lake Edina Project are to improve water quality and the ecological health of Lake Cornelia and downstream Lake Edina.
The Nine Mile Creek Watershed District (District) and City of Edina are undertaking a multi-phased project to improve the health of Lake Cornelia and Lake Edina. To address pollutant loading from stormwater runoff, the District is designing a stormwater filtration project that will be constructed in the fall of 2021. Annual lake herbicide treatments by the City of Edina to control curly-leaf pondweed (2018-2020) have also been implemented and an in-lake aluminum (alum) treatment was conducted by the District in 2020. Beyond this, the District is working with WSB and Associates to manage goldfish in the Lake Cornelia system. The District will also assess the need to improve oxygen conditions in the lake, after evaluating monitoring data. Learn more about these projects below.
Rosland Park Stormwater Filtration BMP Project
To address “external” watershed-based loading from stormwater runoff, the District is designing a stormwater filtration project. This project will be constructed in the city-owned Rosland Park. It will filter out phosphorus from stormwater entering Lake Cornelia. The District will construct the project in fall 2021.
The District is in the process of coordinating the construction of the Rosland Park Stormwater Filtration BMP project. Construction began in early October 2021.
The project will reduce the amount of external phosphorous loads entering Lake Cornelia from Swimming Pool Pond and the larger 410-acre urban subwatershed that drains to the lake (including Southdale Mall) by diverting water leaving the pond through an above ground filtration system after which the water will be treated before discharging to Lake Cornelia.
In addition, during drier periods when water is not discharging from the pond, water will be pumped from the lake to the filtration system to treat the lake water (reducing internal phosphorus loading) before discharging back to the lake.
The stormwater filtration BMP will include three different “vaults” where different innovative filtration media will be used to determine which media results in the best nutrient removal efficiency.
The District has received grant funding for the design and construction of the Rosland Park Stormwater Filtration BMP from the Minnesota Board of Water and Soil Resources through the Clean Water, Land and Legacy Amendment, and Hennepin County through their Opportunity Grant program to offset capital improvement funds utilized by the District to implement the project.
Curly-leaf Pondweed Herbicide Treatment
Another strategy to improve the health of the lakes is to reduce the amount of curly-leaf pondweed, an aquatic invasive plant, in the lake through annual herbicide treatments. This herbicide treatments are being conducted by the City of Edina. Reducing the amount of the invasive plant will help protect and improve the native aquatic plant community and reduce the amount of phosphorus released into the water when the invasive plant dies off and decays in mid-summer. Another strategy to improve water quality is reducing the amount of phosphorus released from the lake bottom sediments, a process called “internal loading”, by conducting an alum treatment.
An herbicide treatment kills undesirable plants. The City Edina annually conducts herbicide treatments on Lake Cornelia that targets invasive curly-leaf pondweed.
Managing the level of curly-leaf pondweed will help protect and improve the native aquatic plant community in Lake Cornelia and reduce internal phosphorus loading.
How does an herbicide treatment work?
After a Minnesota Department of Natural Resources permit is approved, a licensed contractor applies herbicide from a treatment boat. Based on the spring plant surveys done on Lake Cornelia, an herbicide called Endothall was applied to select areas in Lake Cornelia. Herbicide treatments are done in early spring when the water temperature in the lake is generally between 50-60o F. This is when the herbicide is most effective and limits damage to native plants, which start growing later than curly-leaf pondweed.
In-lake Alum Treatment
Alum treatments help control phosphorous levels in lakes. Phosphorous is a nutrient that fuels algae growth. Trained contractors apply the alum using specialized equipment and barges; this equipment ensures the precise placement of the material in the lake. On contact with the water, the liquid alum forms a fluffy aluminum hydroxide precipitate called floc. Aluminum hydroxide (the main ingredient of common antacids like Maalox) binds with the phosphorus to form a compound that does not dissolve in water. The bound phosphorus can no longer fuel algae growth. As the floc settles to the bottom, it removes phosphorus and particles from the water, leaving the lake clearer. The floc then forms a thin layer on the bottom of the lake, and binds with phosphorus in the sediment. The result is a reduction in the frequency and intensity of nuisance algal blooms.
HAB Aquatic Solutions applied alum at Lake Cornelia on May 21, 2020.
Goldfish and Carp Management
To help improve water quality and the ecological health of Lake Cornelia and Lake Edina, the District is undertaking goldfish management.
Goldfish and carp are an invasive species of fish that cause water quality problems due to their bottom feeding behavior. They stir up lake bottom sediments, reducing water clarity and releasing phosphorus. They also uproot aquatic vegetation, destroying habitat for waterfowl and aquatic communities.
The goldfish population was likely started with the release of pet goldfish into one or more of the lakes within the system. Awareness and public education are needed to prevent this from happening in any small ponds that could perpetuate the invasive goldfish population in the Cornelia chain.
A 2018 fish summary indicated that while there were carp in the Lake Cornelia system, they were not abundant. Goldfish, on the other hand, were plentiful and at a level where management of the population is recommended for water quality purposes.
A follow-up goldfish and carp study was completed in 2020 to assess goldfish and carp levels, track inter-waterbody movement through the Lake Cornelia system and test management/removal methods. Surveyors found a large number of young of the year goldfish in the lake, indicating successful goldfish recruitment (survival to the juvenile stage).
The District will continue to study the goldfish and carp in in the Lake Cornelia system in 2021-2022, to determine the best methods for goldfish management.
The District received grant funding for the goldfish studies on Lake Cornelia from the Minnesota Board of Water and Soil Resources through the Clean Water, Land and Legacy Amendment and from Hennepin County through their Aquatic Invasive Species Prevention program.
District Goldfish Study Featured
In this Minnesota Bound episode (filmed in 2020) you can see the goldfish research team in action starting at around 1:20, as they collect information on goldfish in the Lake Cornelia system in Edina.
The City of Edina takes a look at water quality in Lake Cornelia and goldfish in this featured video.
How can you help with the goldfish problem?
If you have unwanted aquarium fish or plants, never dump them in a lake, creek or pond. Always surrender them at a re-home event.
In addition, the District needs volunteer help! Volunteers will bait fish nets with corn. The corn is designed to aggregate carp and goldfish to the nets, as other fish do not feed on the corn. Once the fish are trained to aggregate in the area, WSB will net and remove the carp or goldfish from the lake. Minimum of one week (Monday-Friday) and approximately 5-10 hours per week. Training and materials will be provided. Contact Erica if you are interested in volunteering for a week this summer.
Learn more at about Lake Cornelia and Lake Edina at:
Studies and Reports
Contact Randy, District Administrator, at 952-835-2078.