Collaborative Research: Emergent Hydrological Properties Associated with Multiple Channel-Spanning Logjams

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In this proposal we look to quantify relationships among river shapes, logjams, and the movement of water around jams and through surrounding sediments. In doing so, we will advance understanding of (i) how logjams affect water quality and stream communities in forested watersheds, (ii) the effects of historical and continuing human alteration of river corridors in forested regions, and (iii) how to design artificial logjams in rivers to manage stream temperature, nutrients, and aquatic communities. Our work will be shared with resource managers and practitioners to help with issues associated with river management and restoration. Student participation will include (i) K-12 students via course materials that will be developed in collaboration with elementary school teachers; (ii) undergraduate students involved in field data collection and analysis, including students from diverse backgrounds; and (iii) graduate students responsible for the primary data analysis and interpretation, who will have the opportunity to work with and mentor the undergraduate researchers.The benefits of large wood (LW) in river corridors are numerous, including the potential to enhance hyporheic exchange flow (HEF). Existing work has focused on HEF near single logs or single logjams. However, natural channels in forested regions with minimal human alteration commonly contain abundant dispersed LW pieces and multiple logjams spaced irregularly along the channel. We have little indication of whether multiple channel-spanning logjams produce an additive or nonlinear effect on HEF, but these alternatives have important implications for understanding river ecosystem function and for river management and restoration. Our primary objective is to quantify how HEF changes with increasing channel heterogeneity associated with channel-spanning logjams. We will use field measurements, physical experiments, and numerical models to evaluate the characteristics of HEF associated with different spatial densities of channel-spanning logjams. We expect nonlinear relations between logjams and HEF to result from increased bedforms such as pools that maximize flux rates, increased head gradients associated with decreased downstream spacing between logjam-induced backwaters, and thicker deposits of sand and gravel, which act together to create steep hydraulic head gradients within thick, permeable bed sequences. We also expect a nonlinear relation between logjams and HEF at the transition to anabranching as a result of the enhanced HEF between divided channels. Implications of this work will be with respect to engineered logjams, which are currently being added to rivers, but with no systematic understanding of how the effects of LW addition scale with river size, the volume and spatial distribution of wood addition, or the characteristics of the wood and the channel, constraining our ability to design LW-based river restoration to achieve a desired level of HEF. As part of our research, we will develop curriculum for a local K-12 school.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.