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What makes a river a river?

Author: A guest post from Emily Iskin, Ph.D., AGU Voices for Science fellow and Postdoctoral Research Scholar at Boise State University

Close your eyes and picture a river…go on, do it!

What did you see? Did you picture a clear, deep mountain stream? A raging river in a steep gorge? A creek with grassy banks and forest? Whatever you pictured, it probably included water. Water is often what people think of first when they think of rivers, especially in the American West where drought rages[1] and water supply is the focus of many high-profile discussions[2].

Flow in the Upper Merced River, Yosemite National Park, California (July 2019). © 2023 Emily Iskin Art & Design

But what you might not have pictured are the other components of river corridors – areas that include the river, floodplain, and surrounding valley – that are just as important as flow. River scientists think of rivers in terms of three main flows: water, sediment, and biota. The first is the most recognizable and can often be observed with the naked eye. The importance of the flow of water is not just limited to how much there is, but also when it’s there and how long it lasts, including groundwater. Perennial streams are those that flow year-round, whereas intermittent streams flow seasonally and ephemeral streams only flow after rain events. While there is not always water in these types of river corridors, they are all rivers.

The second flow, sediment, is the material that makes up the streambed, banks, in-channel bars, and underlies the floodplain. Sediment can be made up of rocks of varying sizes, gravel, sand, clay, and/or silt. Streams can also be lined by bedrock, solid rock that does not get carried downstream. Sediment flow is important as it gets transported and stored in the river and on the floodplain, creating habitat and assisting the movement of nutrients in the river corridor.

Cobbles and boulders in Lake Creek, Twin Lakes, Colorado (October 2020). © 2023 Emily Iskin Art & Design.

The third flow, biota, encompasses all life (and death) in a river. This spans grasses to trees to fish to frogs to birds to beaver and their dams to moose to fallen leaves to dead wood. Wait, what? Yes, fallen dead trees, called large wood, are a vital component of river systems in places where there are trees. This wood creates habitat for all sorts of flora and fauna, stores sediment, aids in getting water up on to the floodplain, and can last in rivers for thousands of years[3]. The biota of river corridors makes them into thriving ecosystems and a central part of any landscape.

Thinking and talking about all three flows is important when communicating about rivers, not only for moving river conservation and restoration efforts forward, but also for increasing respect for the rivers as they are and growing the appreciation that all parts of rivers are natural and necessary.

Large wood and coarse particulate organic matter in the Swan River, Montana near Swan Lake (July 2022). © 2023 Emily Iskin Art & Design.

[1] A. Park Williams et al. (2020). Large contribution from anthropogenic warming to an emerging North American megadrought. Science 368, 314–318.

[2] Rothberg, D. (2022, August 4). The Coming Crisis Along the Colorado River (Opinion Guest Essay). The New York Times. 

[3] Wohl, E. (2017). Bridging the gaps: An overview of wood across time and space in diverse rivers. Geomorphology 279, 3–26.

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