A recent research article found that Hurricane Idalia in the Gulf of Mexico (August 2023) was strengthened by a river plume. An extensive riverine plume in the eastern Gulf of Mexico, extending from the Mississippi-Alabama-Florida shelf to the Straits of Florida, produced a ∼20 m thick low-salinity layer and a corresponding warm upper ocean. This created a 10–20 m thick strongly stratified barrier layer below the surface layer that suppressed vertical mixing and became a critical factor contributing to Idalia's rapid intensification under the relatively less than favorable thermal and wind field environments. In other words, though the traditional meteorological indicators of hurricane strengthening did not forecast the degree of intensification that occurred, the river plume blocked the warm ocean water, allowing it to warm further, feeding Idalia (the reference and abstract are at the bottom).
Jing Shi and the other authors, from the University of South Florida, noted that river plumes should be considered in future studies and forecasts, but that the Idalia event was a “perfect storm.” That is, an unusual confluence of atmosphere, ocean, and onshore (i.e., extensive pre-hurricane runoff and river discharge) circumstances produced the intensification.
This immediately reminded me of my own study of the impacts of 2018’s Hurricane Florence on the Neuse River and Neuse estuary. In the Neuse area the traditional indicators of hurricane strength (category on the Saffir-Simpson scale and maximum sustained winds) were unremarkable—in fact, winds along the Neuse did not even qualify for hurricane strength. However, the storm was extraordinarily large in areal extent and very slow moving, so that high winds were present four a good four days, rather than the usual <1 day. The 4 m storm surge was far above anything seen before in the region, coupled with incoming river flows from the Neuse among the highest recorded (probably THE highest, but gages failed before the flood peak arrived). Because the slower movement and higher rainfall is consistent with predictions associated with climate change, I wanted to understand what geomorphic effects of the storm were perhaps harbingers of the future, and which were attributable to specific local characteristics of the Neuse estuary and of the storm track.
I concluded that the large area of the storm, slow forward movement, and extreme rainfall of Florence are likely indicative of a “new normal” with respect to tropical cyclones in the region, but that the geomorphic impacts in the lower Neuse were largely determined by particulars of the Neuse estuary and Florence's storm track—another “perfect storm.” Since that article was published in 2022, by the way, I have quit using the ”new normal” for climate change impacts, as we now have a constantly moving baseline and basically NO normal.
One finding relevant to our beloved swamps was that the lower Neuse upstream of the estuary mostly handled the combined impacts of a huge storm surge from downstream and massive flooding from upstream just fine. This is due to the complex network of channels, subchannels, floodplains, and other features that are able to convey, store, slow, and exchange water as needed—yet another great reason to protect and preserve them!
I have spun off the perfect storm metaphor to argue that landscapes—including, of course, wetlands and swamps—are perfect in the sense that each reflects the combined, interacting influences of a set of environmental conditions (geology, hydrology, climate, soils, biogeography, topography, etc.) and history that makes them unique and idiosyncratic in some respects.
Tupelo-dominated swamp along the Neuse River in western Craven County, N.C.
For example, there are some areas within the lower Neuse River that are almost entirely dominated by water tupelo (Nyssa aquatica). This is a common swamp tree, and known to dominate some stands, but usually (even when dominant) co-occurs with other species such as bald cypress (Taxodium distichum). But some areas along the Neuse and its side-channels contain almost nothing else in the overstory, canopy-tree layer. What is the perfect combination leading to this?
All tupelo, all the time
First, we can start with a regional climate and environmental setting conducive to bottomland hardwood swamps. Within that, N. aquatica (like Taxodium and swamp tupelo, N. biflora) require very specific hydrogeomorphic conditions to become established. It has to be wet but not underwater, and once the seedlings emerge they have to get tall enough fast enough to not get flooded in future high water episodes. The seeds (of all three trees) are dispersed mainly by water, so the location must be such that they can be transported in by flow. But all three species are present in the lower Neuse, and typically occur together in some proportion, so how did tupelo become so dominant?
The most likely answer is logging for cypress. Though some tupelo has historically been cut, cypress is more valuable and sought-after, and most cypress swamps in the southeastern U.S. have been logged at some point. If the cypress were of good commercial quality, most or all of them may have been harvested, leaving only tupelo to re-seed the site (thanks to Dr. Kimberly Meitzen of Texas State University, who studied this along the Congaree River, S.C., and found tupelo replacement of cypress due to cypress clear-cutting).
A fine tupelo cavity tree
I have tried without success to learn something of the specific logging and forestry history of the lower Neuse (what tracts were cut and when). The area was indeed logged (and is still being logged at some sites), and the lumber industry has always been a mainstay of the region, though not so much so now as earlier in the 20thcentury (speaking here of actual trees and wood, as opposed to pulp and paper). But specific land use histories are hard to find. If I found some sawed cypress stumps up in there it would be sufficient evidence to support the logging explanation, and I will look, because that’s what I like to do.
Let’s go stump hunting!
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Shi, J., Hu, C., Cannizzaro, J., et al. 2025. Intensification of Hurricane Idalia by a river plume in the eastern Gulf of Mexico. Environmental Research Letters 20, 024050.
Abstract
Hurricane Idalia formed on 26 August 2023 and three days later rapidly intensified from a Category 1 to Category 4 strength storm in less than 24 h over the west Florida shelf. On August 30, it made landfall along Florida's Big Bend area as a Category 3 hurricane. Strikingly, despite Idalia's moderate intensity and favorable vortex structure, neither upper ocean thermal energy nor environmental vertical wind shear conditions were as favorable during its intensification from Category 2 to Category 4 as earlier in its path, raising the question of what external factors contributed to its extreme intensification during this phase. Using satellite data, underwater glider observations, and numerical model outputs, this study reveals that, in addition to the 2023 marine heatwave, an extensive riverine plume in the eastern Gulf of Mexico, extending from the Mississippi-Alabama-Florida shelf to the Straits of Florida, produced a ∼20 m thick low-salinity layer (∼34–34.5 psu) and a corresponding warm upper ocean (>29 °C, ∼25–30 m thick). This defined a 10–20 m thick strongly stratified barrier layer below the surface layer with buoyancy frequencies exceeding 10−3 s−1 that suppresses vertical mixing and became a critical factor contributing to Idalia's rapid intensification under the relatively less than favorable thermal and wind field environments. Therefore, incorporating the river plume in future forecast models appears to be essential to improve the accuracy of intensity predictions, especially in the areas affected by the plume, where stratification plays an important role in the intensification dynamics.
Phillips, J.D. 2022. Geomorphic impacts of Hurricane Florence on the lower Neuse River: Portents and particulars. Geomorphology 397, 108026.
Abstract
In September 2018 Hurricane Florence had severe impacts on the lower Neuse River and Neuse estuary, NorthCarolina, despite the fact that it was a minor storm in terms of traditional indicators of storm intensity. Thestorm was consistent with recent trends and predictions of tropical cyclone activity driven by Anthropocene climate warming. However, its impacts in the Neuse area were also conditioned by idiosyncratic aspects of the geographic setting and the synoptic situation. Geomorphic changes examined here include erosion of estuarine shoreline bluffs, geomorphic transformations of small freshwater swamps, and effects on the river and floodplain upstream of the estuary. The shoreline changes caused by Florence were unique with respect to previous tropical cyclones and ongoing episodic erosion, due to the extraordinarily high and unusually long duration of storm surge. Transformations of the“ravine swamps”—mainly associated with deposition of >0.6 m of sand on organic muck and open water surfaces—were similarly unprecedented. Despite high river discharges (third highest on record) and the high storm surge, fluvial impacts in the lower river and fluvial-estuarine transition zone were minimal. This is attributable to the morphology of the channel-floodplain system, adapted to Holocene sealevel rise and preserved by wetlands protection programs. The large area of the storm, slow forward movement, and extreme rainfall of Florence are likely indicative of a“new normal” with respect to tropical cyclones in the region. However, the geomorphic impacts in the lower Neuse were largely determined by particulars of the Neuse estuary and Florence's storm track. An exception is the limited impacts on the lower fluvial portion of the river and the fluvial-estuarine transition zone, where there exists a complex mosaic of channels and flowing wetlands capable of accommodating extreme discharges.
