Invasive Species

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Zebra Mussels Confirmed in Lake Roosevelt--What Happens Next?

The Minnesota Department of Natural Resources (DNR) recently confirmed there are zebra mussels in Lake Roosevelt. Based on the sizes of mussels found by RALALA divers and the DNR, the infestation has likely existed for at least two to three years.

The map at left shows search areas and where mussels were detected. Divers caution that other areas may still be affected. They emphasize that their ability to find tiny mussels that blend in with the surroundings is also affected by vegetation and water clarity. RALALA has also received multiple reports this fall of mussels attached to docks and boat lifts. All were in the general area where divers found mussels.

What to Expect

Effects of zebra mussel infestation vary from lake to lake, but here are a few things we might expect:

• Zebra mussels will make the lake clearer because they consume phytoplankton, zooplankton and decaying matter. The clearer water allows sunlight to reach deeper and encourages aquatic plant growth at greater depths, including invasive species such as Eurasian watermilfoil.
• A lower survival rate of fish fry and minnows due to less food supply may eventually result in fewer adult fish.
• Some studies have linked zebra mussel infestations to higher mercury levels in fish.
• Dead mussels and their waste on the lake bottom may contribute to algae blooms.
• The mussel population is likely to expand over time. Many lakes see cycles in which the population explodes and later subsides when the food supply is exhausted. Eventually the population stabilizes.
• Roosevelt will be officially listed as an infested lake. The DNR posted notification signs at Roosevelt and Lawrence boat ramps.

What You Can Do

According to the DNR, there currently is no treatment that meaningfully reduces an established zebra mussel population. However, there are limited actions you can take:

• Expect mussels to attach to docks and lifts. Scraping them off will not reduce the population, but may be necessary to keep equipment functioning. Use gloves and take caution—the shells are sharp.
• If possible, store your boat on a lift. Cleaning mussels off a boat or motor may be a difficult recurring job. At a minimum, tilt outboard motors clear of the water so veligers (juvenile mussels) cannot enter the cooling system and cause serious damage.
• Depending on infestation density, wear gloves when handling docks and lifts and watershoes when wading in the lake.
• State law requires that any dock or lift used in Roosevelt must be stored on land for at least 21 days before it can be moved to another lake.

Prevention and Monitoring

Inspectors at ramps will look for mussels on watercraft and equipment, including bilges, live wells, ballast tanks and jet propulsion systems. More boats may require decontamination. 

The RALALA Board continues to research ways to reduce the impact of zebra mussels and to prevent spread to nearby waters. We will share updates when new information becomes available.

Eurasian Water Milfoil (EWM) was first discovered in South Roosevelt in the summer of 2016 and a few plants were discovered in North Roosevelt later that same year.  In 2023 a patch of was found in Lawrence Lake.  Every year since RALALA has surveyed Roosevelt, Lawrence, Leavitt and Smokey Hollow Lakes for EWM and applied for a permit to treat the plants found with a targeted herbicide.  The amount of EWM found and the density of weed beds has varied widely over the last 10 years, but the treatment program has been very successful in keeping the lakes largely free of this invasive.  The control of EWM has allowed the use of North and South Roosevelt and Lawrence to continue virtually unchanged from the pre-2016 era.  This has, unfortunately, not been without substantial cost to RALALA and the governmental entities which have helped us fund this program.

The specific herbicide applied has changed over the years.  Most recently it has been ProcellaCOR.   ProcellaCOR is a selective, systemic aquatic herbicide used to manage invasive and nuisance freshwater weeds in ponds, lakes, and reservoirs.  It targets specific plant hormones to prevent growth and is known for its effectiveness at low application rates, minimal impact on desirable native plants, and a "reduced risk" classification from the EPA.  Applications are performed by certified professionals.  Changes in DNR policies in recent years have allowed us to treat not just large beds of EWM, but also more isolated patches of plants to avoid them spreading in subsequent years.  The 2024 treatment program resulted in a remarkable reduction in the number of plants found in 2025.  In 2025 we also moved to a twice per year survey on the lakes where milfoil was found in the spring survey, and will, where appropriate do follow up treatments as well.

RALALA is committed to continuing to experiment with new methods of EWM control in order to best preserve our lakes at the lowest practicable cost.