Understanding the physical processes controlling the amount of fine sediment and gravel embeddedness in streambeds

Excess fine sediment (<2mm in diameter, such as silt and sand) impairs over 40,000 miles of streams in the U.S., degrading habitat quality for many aquatic species and costing tens of billions of dollars annually. When fine sediment deposits on a streambed, it can accumulate within pore spaces between coarser streambed particles (>2mm in diameter, such as gravel). This process degrades streambed habitat quality. To date, the only way to assess the amount of fine sediment in streambeds is through direct field measurements. This project re-envisions fine sediment assessment based on the preliminary finding that flood flow conditions are the primary control on the amount of fine sediment in streambeds. Furthermore, this project seeks to make it possible to predict fine sediment accumulation across entire stream networks from widely available stream data, which will open up new avenues of research on the influence of river network branching structure on streambed habitat quality. Ultimately, this will lead to a transformation in our understanding and management of aquatic ecosystems and basin-scale geomorphology. Project activities will also contribute to several additional broader impacts: 1) increasing participation of women and underrepresented minorities in science, technology, engineering, and mathematics (STEM), 2) improving STEM education, 3) increasing public scientific literacy and public engagement with STEM, and 4) developing a diverse, globally competitive STEM workforce. The overarching research question driving this work is: How do flow conditions control the amount of fine sediment present in the streambed? Preliminary results show that the flow conditions that fill a stream to the top of its banks exert a first-order control on the amount of fine sediment in streambeds independent of sediment supply. The main hypothesis to be tested is that the physical process underlying this outcome is related to the amount of sand traveling along the streambed versus suspended higher up in the flowing water when full mobilization of the gravel streambed ceases and any near-bed sand becomes locked into the gravel substrate. This work will demonstrate that this preliminary finding applies broadly across the U.S. and elucidate the underlying processes behind these predictions. This will be accomplished through the following aims: 1) Demonstrate that fine sediment accumulation is predictable from flow conditions throughout a wide physiographic extent by analyzing a U.S. EPA dataset, 2) Measure the temporal variability in fine sediment accumulation from field measurements, and 3) Quantify the fundamental processes relating flow conditions to fine sediment accumulation under different sediment grain size and supply scenarios through laboratory experiments. Broader impacts will involve the creation and evaluation of a hands-on learning experience engaging the public at the Virginia Tech Science Festival and engaging fifth graders from Title I schools during the Hokie for a Day activity, while discussing streambed habitats and developing awareness of scientists from diverse backgrounds. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.