By Kate Isaacson
As a child, one of my favorite Walt Disney movies was “Pocahontas.” The free-spirited young woman who dreams of more inspired me as I grew up and faced my own challenges. These days, as an adult, the songs of “Pocahontas” continue to ignite flames of inspiration within me. When she sings, “What I love most about rivers is, you can’t step in the same river twice, the water is always changing…” It reminds me of the constant ebb and flow of all things in life. Pocahontas was onto something there – but there is a lot more to the story when it comes to evolving waterways.
Water molecules continuously flow through river systems, but it is not just the water that is in consistent dynamic change. The sediments, gravels, woody materials and aquatic organisms are also constantly moving. And that is a good thing.
Fluvial geomorphology is a scientific study that has made headway in recent years to bring river movement concepts into common knowledge. The term refers to natural river movement processes, such as erosion, transportation, floodplain formation and deposition of sediments. A central theme of many river conferences recently has been ‘freedom space’ and allowing rivers room to roam as they did before vast urbanization choked their banks. But why do rivers need to move?
I asked this question to Bill Hoblitzell, consulting watershed scientist with Lotic Hydrological, who eloquently answered, “In addition to water, [rivers] carry a lot of ‘stuff,’ and that stuff is sediment. Each year, the river moves a load of fine sediment -sand and silt-and coarse sediment – gravels, cobbles, even boulders- downstream. During the high snowmelt periods, (the river) picks it all up and moves it a little further down the line. This repeated physical process then sets the stage for all the biological processes – fisheries habitat, spawning gravel replenishment, new riparian colonization, etc.”
Riparian zones, the vegetated buffer along the riverside, include specialist plants, which offer substantial ecological services, such as bank stabilization, stormwater runoff filtration, floodwater absorption and temperature reduction, as well as wildlife habitat, breeding and rearing grounds and food. Riparian plants are some of the most important species in our mountain ecosystems, and they require recharge from river movement to thrive. As Bill explains, “The riparian plant community especially depends on a patchy mosaic of different habitats, and the short- and medium-term disturbances created by the river, such as scouring a meander, bank erosion, overbank flooding. All this sets the stage for the diverse plant and animal communities that come later.”
So exactly how far do rivers need to roam? The answer: it completely depends on the unique geology in the area. For example, in deep bedrock canyons, the rock formations are typically more resistant to erosion and river movement from side to side is slow. In areas like Edwards, where the substrate consists of fine sediments dropping out of the system, as the steep grade significantly decreases, and a slower flow allows for greater movement of the river side to side. This movement can be vast (inches, feet or more) and as Bill says, “constant meandering is the norm.” The predominant factors that influence the degree of river movement are the soil type (which determines how well the soil holds together), resistance from plants and size of the river.
We are learning the tough lesson that we cannot control the powers of Mother Nature. She always wins one way or another, because she plays the long game and river movement is a classic example of this. Rivers create energy with all of that moving water and when rivers are constrained, the energy builds and eventually must be released, often in violent ways. Allowed to migrate naturally, river systems function the way they need to, and vegetation and wildlife subsequently flourish.
Humans aren’t the only creatures that affect change in rivers, however. Busy beavers bustling about do modify the ecosystem and are often cited for negative stream impacts. They have earned a bad reputation, but are actually, “an integral part of the mountain river system where we live,” as Bill explains. “Overall, beavers trap and slow sediment transport in the watershed, increase the complexity of channels and riparian zones, and provide a greater diversity of aquatic habitat types for other species. They need a healthy food supply to thrive, usually in the form of willows and woody riparian vegetation. If beaver communities are present and sustainable, it often means your riparian zones are in good condition.”
Though rivers are often thought of simply as water corridors, there is so much more to stream and river systems. Plants, wildlife and human-interaction all play major roles in the health of our rivers, along with the energy they create and distribute.
Kate Isaacson is the Projects & Events Manager for Eagle River Watershed Council. The Watershed Council has a mission to advocate for the health and conservation of the Upper Colorado and Eagle River basins through research, education, and projects. Contact the Watershed Council at (970) 827-5406 or visit erwc.org.