Justin C. Stout

Identifying Sediment Sources and Sinks in the Root River, Southeastern Minnesota

Excessive loading of fine sediment is a prominent cause of river impairment, not only due to direct effects on biota and habitat but because sediment is often laden with excess nutrients, metals, and toxic substances. Determining the sources and transport pathways of sediment has proven challenging. The Root River watershed in southeastern Minnesota was listed under section 303d of the U.S. Clean Water Act as having forty-three impaired reaches, raising these questions: Where is the fine sediment coming from? What proportions of the sediment are from uplands versus near-channel erosion? How much of the excess sediment loading is caused by modern land use and water management versus the legacy of past land use? Managing fine sediment at the watershed scale requires that we identify potential sources and sinks throughout the watershed, measure source contributions, and understand transport pathways of fine sediment. Here we utilize sediment fingerprinting techniques involving long- and short-lived radionuclide tracers, specifically beryllium-10 (10Be), excess lead-210 (210Pbex), and cesium-137 (137Cs), in combination with other supporting data sets to address the preceding questions. We document a shift in hydrologic regime and that sediment fluxes are sensitive to both magnitude and sequence of flood events. Geomorphic analysis indicates that many river reaches have accessible near-channel sources that contribute the dominant proportion of the washload flux in subwatersheds. Lastly, geochemical tracer analyses of floodplains and hillslope soils indicate that historic erosion has been variable across the landscape and the majority of suspended sediment in the river today is sourced from floodplains and terraces.

Predicting natural instream woody-habitat loads across large river networks

Past waterway management practices worldwide involved extensive removal of instream woody habitat (IWH) and riparian vegetation. The importance of instream woody habitat for healthy aquatic ecosystems has now been recognised, with management approaches reversed to reintroduce instream woody habitat and replant riverbanks. Knowledge of natural or pre-disturbance IWH loads is useful to guide such restoration programs; however, such datasets are often unavailable. In this study, natural IWH loads were mapped along 105km of undisturbed rivers in south-eastern Australia. This field dataset was modelled, using boosted regression trees, against geomorphic, environmental and climatic variables to predict natural IWH loads in rivers across Victoria. Mapped natural IWH loads averaged 0.029m3m–2 (±0.005), ranging from 0.083 to 0.002m3m–2. Natural IWH volumes were predicted to range from 0 to 0.102m3m–2. Distinct IWH loading trends were noticeable over larger spatial scales. Eastern Victoria showed relatively lower natural IWH loads than did western Victoria. Because many stream restoration efforts do not have a quantifiable knowledge of natural IWH load, the results of the present study provide some guidance. The predicted IWH loadings are a useful first step in identifying broad areas for further investigation and a natural condition base for current IWH condition modelling.

Using the Weibull distribution to improve the description of riverine wood loads

Abstract Reporting uncertainty in environmental measurements and estimates is important for cross‐comparison and inter‐comparison of sites and other spatial units. One such measure is the load of large in‐stream wood in river systems. In this paper we propose the use of the Weibull distribution to describe the central tendency and variability of wood loads along a river reach. We illustrate the link between the average wood load and the central tendency or scale parameter of the Weibull distribution. The shape of the Weibull distribution is strongly related to the ability of rivers to transport and rearrange the wood in a reach. We use six Victorian rivers to test the fit of the Weibull distribution, showing that the Weibull is a useful and flexible distribution that provides common reporting metrics useful for future studies. Using common reporting metrics provides a stronger tool for comparisons of wood loads between rivers and with reaches. Copyright