Abstract The Middle Fork Nooksack River drains the southwestern slopes of the active Mount Baker stratovolcano in northwest Washington State. The river enters Bellingham Bay at a growing delta 98 km to the west. Various types of debris flows have descended the river, generated by volcano collapse or eruption (lahars), glacial outburst floods, and moraine landslides. Initial deposition of sediment during debris flows occurs on the order of minutes to a few hours. Long-lasting, down-valley transport of sediment, all the way to the delta, occurs over a period of decades, and affects fish habitat, flood risk, gravel mining, and drinking water. Holocene lahars and large debris flows (>10 6 m 3 ) have left recognizable deposits in the Middle Fork Nooksack valley. A debris flow in 2013 resulting from a landslide in a Little Ice Age moraine had an estimated volume of 100,000 m 3 , yet affected turbidity for the entire length of the river, and produced a slug of sediment that is currently being reworked and remobilized in the river system. Deposits of smaller-volume debris flows, deposited as terraces in the upper valley, may be entirely eroded within a few years. Consequently, the geologic record of small debris flows such as those that occurred in 2013 is probably very fragmentary. Small debris flows may still have significant impacts on hydrology, biology, and human uses of rivers downstream. Impacts include the addition of waves of fine sediment to stream loads, scouring or burying salmon-spawning gravels, forcing unplanned and sudden closure of municipal water intakes, damaging or destroying trail crossings, extending river deltas into estuaries, and adding to silting of harbors near river mouths.

Abstract Flood-control reservoirs designed and built by federal agencies have been extremely effective in reducing the ravages of floods nationwide. Yet some structures are being removed for a variety of reasons, while others are aging rapidly and require either rehabilitation or decommissioning. The focus of the paper is to summarize collaborative research activities to assess sedimentation issues within aging flood-control reservoirs and to provide guidance on such tools and technologies. Ten flood-control reservoirs located in Oklahoma, Mississippi, and Wisconsin have been examined using vibracoring, stratigraphic, geochronologic, geophysical, chemical, and geochemical techniques and analyses. These techniques and analyses facilitated: (1) the demarcation of the pre-reservoir sediment horizon within the deposited reservoir sediment, (2) definition of the textural and stratigraphic characteristics of the sediment over time and space, (3) the accurate determination of the remaining reservoir storage capacity, (4) the quantification of sediment quality with respect to agrichemicals and environmentally important trace elements over both time and space, and (5) the determination of geochemical conditions within the deposited sediment and the potential mobility of associated elements. The techniques employed and discussed here have proven to be successful in the assessment of sediment deposited within aging flood-control reservoirs, and it is envisioned that these same approaches could be adopted by federal agencies as part of their national reservoir management programs.

Abstract We investigated the viability of ground-penetrating radar (GPR) as a method to estimate the quantity of sediment stored behind the Merrimack Village Dam on the Souhegan River in southern New Hampshire. If the predam riverbed can be imaged, the thickness and volume of the reservoir deposit can be calculated without sampling. Such estimates are necessary to plan sediment management after dam deconstruction. In May 2008, we surveyed six cross sections with a Mala Geosciences ProEx 100 MHz GPR. In a related study, topographic surveys were conducted in 2008–2009 to monitor the sediment flux associated with the removal of the Merrimack Village Dam in August 2008. Within a month of the removal, these surveys mapped the predam riverbed in the uppermost cross sections in the former impoundment. We compared these surveys to our interpreted GPR images for one cross section to determine a calibrated velocity for the impounded sand of 0.043 ± 0.020 m/ns. We also estimated the radar velocity of the deposit by analyzing hyperbolic reflections in the GPR images, and found a similar result (0.039 m/ns). Using the calibrated velocity, we estimated a total volume of sediment stored behind the Merrimack Village Dam of 66,900 ± 9900 m 3 , which compares well to a previous estimate (62,000 m3) based on a depth-to-refusal survey. Our findings indicate that GPR is a useful technique for quantifying impounded sediment prior to dam removal in reservoirs containing 1–10 m of sand overlying a coarser predam riverbed, but it may be less effective in settings with finer and/or thicker impounded sediment.

Abstract The accurate prediction of sediment erosion after dam removal is critical to quantifying the impact of dam removal on the reservoir and downstream environment. A variety of methods can be used to estimate this impact, and one of the most common is to use a one-dimensional mobile bed sediment transport model. I describe a one-dimensional sediment transport model (SRH-1D) and use it to simulate a laboratory experiment of incision through a reservoir delta deposit. The model allows the user to specify the erosion width through the deposit as a function of the flow rate. The model is shown to predict the vertical incision and downstream sediment load with reasonable accuracy if the erosion width is specified. Sensitivity tests to the transport equation parameters, erosion width, and angle of repose are conducted. The sediment loads exiting the dam are shown to be sensitive to the critical shear stress, but they are relatively insensitive to changes to the erosion width and angle of repose. One-dimensional models are shown to require the specification of the erosion width, but the results are not considered to be extremely sensitive to its value, so long as it is approximately equal to the observed river width under the same flow conditions. Further work on modeling of bank erosion is necessary to more accurately predict the long-term evolution of reservoir deposits.

Abstract The removal of obsolete and unsafe dams for safety, environmental, or economic purposes frequently involves the exploration of sediments trapped within the impoundment and the subsequent assessment of sediment management needs and techniques. Sediment management planning requires a thorough understanding of the watershed’s surficial geology, topography, land cover, land use, and hydrology. The behavior of sediments is influenced by their age, consolidation, and stratigraphy. All watersheds have a history that helps forecast sediment loads, quality, gradation, and stratigraphy. Impounded sediment deposits may include coarse deltas and foreset slopes, fine or coarse bottom deposits, cohesive or organic matter, and wedge deposits immediately behind the dam. Some watersheds have anthropogenic pollutants from agricultural activities, mining, industries, or urban runoff. The volume and rate of sediment release during and after small dam removal can be limited by active management plans to reduce potential downstream impacts. Management strategies include natural erosion, phased breaches and drawdowns, natural revegetation of sediment surfaces, pre-excavation of an upstream channel, hazardous waste removal or containment, flow bypass plans, and sediment dredging.