Throwback Thursday: How did streams and rivers flow before humans started changing things?

Article: Zimmerman, J. K., Carlisle, D. M., May, J. T., Klausmeyer, K. R., Grantham, T. E., Brown, L. R., & Howard, J. K. (2018). Patterns and magnitude of flow alteration in California, USA. Freshwater Biology.

Natural flows? I want it that way
Part of the Merced River running through Yosemite Valley in California (Credit: Chris Chabot)

Whether it’s a song you haven’t heard in a while or an Instagram post of a goofy childhood photo, most people love a good throwback. One area where that hasn’t always been the case, though, is in water resource management. As we’ve built cities, farms, and roads, we have also done a lot to change the way that water naturally flows through the environment. We have moved water to places where people need it, stored it behind dams, and changed the path that it takes in order to prevent flooding. We can thank this engineering work for being able to turn on our faucets and know that water will come. However, these changes have not been universally beneficial. Scientists are finding increasingly more ways that changing the movement of water in rivers and streams can be ecologically damaging. Studies have shown how it can hurt the native wildlife and plants that live in and around a river.

Rivers and streams could use a throwback. That is, in an effort to support the health of these ecosystems, there is growing interest in managing water in ways that returns river and stream flow to something that resembles what it was before humans came along. With some smart engineering and policy, there are ways to make this work and still meet human demands for water. But one huge problem stands in the way: in many places, we have changed the environment so much and for so long we simply do not know what the natural flows were like. While some rivers have gages that record flow, many of these rivers have likely already been changed by humans in some way and there are many other rivers for which we have no records of flow. This recent study by Julie Zimmerman and other scientists tries to solve that problem.

The Shasta Dam across the Sacramento River in Northern California (Credit: US Bureau of Reclamation)
California Streamin’: California as a case study
A portion of the California Aqueduct, which transports water from Northern California, across the San Joaquin Valley, to Southern California (Credit: lan Kluft)

For this study, the scientists looked at rivers and streams across the state of California. While most people think of year-round sunshine when they think of California, the state actually has one of the most variable climates from year to year of any state in the US. California experiences large fluctuations in annual rainfall, which means there are some really wet years, some really dry years, and it can be difficult to tell which one any given year is going to be. All of this is on top of the wide variety of the type of environments in the state ranging from deserts to forests and coasts to mountains. This makes managing water particularly tough in California. But it also means that natural flows will also show similar variability, which makes it a good test for the scientists’ work.



If you wanna be my model, you gotta help me predict natural flows

 Because there is not data for natural flows of all rivers and streams, the scientists had to figure how to use data that they do have to get the data that they want. It’s a little bit like a puzzle. In order to do this, they turned to data science. In particular, they used a random forest model, (which is not actually a model of random vegetation as I mistakenly assumed the first time I heard about it– If you are interested, you can read more about this type of model here.) The basic idea is that they used a statistical computer model to establish relationships between flows and stream/river characteristics (like surrounding land or climate) for natural streams and rivers. Once the computer model established those relationships, the scientists could use the characteristics of the human-impacted rivers to predict what their natural flows would be. With these natural flow predictions, they were then able to compare them to the actual flows and categorize the human impact as making the flows much higher (inflated), much lower (depleted), or not causing a large change.


Turn and face the strange: Changes to flows from human activity

When the researchers compared the predicted natural flows to the actual flows, they found that 95% of the streams and rivers showed at least one instance where the flow was very different from natural flow and in 11% of the rivers and streams this happened frequently. It was also common that rivers experienced both periods of inflation and depletion, with inflation being more common during the summer months. This makes sense since summers in California are typically very dry, but with human intervention, water is stored during the winter and then released during the summer to ensure availability to people who need it. The results also reflected another regional pattern of management practices. In Northern California, depletion was more common whereas in Southern California, inflation was more common. This result is in line with the fact that most of California’s rain falls in the northern part of the state and the water is then transported to the drier southern part of the state where a lot of people live.

Instances of altered and unaltered mean monthly flow (top row), maximum flow (middle row), and minimum flow (bottom row). Cases of depletion are shown in red, with larger dots indicating it happens more frequently and darker shade indicating it the depletion is severe. Cases of inflation are shown in blue, with larger dots indicating it happens more frequently and darker shade indicating it the inflation is greater. White dots indicate where the flow is not different from the natural flow. (Credit: Zimmerman et al 2018)

Overall, this study showed that humans have had a significant impact on the way that water moves through the environment, which we know can impact the ecological health of an environment. We are moving towards new ways to manage water that help minimize these impacts, and this work is an important step in getting us there. Hopefully, natural flows will soon be a current trend and not just a throwback of the past.

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Jeannie Wilkening

Jeannie Wilkening

I am currently a PhD student in Environmental Engineering at UC Berkeley where my research focuses on ecohydrology, which means I look at interactions between ecosystems and the water cycle. Before coming to Berkeley, I did my undergraduate in Chemical Engineering at University of Arizona and an MPhil in Earth Sciences at University of Cambridge, where my research focused on biogeochemical cycling in salt marshes. When I'm not in the lab, I enjoy knitting, hiking, watching too much Netflix, and asking strangers if I can pet their dog. Twitter: @jvwilkening

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