Severe flooding impacts aquatic life

June 12, 2024  |  By Michael J. Caduto  |  The Outside Story 

Illustration by Adelaide Murphy Tyrol

July 2023 was the hottest month ever recorded worldwide, according to the National Oceanic and Atmospheric Administration, and rising temperatures associated with climate change have dramatically increased atmospheric moisture, causing more frequent and severe storms. 

During Vermont’s July 10-11 flood last year, at peak flow more than 4 billion gallons of water entered Lake Champlain per hour, eventually raising the water level by three feet and reaching a record level on July 23. 

In New Hampshire, a full summer’s worth of rain fell in July, and the state issued 38 flash flood warnings, exceeding the previous annual record of 36 such warnings. 

Floodwater erodes riverbanks, transporting enormous volumes of sand, silt, and loam. Silt coats the gills of fish and aquatic insects, clouds the water, and reduces sunlight reaching the river bottom, where it is essential for growth of the algae that form the base of aquatic food chains. While aquatic species are adapted to recover from annual spring flooding, the increased severity – and unseasonal timing – of floods like the 2023 event can cause lasting damage. 

“During major flood events, in-stream habitat changes significantly,” said Jim Deshler, an aquatic biologist at the Vermont Department of Environmental Conservation. “Large boulders are mobilized and rolled downstream, wood already in the stream is picked up and relocated, and stream banks fail, adding sediment and new wood to the channel.”

Flood surges damage and uproot plants in aquatic and riparian habitats and scour algae from rocks. Macroinvertebrates and fish caught in flood waters are forced downstream and may become stranded in floodplains or die, Deshler explained.

Bottom-dwelling insects, worms, crustaceans, and other lifeforms are adapted to survive seasonal flooding, and can recover over time. Upstream populations can be crucial for repopulating depleted insect communities downstream as insects walk, fly, or “drift” down with the current. Mayflies, midges, and blackflies recolonize quickly after floods, but caddisflies, flatworms, and amphipods are depleted from local aquatic communities for a time. 

Despite these species’ ability to recover, frequent and extreme flooding decreases species diversity and significantly increases the recovery time for many species. 

Deshler’s team surveyed seven sites immediately after the July 2023 flood and monthly through October 2023, and then compared their findings with numbers documented during pre-flood sampling. Immediately after the flood, they found an 86% decrease in the density (or total number) of organisms and a 23% decrease in the richness (number of unique species). Deshler said the densities returned to expected levels by October, with larger streams recovering more quickly than smaller streams. He said this was likely due to upstream populations of aquatic species continuing to drift downstream.  

The Vermont Department of Fish and Wildlife survey on the impacts of 2011’s Tropical Storm Irene on brook trout populations sheds light on how severe flooding impacts fish. Post-flood populations of older trout in the Mad River and Dog River watersheds in north-central Vermont were 41 to 64% lower than pre-flood numbers. Immature fish were especially hard hit: more than 60% of the population was depleted in some locations, while in others the immature trout were wiped out.

“Young fish are generally more susceptible to being lost during flood events, specifically the ever-important young-of-the-year,” Deshler said. “Mortality of adult trout occurs also, especially in a flood at the scale of the one we had in July 2023. Trout populations are expected to recover in two to three years if habitats are left intact after flooding.”

He noted that human responses to flooding – including removing wood and channelizing (straightening, dredging, and confining) streams and rivers – pose the greatest threat to aquatic organisms. “Historic stream channelization decreases flood resiliency, leading to higher flood levels downstream,” Dreshler said. “A stream that is incised and channelized is like a firehose, sending all the water downstream at a high rate of speed, inundating downstream communities, and eroding stream banks upstream.”

The post-flood period is an opportunity to observe what streams and rivers are telling us about their dynamics and to restore habitat sustainably to help prepare for future floods. Protecting and restoring floodplains provides space for storing and slowly releasing floodwaters. Improving habitat and flood resiliency are critical to preserving aquatic life – especially as floods become more common, severe, and unseasonal. 

“It’s important to remember that we all live in a watershed,” Deshler said. “The actions a community performs on a local stream have an effect on our upstream and downstream neighbors.”

Michael J. Caduto is a writer, ecologist, and storyteller who lives in Reading, Vermont. He is author of “Pond and Brook: A Guide to Nature in Freshwater Environments.” The Outside Story is assigned and edited by Northern Woodlands magazine and sponsored by the Wellborn Ecology Fund of the New Hampshire Charitable Foundation.