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Measuring suspended-sand fluxes in rivers remains a scientific challenge due to their high spatial and temporal variability. To capture the vertical and lateral gradients of concentration in the cross section, measurements with point samples are performed. However, the uncertainty related to these measurements is rarely evaluated, as few studies of the major sources of error exist. Therefore, the aim of this study is to develop a method determining the cross sectional sand flux and estimating its uncertainty. This SDC (for Sand Discharge Computing) method combines suspended-sand concentrations from point samples with ADCP (Acoustic Doppler Current Profiler) high-resolution depth and velocity measurements. The MAP (for Multitransect Averaged Profile) method allows to obtain an average of several ADCP transects on a regular grid, including the unmeasured areas. The suspended-sand concentrations are integrated vertically by fitting a theoretical exponential suspended-sand profile to the data using Bayesian modelling. The lateral integration is based on the water depth as a proxy for the local bed shear stress to evaluate the bed concentration and sediment diffusion along the river cross-section to evaluate the bed concentration and sediment diffusion along the river cross-section. The estimation of uncertainty combines ISO standards and semi-empirical methods with a Bayesian approach to estimate the uncertainty due to the vertical integration. The new method is applied to data collected in four rivers under various hydro-sedimentary conditions: the Colorado, Rhône, Isεave;re and Amazon Rivers, with computed flux uncertainties ranging between 18 and 32 %. The relative difference between the suspended-sand flux in 21 cases calculated with the proposed SDC method compared to the ISO 4363 method ranges between -16 and +3 %. This method, which comes with a flexible, open-source code, is the first proposing an applicable uncertainty estimation, that could be adapted to other flux computation methods. [ABSTRACT FROM AUTHOR]

Closure of Glen Canyon Dam in 1963 transformed the Colorado River by reducing the magnitude and duration of spring floods, increasing the magnitude of base flows, and trapping fine sediment delivered from the upper watershed. These changes caused the channel downstream in Glen Canyon to incise, armor, and narrow. This study synthesizes over 45 yr of channel-change measurements and demonstrates that the rate and style of channel adjustment are directly related to both natural processes associated with sediment deficit and human decisions about dam operations. Although bed lowering in lower Glen Canyon began when the first cofferdam was installed in 1959, most incision occurred in 1965 in conjunction with 14 pulsed high flows that scoured an average of 2.6 m of sediment from the center of the channel. The average grain size of bed material has increased from 0.25 mm in 1956 to over 20 mm in 1999. The magnitude of incision at riffles decreases with distance downstream from the dam, while the magnitude of sediment evacuation from pools is spatially variable and extends farther downstream. Analysis of bed-material mobility indicates that the increase in bed-material grain size and reduction in reach-average gradient are consistent with the transformation of an adjustable-bed alluvial river to a channel with a stable bed that is rarely mobilized. Decreased magnitude of peak discharges in the post-dam regime coupled with channel incision and the associated downward shifts of stage-discharge relations have caused sandbar and terrace erosion and the transformation of previously active sandbars and gravel bars to abandoned deposits that are no longer inundated. Erosion has been concentrated in a few pre-dam terraces that eroded rapidly for brief periods and have since stabilized. The abundance of abandoned deposits decreases downstream in conjunction with decreasing magnitude of shill in the stage-discharge relations. In the downstream part of the study area where riffles controlling channel elevation have not incised, channel narrowing has resulted from decreased magnitude of peak discharges and minor post-dam deposition. These physical changes to the aquatic and riparian systems have supported the establishment and success of an artifact ecosystem dominated by non-native species. Models for the channel response downstream from large dams typically consider factors such as the degree of sediment deficit, the pre-dam surface and subsurface grain size, and the magnitude of post-dam average flows. These results suggest that it is also necessary to consider (1) the possibility of variable responses among different channel elements and (2)the potential importance of exceptional flows resulting from management decisions. Keywords: channel adjustment, erosion, dams. geomorphology, alluvial deposits.

Before Glen Canyon Dam was completed upstream from Grand Canyon, floods scoured sand from the channel bed and deposited sand on bars within recirculating eddies. After completion of Glen Canyon Dam in 1963, peak discharge of the mean annual floods dropped from about 2600 to 900 [m.sup.3]/s, and 85% of the sediment supply was eliminated. Under the postdam flow regime, sand bars in eddies have degraded. In an experiment to study, in part, the effects of floods in rebuilding these bars, a controlled flood was released from Glen Canyon Dam in late March and early April 1996. Although fluvial sequences characteristically fine upward, the deposits of the experimental flood systematically coarsen upward. Measurements of suspended-sediment concentration and grain size and of bed-material grain size suggest that the upward coarsening results from the channel becoming relatively depleted of fine-grained sediment during the seven days of the high-flow experiment. Predam flood beds of the Colorado River also coarsen upward, indicating that supply-limitation and grain-size evolution are natural processes that do not require the presence of a dam.

Large rock-avalanche deposits are extremely useful tools in paleogeographic and tectonic reconstructions. These deposits are large ([is greater than]0.1 [km.sup.3]), semicoherent 'snapshots' of a source area that were emplaced instantaneously in a sedimentary basin. A rock-avalanche deposit with a known age in a sedimentary basin requires the restoration of that basin to a position adjacent to the rock-avalanche source area at the time of emplacement. The Miocene sedimentary section of the Amargosa Chaos and Sperry Hills basins in the southern Death Valley region contains deposits of large rock-avalanches from both spatially and temporally separated source terranes. The rock-avalanche deposits in the Amargosa Chaos basin require the restoration of the basin to within 10 km of the Kingston Range prior to 7.8 Ma. West-derived alluvial fan sediments in this basin can be confidently tied to the southern Panamint Mountains, with early east-derived alluvial fan sediments (pre-7.8 Ma) tied to the Kingston Range. Moreover, the pre-7.8 Ma stratigraphy of the Amargosa Chaos basin is equivalent to the pre-7.8 Ma stratigraphy of the Miocene sedimentary section in the Sperry Hills. Prior to [is similar to]7.8 Ma, the Amargosa Chaos and Sperry Hills basins were both parts of a larger sedimentary basin. Sedimentologic constraints on the geometry of the Amargosa Chaos basin indicate that throughout its history it was a half-graben opening along a west-dipping normal fault with substantial vertical relief in the uplifted footwall. Evidence for this topography exists in the form of multiple large rock-avalanche deposits derived from footwall bedrock. The axis of this basin trends northwest with the active basin-bounding normal fault on the northeast margin. At [is similar to]7.8 Ma, the extension direction in the southern Death Valley region changed from southwest to northwest. Younger sediments, including three younger rock-avalanche deposits, record the motion of the Amargosa Chaos basin away from the Sperry Hills basin. Top-to-the-northwest displacement of 30 km along the Amargosa detachment system is required between 7.8 Ma and 4.9 Ma between the Sperry Hills basin and the location of Amargosa Chaos basin in the southern Black Mountains. Continued, post-4.9 Ma, northwest-directed extension is required between the Amargosa Chaos basin and the southern Panamint Mountains. In addition, 15 km of right-lateral slip on the southern portion of the Grand View fault is required from 7.8 to 3 Ma to move the Sperry Hills basin away from its pre-7.8 Ma position adjacent to the Kingston Range.

Morphological change in river channels is frequently evaluated in the context of mass balance sediment budgets. In a closed sediment budget, measurements of sediment influx and efflux are coupled with measured changes in channel topography to provide both spatial and temporal resolution, and independent estimates of the mass balance. For sediment budgets constructed over long river segments (~102 channel widths or greater) and long periods (~2 years or longer), spatial and temporal accumulation of measurement uncertainty, compounded by inadequate sampling frequency or spatial coverage, may produce indeterminate results. The degree of indeterminacy may be evaluated in the context of a signal‐to‐noise ratio (SNR), which is a function of the magnitude of the mass balance and the magnitudes of potential systematic uncertainties associated with measurements and incomplete sampling. We report on a closed sand budget consisting of measurements of flux and two morphological surveys for a 50‐km segment of a large river over a 3‐year period. Accurate reporting of the magnitude and sign of the change in sand storage was only possible by using state‐of‐the‐art techniques with high temporal frequency and large spatial extent. Together, a sand flux and morphological mass balance revealed that sand evacuation was temporally concentrated (~100% of mass change occurred during 19% of the study period) and highly localized (70% of mass change occurred in 12% of the study segment). A SNR analysis revealed that uncertainty resulting from undersampling may approach or exceed that caused by measurement uncertainty and that daily sampling of suspended‐sand concentration or repeat mapping of at least 50% of the river segment was required to determine the sand budget with SNR > 1. The approach used here to analyze sand budget uncertainty is especially applicable to other river systems with large temporal variability in sediment transport and large spatial variability in erosion and deposition. © 2018 John Wiley & Sons, Ltd. Comprehensive sand budget based on high‐resolution measurements of sediment flux, channel morphology, and bed composition for 50‐km segment of Colorado River demonstrates erosion. Uncertainty, evaluated by a signal‐to‐noise analysis shows that sub‐daily sampling of suspended‐sediment concentration and repeat mapping of at least 50% of the study area are required to determine a sand budget with signal‐to‐noise ratio > 1. [ABSTRACT FROM AUTHOR]

Turbidity, a measure of water clarity, is monitored for a variety of purposes including: 1) to help determine whether water is safe to drink; 2) to establish background conditions of lakes and rivers and detect pollution caused by construction projects and storm water discharge; and 3) to establish connections with aquatic biological properties, such as primary production and predator-prey interactions. Turbidity is normally measured with an optical probe that detects light scattered from particles in the water. Probes have defined upper limits of the range of turbidity that they can measure. The general assumption is that when turbidity exceeds this upper limit, the values of turbidity will be constant, i.e., the probe is "pegged"; however, this assumption is not necessarily valid. In cases where turbidity greatly exceeds the upper measurement limit, turbidity probes can falsely report incorrectly low values of turbidity that appear to be within the limits of the probe. In rivers with limited variation in the physical properties of the suspended sediment, an increase in suspended-sediment concentration will initially cause a linear increase in turbidity. When the suspended-sediment concentration in these rivers causes turbidity levels that exceed the upper measurement limit of a probe, turbidity probes do not necessarily "peg" at a constant value. Data from the Colorado River in Grand Canyon, Arizona, USA and a laboratory experiment both demonstrate that when turbidity exceeds instrument-pegged conditions, increasing suspended-sediment concentration (and thus increasing turbidity) may cause optical probes to record decreasing "false" turbidity values that appear to be within the valid measurement range of the probe. Therefore, under high turbidity conditions, other surrogate measurements of turbidity (e.g., acoustic attenuation measurements or suspended-sediment samples) are necessary to correct these low turbidity measurements and accurately measure turbidity. [ABSTRACT FROM AUTHOR]