1. Collaborative Research: Unraveling the Controls on the Origin and Environmental Functioning of Oxbow Lakes
National Science Foundation Geomorphology and Landuse Dynamics Program (#1911321), $245,526, 2023-2026
Oxbow lakes are characteristic and environmentally important features of meandering river floodplains. They function as critical habitat for many species and as highly effective sinks for sediment-associated contaminants. In spite of important breakthroughs in our understanding of oxbow evolution, a key part of their life cycle – their origin – is one that is often over generalized and over-looked. Our aim is to test the hypothesis that oxbow formation occurs because geometrically-forced flow instabilities within the limbs of high-angle bifurcations lead to rapid plugging. We propose to couple field measurements and numerical modeling in the study of meander cutoffs on the West Fork White River, Indiana. We focus our attention on the development of sediment plugs within both entrances of cutoff channels. We will assess the importance of bifurcation geometry on plug formation and subsequent oxbow alluviation by examining cutoffs that span a range of geometries. We will also assess the importance of plug formation on the inherited capacity for oxbow contaminant storage, leveraging the history of polychlorinated biphenyl (PCB) contamination on the West Fork White River. Our integrated field and numerical approach will inform experiments that will be used to hindcast and forecast plug development and oxbow formation and quantify their tendency to sequester sediment and pollutants.
Intellectual Merit
Studies highlighting flow dynamics within cutoff-induced bifurcations may give the appearance that we can explain the processes that transition bifurcations into oxbows. But the lack of any systematic or coherent examination of sediment transport and sedimentation at the bifurcation has meant that the field of oxbow science has been stuck. Despite significant advances in understanding the stability of bifurcations over the last twenty years, the study of bifurcation instability has rarely been attempted. Furthermore, most work on bifurcation dynamics focused on idealized geometries. By flipping the perspective and looking at natural geometries, we aim to define under what conditions bifurcations are unstable and form sediment plugs within channel entrances and exits that disconnect the cutoff and form an oxbow lake. This has the potential to transform our understanding of how oxbows form, something that has been elusive. The results of the work will allow us to explain distinct patterns of oxbow alluviation and how oxbow production defines the ecological, geomorphological, and sedimentological characteristics of floodplains.
Broader Impacts
Our plans for broader impacts are twofold. First, we will support environmental management efforts for the West Fork White River. Given long-term impacts of PCB pollution on the West Fork, the Indiana Department of Environmental Management (IDEM) monitors contaminants to use as ecological indicators. We will support IDEM by supplementing fish surveys with estimates of total PCB storage within oxbow and riverbed sediment during each year of the project. Second, we will provide experiential learning experiences for underrepresented high school students and undergraduates. Resolving environmental injustice will require empowering and educating underrepresented and marginalized communities, the success of which involves training a new and diverse generation of environmental researchers. To address this challenge, we are collaborating with two local organizations to deliver a summer camp for high school students from environmental justice communities in the Indianapolis metropolitan region. We will support the camp by delivering two day-long workshops on the importance of rivers and watersheds to society. Further, scholars have demonstrated the effectiveness of early research experiences in inspiring students from underprivileged backgrounds to pursue higher education and science careers. Motivated by this work, we will recruit three summer undergraduate research interns from the Williams Summer Science Program (SSP), which provides an immersive five-week summer learning experience for incoming first-year undergraduates from groups historically underrepresented in the sciences. The Co-PIs will share mentoring and advising responsibilities, and SSP interns will present their findings at suitable meetings of the American Geophysical Union.
2. Combining Theory, Deep Learning, and Lidar to Test Climate and Slope Controls on Tree Throw Production on Hillslopes
National Science Foundation Geomorphology and Landuse Dynamics Program (#1812019), $409,423, 2022-2025
When trees fall over and uproot, they suddenly and stochastically heave soil and rock from deep in the soil mantle to the surface. This process, called tree throw, is an important contributor to hillslope sediment transport and influences soil characteristics, yet we do not know tree throw production rates, probability distributions of those rates, or how those rates vary spatially. Building on recently published data and theory, we seek to answer those questions and motivate the following hypothesis: There is a positive relationship between tree throw production rates and land surface slope that is modulated by the regional wind climate (e.g. strong westerly winds generate more tree throws on east facing slopes). To answer these questions and test our hypothesis, we will train a convolutional neural network (CNN) to identify the geomorphic signature of tree throw (a pit-mound couplet) from widely-available lidar data. We will deploy the CNN on data from five sites with different wind climates (magnitude and direction) throughout the eastern United States. At three of the five sites, we will collect drone-based lidar and assess the CNN predictions. Combining maps of pit-mound couplets and theory, we can extract tree throw production rates, distributions, and spatial patterns. To test our hypothesis, we will compare statistics of wind direction and magnitude against tree throw production rates on different slopes and slope aspects.
Intellectual Merit
Tree throw produces, mixes, and moves sediment; creates canopy gaps and habitat microniches; and influences forest carbon dynamics at local and global scales as major blowdown events potentially cause interannual variability in carbon respiration from Earth’s forests. Forests cover 30% of Earth’s surface and hold ~45% of terrestrial carbon, and tree throw is a common ecogeomorphic process among forests, yet we know little about its frequency and spatial patterns. Tree throw occurs when extreme atmospheric events exert forces on forest canopies that can exceed soil and root strengths. The uprooting creates a topographic signature in forest floors, which creep-like processes rework and degrade. Our work recognizes that the statistics and spatial patterns of topographic roughness contain process information of tree throw rates and the events that drive it. We will establish new methods for automated mapping of pit-mound couplets in topographic data and theory to interpret roughness in process-based terms. Given that the USGS 3D Elevation Program will have continental US-wide lidar data by 2023, one could deploy this method across very large areas. Our work connects statistics of topography to the atmospheric events that drive them and paves the way to use tree throws as a proxy for extreme wind. We will establish a connection between the atmosphere, ecology, and geomorphology – a nexus highlighted in the National Academies’ survey on Catalyzing Opportunities for Research in Earth Sciences.
Broader Impacts
The main broader impact activity of this proposal is to provide K-12 teachers with an experiential learning opportunity and in-hand curriculum to take back to their students. This activity will be achieved as part of Indiana University’s award-winning Education for Environmental Change program, which has brought >180 K-12 teachers, from urban and rural areas, to campus since 2017 for a week-long workshop interacting with IU faculty engaged in environmental research. The program will bring teachers to the IU Research and Teaching Preserve, a hardwood forest at the north end of campus, to explore tree throws and their connection to ecology and hydrology. Teachers will be provided with grade-specific curriculum that incorporates some of the data generated as part of the research. In addition, the proposal will support the training and mentoring of a postdoctoral scholar and contribute to advancing NSF’s Big 10 Ideas by ‘Harnessing the data revolution’.
3. How Farming Decisions Influence Soil Erosion In Marginal, Agricultural Floodplain Agroecosystems
United States Department of Agriculture, Agriculture and Food Research Initiative, Water Quality Program, $338,419, 2023-2027
Soil erosion is conventionally studied on relatively steep hillslopes, but it also occurs on low-sloped agricultural floodplains because of the existence of channels carved into the surface that regularly convey floodwaters from the adjacent river. Compared to hillslopes, the soil-erosion mechanisms and processes developing and sustaining floodplain channels are poorly understood. We hypothesize that agricultural floodplains experience significant soil erosion because focused erosion from regular flooding in locations without crop cover allow channels to form. We will test this hypothesis on the agricultural floodplain along the East Fork White River in Indiana with state-of the-art drone-based lidar, numerical modeling, and state-wide lidar elevation differencing to measure and simulate soil erosion. Our overall goal is to understand soil erosion in channels on low-sloped agricultural floodplains.
The Specific Aims of this Proposal
- Aim 1: Measure water flow and soil erosion on floodplains with novel measurement technologies, such as drone-based lidar.
- Aim 2: Simulate how different vegetation affects soil erosion/deposition on floodplains using a 2-D hydrodynamic-vegetation-sediment transport model.
- Aim 3: Use state-wide lidar, collected in Indiana from 2017 and 2011-2013, to assess the extent of erosion on agricultural floodplains and the role of floodplain channels.
This proposal addresses the program area priority on "mitigation and/or measurement of soil erodibility and erosion to sustain agroecosystems." The rationale for our work is that knowledge of how the soil erodes in agricultural floodplains in response to farmers’ planting decisions can inform how marginal floodplain agroecosystems can be sustainably managed in the future to minimize soil loss.