How the Floods of 2023 Affected Fish

FEBRUARY 2024

The Summer 2023 floods brought devastation to so many throughout the Lake Champlain Basin—floodwaters inundated homes and businesses, destroyed crops and livelihoods, and washed away roads. Rivers throughout the basin rushed to levels not seen in nearly 100 years, with staggeringly high flow ultimately pouring into Lake Champlain. Though not to the same level of extremity, the heavy rains that we saw in December 2023 raised rivers throughout the basin past flood stage yet again. The effects of these floods on buildings and infrastructure are usually easier to see and to quantify than the impacts on things below the water. What effect did the floods of 2023 have on fish, from the headwaters to the lake?

According to Aquatic Habitat Biologist with the Vermont Department of Fish and Wildlife, Will Eldridge, the July and the December floods had major implications for fish populations throughout the Lake Champlain Basin. What exactly that was depends largely on what kind of habitat the fish were in when the waters came rushing. Smaller streams and rivers in mountainous regions tended to be more challenging habitats. Here, floodwaters are fast and small juvenile fish are more likely to be pummeled.

Headwaters are also home to fish that are particularly vulnerable to flooding like trout. As the floods of 2023 submerged farm fields and parking lots, trout were among the most likely species to have “ridden” the floodwaters into unsuitable habitats where they often die as the waters recede. On top of this, species such as brook trout spawn in the fall in streams by laying eggs into nests called redds, which are built in streambed gravel by females using their tails to loosen up the substrate. Under the gravel, fertilized eggs incubate and hatch into fry by late winter. While most fish had a fighting chance in summer floods, the December 2023 floods may have disrupted swaths of developing brook trout redds.

Downriver, floods have more mixed effects on fish. From a water quality perspective, fish suffer along with the rest of us: as floodwaters collect everything from manure to sediments, from waste to oil, pollutants can have both short-term and long-term effects on fish health. Swift-moving sediments can be particularly challenging in the short-term, as they can irritate and batter sensitive gills.

While water quality can decrease to the detriment of fish, powerful floods can shift the habitat dynamics downstream as rivers and floodplains widen into Lake Champlain in ways that actually benefit fish. Rising waters open up previously unavailable habitat by connecting waterways, and with new habitat comes new food sources. “Floods stimulate the entire aquatic food web,” says Will, “and fish are a part of that.”

A unique thing about 2023 was the fact that the Lake Champlain Basin weathered multiple flood events--in both the summer and in the winter of the same year. While we have data on fish populations rebounding after floods from previous years, we lack data on how fish respond to back-to-back floods, and whether or not multiple floods can add compounding stressors to individual fish health and overall fish populations. “We expect them to bounce back,” notes Will, but as with many of the long-term effects of the floods, “[…] time will tell.”

The floods of 2023 highlighted the importance of working with natural hydrology rather than trying to dominate it. This means avoiding dredging rivers and streams wherever possible and leaving things like boulders and logjams, as these are crucial for fish to take cover. In the Vermont Legislature, the Senate Natural Resources & Energy Committee is working on a bill, S.213, the Flood Safety Act. which advanced in the senate on 2/21/2024. This bill takes important steps to reduce the risk of future flooding and strengthen our communities’ resilienceto address climate resilience through expansion of wetlands and protection of river corridors. A proposed state policy of a “net gain” of wetlands will focus on protection of current wetlands and restoration of degraded wetlands to absorb floodwaters and provide habitat. Further, the measure seeks to limit development in river corridors, which would also help floodwaters dissipate and minimize property damage and debris from washing downstream.

Wetland protection and floodplain connectivity (or space for overflow that is directly adjacent to rivers) is key for protecting all of us from the worst impacts of floods. When fish get swept away into farm fields and parking lots, they have almost no chance of survival and the humanmade area is now damaged by floods. When fish are able to take refuge in wetlands, they are more likely to survive and thrive as the floods carve out new habitats. Floods are a natural part of river processes—they are how boulders are moved, logjams are formed, and channels are carved. As the frequency and intensity of these flood increase with climate change, we need to let floodplains do their job of taking in floodwaters—for the sake of the fish, and for our sake.

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