When Leaves Fall, So Can Lake Oxygen Levels

Nutrient-rich foliage should be diverted from our waterways.

This story was written by the Our Gem Collaborative team for the CDA Press on Nov. 8, 2020. Read the original article.

While autumn is an especially spectacular season in North Idaho, the colorful leaves decorating our trees can threaten our lakes and rivers once they fall to the ground. Organic debris contains high levels of nutrients such as phosphorus and nitrogen which can impair water quality.

Phosphorus and nitrogen are essential nutrients, meaning all living things need them to grow. When leaves and pine needles shed in cool fall weather, they decompose and release bound nutrients. When it rains, fallen leaves may wash into lakes and rivers through the storm drain system. Excess nutrient loads into aquatic ecosystems cause increased algae and aquatic plant growth. Microbial degradation of this excess organic material leads to very low levels of dissolved oxygen in the water, which is hazardous to aquatic organisms. In Coeur d’Alene Lake, depletion of dissolved oxygen can also promote geochemical reactions that release mining-related hazardous metals from lakebed sediments.

Nutrient inputs to waterbodies are a concern throughout the year but especially in autumn when leaves fall and wash into our lakes and rivers. A 2016 United States Geological Survey (USGS) study in Madison, WI found stormwater nutrient levels peak during fall months when large amounts of organic detritus littered streets. This nutrient load spike can be managed, however. The study found leaves contributed 56% of annual total phosphorus load in urban stormwater compared to 16% when streets were cleared of leaves prior to rain events.

“Our study found leaf removal is one of the few treatment options available to environmental managers for reducing the amount of dissolved nutrients in stormwater,” said William Selbig, USGS scientist and author of the report.

The city of Coeur d’Alene implements this management strategy with the annual Leaf-Fest. Residents are encouraged to rake their leaves and pine needles into the street for the city to remove. City trucks pick up about 700 loads of leaves per year, equating to about 2,000 tons of leaves that would otherwise wash through the stormwater system into Coeur d’Alene Lake and Spokane River, or would be bagged and sent to the landfill. The leaves are taken to the Coeur d’Alene Airport, where they are tilled into the ground to yield a nutrient-rich soil for use in future city projects.

Join Leaf-Fest 2020

Leaf-Fest 2020 runs from Nov. 12 through Dec. 4, starting south of Sherman and moving north. Crews push leaves into large piles to pick up then street sweepers clean up any remaining leaves.

Here are some tips for Coeur d’Alene residents to participate:

• Move vehicles, trailers, basketball hoops, etc. off the street.
• Keep leaves one foot from the curb to allow for stormwater flow.
• Keep leaves away from storm drains.
• Do not mix branches, debris or other trash in with the leaves.
• There is only enough time for one pass through each neighborhood, so rake your leaves into the street now.