This study uses Fossil Creek, Arizona as a model system for studying how restoration of flow and removal of non-native fish affects the physical and biological structure of streams. For the last century the majority of water was diverted from Fossil Creek for hydropower production. Water was returned in 2005 as part of a river restoration program that included removing non-native fish. Fossil Creek was targeted for restoration because it retains native fish and has unique travertine dams creating steep waterfalls and deep pools. This study will test how increased flow affects travertine formation, river productivity and biodiversity. The integration of geomorphology and ecology will advance our understanding of the interplay of physical and biological process in structuring stream ecosystems. Fossil Creek will serve as a national case study in river restoration by testing whether restoration goals are met. Fossil Creek is a living laboratory where hundreds of high school, undergraduate and graduate students are learning to conduct basic and applied research. This project will train a new generation of students skilled at communicating their science to managers and the public. Public outreach includes a PBS video, an article in Scientific American and outreach materials for visitors to Fossil Creek. Together these materials will educate the public about the importance of science in river restoration.
The solutions to many problems in river management involve understanding how living organisms are affected by physical, non-biological processes, such as water flow and the movement of sediment downstream. Recently, river scientists have begun to appreciate how living organisms are not only affected by physical processes, but also can influence how those processes work, and thus help shape their own physical environment. Streams where travertine rock crystallizes from spring water rich in dissolved ions provide an excellent example. In these streams, aquatic plants and animals can influence the formation and growth of travertine dams, creating a channel pattern of alternating steps and pools, which in turn provide better habitat than the same stream would without the steps and pools. In this project, we took advantage of the decommissioning of a hydro-electric project on Fossil Creek, Arizona, where restoration of CaCO3-rich spring flow after nearly a century of flow diversion, triggered rapid re-growth of travertine dams. We explored the interactions between biological and physical processes in the development of the step-pool channel pattern, seeking evidence for feedbacks. We considered three ways that biological and physical processes might interact: first, the situation where living organisms have a minor effect on the rates of physical and chemical processes that produce travertine dams, which mostly depend on non-biological factors such as water chemistry and flow velocity; second, where biological processes combine with physical processes in mutually reinforcing feedback loops; and third, where the effects of organisms on their environment work in opposition to physical processes, such that the travertine step-pool pattern reflects a balance between dominantly-biological processes that build travertine dams and dominantly-non-biological processes that erode travertine dams. We considered three phases of a life cycle of travertine dams: formation, growth, and the destruction by floods. Where dams form depends on physical factors, such as narrow points along the river, and biological factors, such as where logs and other woody debris collect. We measured changes in travertine thickness on a bedrock step, and found evidence for a mutually reinforcing, positive feedback between flow velocity and travertine deposition, a classic example of physical feedback thought to occur without any influence of living organisms. However, we measured the organic content of travertine samples and found that the growth of algae, contributes substantially to travertine accumulation. Plus, the rate of growth is most rapid during seasonal algal blooms. To quantify the growth of travertine dams, we embedded magnets into nascent travertine dams. We calculated growth rates from changes over time in the intensity of the magnetic field at the dam surface with a magnetic locator. We recorded a range of hydraulic and travertine composition variables to characterize the mechanism of growth: non-biological chemical precipitation, algal growth, trapping of organic material, or growth of large plants. We find: (1) rapid growth of travertine dams following restoration of the spring-fed water flow; (2) dam growth rates decline downstream, consistent with the loss of dissolved chemical constituents because of upstream travertine deposition, but also parallel to a decline in organic content in dam surface material and a downstream shift in dominant type of biological influence on travertine deposition; (3) biological influences are associated with faster travertine dam growth rates than non-biological physical factors; and (4) where hydraulic attributes, such as flow depth and velocity, are correlated with faster growth rates, those physical processes appear to spark increased biological influence on travertine growth. We conclude that the strong influence of living organisms on rates of travertine growth, coupled with the beneficial effects of travertine on ecosystem health, demonstrate a positive feedback between biology and geomorphology. During our study, several floods occurred causing erosion of travertine. We used measurements of travertine dam growth and erosion, together a record of river flows, to explore how the river channel is shaped by flows of different sizes. A long-standing assumption in river science has been that moderate-size flows are responsible for shaping rivers, because low magnitude flows are too weak to have much effect, even though they occur most of the time, and high magnitude flows, do not occur frequently enough to control river form, despite the great power they have to cause erosion. Our results suggest a new way to think about the trade-offs between flow magnitude and frequency in controlling river channel form. Instead of one dominant moderate-sized flow, there may be two characteristic flows that together create the channel form in travertine streams: a frequent low-magnitude flow that supports the biological processes that drive travertine dam growth, and an infrequent, high-magnitude flow, capable of eroding travertine and counteracting the influence of organisms on the channel form. On average, the channel form, and functions of the river such as providing ecological habitat and conveying flood waters, reflect a dynamic balance between the constructive effects of organisms and the destructive effects of non-biological forces.
