Neighbors noticed a smell and hundreds of dead fish in Gages Lake and Valley Lake. The Lake County Health Department and Illinois Department of Natural Resources are investigating (Shardaa Gray/CBS Chicago). YouTube Tips ⓘ
A fish kill incident, likely caused by hot weather followed by rain, and a number of other factors, has caused a terrible odor in the neighborhood near Valley Lake in Wildwood and the Gages Lake census-designated place (CDP) in Warren Township.
PSA: The Wildwood Park District is aware of the sightings and reports of numerous dead fish at Valley Lake. We are working in communication with our treatment company and environmental authorities to address the issue.
To avoid a dissolved oxygen crash and fish kill, our treatment company (ILM) purposesly did not overtreat during their last two visits to keep that environment balanced. This current situation can partially be attributed to current low dissolved oxygen levels with the previous hot temperates and sudden rain that can cause the lake to “flip”.
— Wildwood Park District/Tuesday Aug. 19, 2025 11:47 AM/Facebook
The Wildwood Park District updated the community Wednesday, August 20, 2025, regarding the fish-kill incident at Valley Lake in Wildwood that occurred this week. The park district is working with the Lake County Health Department and has been in contact with the Illinois Department of Natural Resources in efforts to identify the potential cause of this event and to assess current water conditions. According to the Lake County Health Department, dissolved oxygen (DO) readings at Valley Lake Wednesday afternoon, August 20, 2025 measured values of 1.5 mg/L of dissolved oxygen at the surface and down to 0.6 mg/L at a depth about one foot down under the lake surface.
Typically, levels below 5 mg/L can begin to stress fish, so the low dissolved oxygen levels would contribute to a fish kill, according to the Wildwood Park District. There can be several contributors for for a Dissolved Oxygen crash. In the summer, water temperatures are higher, and water holds less oxygen at higher temperatures below the water surface. As temperature increases, the solubility of oxygen in water decreases, so less oxygen is capable of dissolving in sub-surface water. Colder water holds more dissolved oxygen.
In addition, heavy rain events that occurred last weekend and Monday can bring in large amounts of organic material, which can be oxygen-consuming, which can cause the fall of dissolved oxygen in lake water. Introduction of a large amount of decaying biological material to a lake can cause a proliferation of microorganisms using up available oxygen in the process of breaking down organic matter.
Drought, harmful algal bloom, overpopulation, or a sustained increase in water temperature can can all contribute to low dissolved oxygen conditions in lake water. Algae blooms are a natural occurrence in nutrient-rich lakes and rivers; however, in populated suburban neighborhoods, fertilizer or animal waste runoff into a lake can cause a spike in nutrient levels leading to algae blooms.
There is another situation of hot weather and rain conditions that can cause fish kills that usually occur in late summer or early fall. Hot weather increases temperatures especially at the surface layer of water, since warmer water rises to the surface, and the surface water is additionally heated by hot air and sunlight. Contrary to the warm conditions causing less dissolved oxygen water, there are situations when the layer of warm water at the surface of a lake can have more dissolved oxygen because of contact and exchange with hot air just above the surface. The oxygen exchange from air to water is also enhanced if water is physically agitated by winds. Air is about 21 percent oxygen and 78 percent nitrogen. The exchange of air and water at the surface, brings higher levels of dissolved oxygen at the water surface, which might cause more live fish than normal to be located near the water surface to support life.
If heavy winds or cold rain occurs and water agitation is excessive, the water layers can mix and cause critical problems for fish. If the layer of low oxygen water below is mixed into the warm surface layer, the mixing can reduce oxygen levels throughout all depths, disrupt oxygen levels that support life at the surface, and lead to fish kill.
Infectious diseases and parasites can also lead to fish kill. Toxicity is a less common cause of fish kill, and is often associated with man-made water pollution.
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