Drought and Flooding: Two Sides of the Same Coin
Exploring the effects of droughts and floods in western river ecosystems
Dear Readers,
This week we are going to be discussing the subject of drought and flooding events in river ecosystems across the western region. Why western river ecosystems? Well, as Edward Abbey would say “nowhere is water so beautiful as in the desert, for nowhere else is it so scarce.”1 Scarce indeed! I’ve had the opportunity to explore many of our western river ecosystems, where sandstone stains along towering river canyons act as historical markers for water flows that no longer exist. At the same time, I’ve driven down a washboard dirt road through Utah’s desert and have witnessed the power of water following a summer monsoon. You can never forget that red water—pushing sediment to new locations, taking with it seeds of desert flowers and cacti to propagate the next generation. Having been witness to both droughts and floods in western river ecosystems, I can’t help but dive into their implications!
While drought and flooding are often viewed as opposing events with contrasting consequences, their effects on ecosystems are surprisingly intertwined! Droughts are easily defined as prolonged periods with no rainfall, whereas floods are defined as an excess of water beyond the ecosystem's normal confines.2 As you might expect, both are related to precipitation patterns. These precipitation patterns include both rain and snowfall. Snowmelt and glacial melt events also affect streamflows throughout river ecosystems. River flow variations caused by precipitation patterns control the creation and maintenance of river and floodplain habitats, as well as the biodiversity observed throughout river ecosystems.3 To hone in on some of the many aspects of this subject, this article will delve into the effects of floods and droughts on riparian vegetation, invertebrates, vertebrates, and carbon sequestration in western river ecosystems. Given my botany background, we’ll start with riparian vegetation!
Riparian Vegetation
Plants are well adapted to their habitats, where many species have become specialists in dealing with both flooding and drought events. Riparian plants, which are always associated with river networks, are no exception to these adaptations! Riparian conditions are often harsh in western river ecosystems, where riparian plants can experience extreme variation in hydraulic events. Despite the riparian plant species' specialized adaptations to their environment, they are still susceptible to adverse effects from flood-drought events. Prolonged drought events, which are common in western river ecosystems, have been shown to negatively impact plant species richness, biomass, and seedling survival.4 Similarly, studies that have addressed flooding events in riparian ecosystems have found that species richness is generally reduced at flooded sites in nutrient rich-catchments, where it increases at sites in desert and semi-arid climate regions.5 Riparian vegetation's response to both drought and flood events inadvertently relates to their effects on soil processes. However, the effects of droughts and floods can be variable depending on soil texture throughout river ecosystems. Effects on the vegetation often cause feedback effects on other ecosystem processes, such as biodiversity and biogeochemical cycles.
Invertebrates
Aquatic invertebrates are another essential component of western river ecosystems. These particular invertebrates contribute to nutrient cycling and the turnover of organic material, where drought and flooding events pose interesting changes to the microhabitats that invertebrates rely on. 6 Similar to riparian vegetation, many of these aquatic invertebrates have evolved to live in fluctuating stream flows, as stream flow is historically linked to precipitation patterns. With that being said, evolution is a long process, in which it often takes multiple generations to arrive at a well adapted species for a specific habitat. Although drought and flooding events are normal processes in river ecosystems, the frequency and length of flood and drought events in western river ecosystems is shifting. This inevitably has implications for the current aquatic invertebrate generation in western river ecosystems. In general, the response of aquatic invertebrates to drought stress is often a decline in both density and richness, whereas the same is often true for flooding events7 Although many of the effects of floods and droughts are transient, they can have long-term consequences for aquatic invertebrate populations and the ecosystem processes that these communities regulate. This is also true for aquatic vertebrates.
Vertebrates
For all of you anglers out there, this one is for you! I’m sure through the years of fishing throughout different river ecosystems, you are familiar with some of the impacts of droughts and flood events on vertebrates. When the snowmelt is high one year, you may not have any bites due to the muddy nature of the water. Alternatively, when the water is low, there are often restrictions of angling due to the temperature of the water. These are well-known effects. However, other effects that are not as well known relate to the effects of fish egg survival under flood-drought events. Egg hatching often declines under these ecosystem events in river systems, where droughts create heat stress for egg development, and floods cause sediment buildup. The sediment buildup from flooding events is particularly interesting, as fish eggs are buried under sediment, causing a lack of oxygen, which is essential for the hatching eggs' survival. Aside from fish, almost all other vertebrate species are impacted by flood-drought events. As discussed above, plant communities in western river ecosystems generally respond to drought with a shift in species richness and composition, which drives the assemblage of vertebrate communities. Birds—for example—often rely on surrounding vegetation as a source of both food and habitat. As riparian vegetation shifts in response to these events, we can also expect the surrounding vertebrate community to respond to that shift. That includes observing a decline in some species, while observing an increase in other species, with the latter typically being non-native species.
Carbon Sequestration
As you can imagine, plant, invertebrate, and vertebrate communities all have an impact on broader ecosystem processes. Where plants have a role in carbon sequestration and nutrient cycling, invertebrate and vertebrate dynamics also play a role in these biogeochemical processes. As these three components shift in response to drought and flood events, we should expect the biogeochemical cycles of western river ecosystems to begin to shift, namely carbon cycles. One example of this can be seen in Quercas robur, a riparian oak species, where carbon sequestration was higher during flood years versus drought years.8 Moreover, rivers transport nearly 200 million tons of carbon each year, and stores this carbon as ocean sediments. 9 One may infer that flood-drought events may negatively affect the carbon load that is transported to the ocean, where a lack of water may decrease sediment movement and an overload of precipitation pushes sediment to the banks of the river. However, during flood events, carbon is also transported throughout the path of that water. What happens to that carbon during extreme floods is still a widely studied topic that needs more research to determine the fate of that carbon.
Well, I’m sure you are finding yourself a bit lost in this article if a lot of this is new to you! So, let’s summarize the main points. 1.) Flood-drought events are common in western river ecosystems, and will continue to persist into the future. 2.) Flood-drought events have similar impacts on ecosystem processes as they relate to riparian vegetation, invertebrates, vertebrates, and biogeochemical cycles. 3.) Current research on the fate of carbon during flood-drought events is lacking, and it is an important topic to contribute to our understanding of carbon feedbacks in western river ecosystems. 4.) Due to water scarcity and precipitation patterns, western river ecosystems are particularly important and unique in acting as ecosystems to study these dynamics. I hope you have found this article as interesting as I have found it to be! Stay tuned for next week's article, where we will dive into a simpler subject. Hang in there while I learn to write with a more creative tone and less of a scientific approach! As always, stay safe and happy!
Thanks for reading,
Darby
Edward Abbey. 1984. Essay in Desert Images, collected in Beyond the Wall: Essays from the. Page 82.
Cooley, H. 2006. Floods and droughts (pp. 91-142). Island Press: Washington, DC, USA.
Zeiringer, B., Seliger, C., Greimel, F., and Schmutz, S. 2018. River Hydrology, Flow Alteration, and Environmental Flow. In: Schmutz, S., Sendzimir, J. (eds) Riverine Ecosystem Management. Aquatic Ecology Series, vol 8. Springer, Cham. https://doi.org/10.1007/978-3-319-73250-3_4
Garssen, A. G., Verhoeven, J. T., and Soons, M. B. 2014. Effects of climate‐induced increases in summer drought on riparian plant species: A meta‐analysis. Freshwater Biology, 59(5), 1052-1063.
Garssen, A. G., Baattrup‐Pedersen, A., Voesenek, L. A., Verhoeven, J. T., and Soons, M. B. 2015. Riparian plant community responses to increased flooding: A meta‐analysis. Global Change Biology, 21(8), 2881-2890.
MALMQVIST, B. 2002. Aquatic invertebrates in riverine landscapes. Freshwater biology, 47(4), 679-694.
Halse, S. A., Cale, D. J., Jasinska, E. J., and Shiel, R. J. 2002. Monitoring change in aquatic invertebrate biodiversity: sample size, faunal elements and analytical methods. Aquatic Ecology, 36(3), 395-410.
Shupe, H. A., Jensen, K., Oldeland, J., and Ludewig, K. 2022. Droughts decrease and floods increase carbon sequestration rates of Quercus robur in hardwood floodplain forests. Trees, Forests and People, 9, 100294.
Verchot, M. 2015, May 14. How rivers bury carbon at sea. Scientific American. Retrieved August 29, 2022, from https://www.scientificamerican.com/article/how-rivers-bury-carbon-at-sea/