How Do Severe Floods Impact Wood Turtles?

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Last summer, parts of Vermont were hit by record-breaking torrential rains, with some areas getting over 9 inches in less than two days. Villages flooded, state highways washed out, businesses were destroyed, and some people lost their lives or livelihoods. These floods were described as a once-in-every-500-year event. Exactly one year later, it happened again. Not even a month later, the city of St. Johnsbury, VT, had a one-in-a-1000-year flood. There is no question that heavy rains and extreme floods are more common here than they were 50 years ago. These recent floods are also causing a re-think about whether, where, and how to rebuild. Humans weren’t the only ones affected by these floods, however, and many people have asked me the same question: how do extreme floods affect Wood Turtles?

A small tractor clears water from a business as flood waters block a street, Wednesday, July 12, 2023, in Barre, Vt. Following a storm that dumped nearly two months of rain in two days, Vermonters are cleaning up from the deluge of water. (AP Photo/Charles Krupa)

Out of all the turtles native to New England, I suspect Wood Turtles are the species most directly impacted by extreme floods. As valley stream specialists, the rivers most prone to extreme floods are the same rivers where Wood Turtles thrive. Their bodies are even built for it. If you think back to high school geometry, you may recall that triangles and pyramids are among the strongest shapes. I don’t think it is a coincidence that a Wood Turtle’s shell forms pyramids on each scute (armored plate), which may help soften the blow when they are caught in strong currents and smashed into boulders. Yet, severe floods are dangerous to Wood Turtles. In one study, ~40% of Wood Turtles were washed downstream annually during late winter/early spring floods, some of which were killed or buried by sediments. In the weeks following last year’s July floods, we saw an uptick in Wood Turtles hit by cars. We attributed this to the turtles crossing roads on their journeys back home after being washed downstream. 

This Wood Turtle was injured by a car in an unusual area for the species to be, suggesting the turtle might have been displaced by severe floods that swept through that area very recently. The turtle is now being cared for by a wildlife rehabilitator. Photo by Lindsay Scott
This Wood Turtle was injured by a car in an unusual area for the species to be, suggesting the turtle might have been displaced by severe floods that swept through that area very recently. The turtle is now being cared for by a wildlife rehabilitator. Photo by Lindsay Scott

Wood Turtles also usually lay their eggs in floodplains. Their nest sites often flood in the winter or early spring but tend not to in the summer when eggs are present. While turtle eggs can survive brief floods, many nest sites last year spent days, or even weeks underwater. It didn’t come as a shock, then, that I didn’t encounter a single young-of-the-year hatchling last fall. But it isn’t all bad news. While the recent severe summer floods were a bad thing for Wood Turtle nests, they were a good thing for nesting habitat. 

Invasive plants such as Japanese knotweed are one of many reasons why nesting resources are limited along the banks of many Wood Turtle streams. Photo by Kiley Briggs

Quality nest sites are a limited resource along many valley streams, forcing Wood Turtles to nest in dangerous settings such as residential areas or along roads. The farther a Wood Turtle travels from a stream to nest, the more likely she will be hit by cars or farm equipment. When humans straightened rivers in the past couple centuries, those rivers also lost their natural sand and gravel deposits. Many rivers that still have prominent sand and gravel bars are also overrun with invasive vegetation that shade out potential nest sites. The result: far fewer places for Wood Turtles to nest.

After the first severe flood last year, however, I saw new sand and gravel bars all over the place, including along streams that lacked quality nesting habitat before. With each subsequent flood, those beaches collected more sand, and cropped up in new places. Many Wood Turtle eggs were damaged or destroyed by these floods, but after floodwaters receded, there were far more places for the turtles to nest. 

The stream pictured here had no obvious high quality nest sites for Wood Turtles prior to the extreme floods in July of 2023. Once floodwaters receded, however, several prominent sandbars were deposited along the streambanks that could be very attractive to nesting turtles. Photo by Kiley Briggs
The stream pictured here had no obvious high quality nest sites for Wood Turtles prior to the extreme floods in July of 2023. Once floodwaters receded, however, several prominent sandbars were deposited along the streambanks that could be very attractive to nesting turtles. Photo by Kiley Briggs

Overall, I suspect these severe floods are a bad thing for Wood Turtles, especially if they continue to increase in frequency and intensity. But, as is often the case in nature, the full truth of the matter is nuanced and complicated. Some turtles were probably drowned, and others were killed by cars while walking back home after being displaced. Flooded eggs may also not have fared well. Are those strains on Wood Turtle populations offset by the improved nesting habitat the floods created? Maybe so, if the floods only happen every now and then, but climate models suggest these events will become more common and more severe.

One recent and widely-cited study examined how habitat suitability for Wood Turtles will change under different climate scenarios. Researchers involved with that study painted a pretty clear picture about how a warming climate will reduce habitat quality for Wood Turtles, especially in southern parts of their range. However, that study didn’t mention floods a single time, and in the Northeast, increased flooding is part of the equation now. If we are to truly understand how a changing climate will impact species such as the Wood Turtle, flooding needs to be a part of the discussion. Gaining a better understanding of how these floods will impact the Wood Turtle will also help us develop and implement habitat management practices to help Wood Turtles survive our rapidly changing world.