Michael Bliss Singer

Modeling decadal bed material sediment flux based on stochastic hydrology

Estimates of decadal bed material sediment flux and net storage are derived by driving sediment transport calculations with a stochastic hydrology model. The resulting estimates represent the whole distribution of sediment flux based on natural variability in channel characteristics (gradient, width, and bed grain size) and the magnitude, duration, and interarrival time of flood events. A procedure for calibrating a fractional sediment transport equation of a commonly used form to bed material grain size distributions (BMGSDs) at cross sections is presented. The procedure was applied to the Sacramento River channel network to compute estimates of annual total and annual peak bed material discharges into and through the main stem over a 30-year period. Main stem bed material budgets were evaluated to identify reaches in states of net accumulation or scour. Simulations highlight large imbalances in sand and gravel storage throughout the Sacramento River, which can be explained by a combination of local hydraulics and BMGSDs and for which there is at least some empirical support.

Identifying eroding and depositional reaches of valley by analysis of suspended sediment transport in the Sacramento River, California

Spatial patterns in suspended sediment transport and storage along the Sacramento River were assessed by evaluating the suspended sediment budget for the main channel accounting for all tributaries and diversions. Time series analysis was employed to quantify the relationship between streamflow and suspended sediment concentration for gauging stations along the main channel and signature tributaries. Sediment concentration records (of 2-yr duration) were extended using Box-Jenkins transfer function models to calculate annual rates of suspended sediment discharge over a 32-year period since dam construction on the Sacramento River. The suspended sediment budget was evaluated to identify reaches of net erosion or deposition. The results of the budget suggest the influence of tectonics and anthropogenic channel modification.

Enduring legacy of a toxic fan via episodic redistribution of California gold mining debris

Significance This paper is of fundamental interest to the millions of residents living at the downstream end of this and other global river basins beset by industrial metals mining. Sediment-bound Hg has contaminated food webs of the San Francisco Bay-Delta, but the dominant geographical sources of Hg to downstream ecosystems in this and similar river basins are debated. Likewise, the processes by which Hg is delivered to lowlands and the patterns of its floodplain deposition are poorly understood. This research addresses a gap in generic theory of postmining fan evolution that enables anticipation, prediction, and management of contamination risk to food webs. The interrelationships between hydrologically driven evolution of legacy landscapes downstream of major mining districts and the contamination of lowland ecosystems are poorly understood over centennial time scales. Here, we demonstrate within piedmont valleys of California’s Sierra Nevada, through new and historical data supported by modeling, that anthropogenic fans produced by 19th century gold mining comprise an episodically persistent source of sediment-adsorbed Hg to lowlands. Within the enormous, iconic Yuba Fan, we highlight (i) an apparent shift in the relative processes of fan evolution from gradual vertical channel entrenchment to punctuated lateral erosion of fan terraces, thus enabling entrainment of large volumes of Hg-laden sediment during individual floods, and (ii) systematic intrafan redistribution and downstream progradation of fan sediment into the Central Valley, triggered by terrace erosion during increasingly long, 10-y flood events. Each major flood apparently erodes stored sediment and delivers to sensitive lowlands the equivalent of ∼10–30% of the entire postmining Sierran Hg mass so far conveyed to the San Francisco Bay-Delta (SFBD). This process of protracted but episodic erosion of legacy sediment and associated Hg is likely to persist for >104 y. It creates, within an immense swath of river corridor well upstream of the SFBD, new contaminated floodplain surfaces primed for Hg methylation and augments/replenishes potential Hg sources to the SFBD. Anticipation, prediction, and management of toxic sediment delivery, and corresponding risks to lowland ecology and human society globally, depend on the morphodynamic stage of anthropogenic fan evolution, synergistically coupled to changing frequency of and duration extreme floods.

Hydrologic versus geomorphic drivers of trends in flood hazard

Flooding is a major hazard to lives and infrastructure, but trends in flood hazard are poorly understood. The capacity of river channels to convey flood flows is typically assumed to be stationary, so changes in flood frequency are thought to be driven primarily by trends in streamflow. We have developed new methods for separately quantifying how trends in both streamflow and channel capacity have affected flood frequency at gauging sites across the United States Flood frequency was generally nonstationary, with increasing flood hazard at a statistically significant majority of sites. Changes in flood hazard driven by channel capacity were smaller, but more numerous, than those driven by streamflow. Our results demonstrate that accurately quantifying changes in flood hazard requires accounting separately for trends in both streamflow and channel capacity. They also show that channel capacity trends may have unforeseen consequences for flood management and for estimating flood insurance costs.

Floodplain ecohydrology: climatic, anthropogenic, and local physical controls on partitioning of water sources to riparian trees

Seasonal and annual partitioning of water within river floodplains has important implications for ecohydrologic links between the water cycle and tree growth. Climatic and hydrologic shifts alter water distribution between floodplain storage reservoirs (e.g., vadose, phreatic), affecting water availability to tree roots. Water partitioning is also dependent on the physical conditions that control tree rooting depth (e.g., gravel layers that impede root growth), the sources of contributing water, the rate of water drainage, and water residence times within particular storage reservoirs. We employ instrumental climate records alongside oxygen isotopes within tree rings and regional source waters, as well as topographic data and soil depth measurements, to infer the water sources used over several decades by two co-occurring tree species within a riparian floodplain along the Rhône River in France. We find that water partitioning to riparian trees is influenced by annual (wet versus dry years) and seasonal (spring snowmelt versus spring rainfall) fluctuations in climate. This influence depends strongly on local (tree level) conditions including floodplain surface elevation and subsurface gravel layer elevation. The latter represents the upper limit of the phreatic zone and therefore controls access to shallow groundwater. The difference between them, the thickness of the vadose zone, controls total soil moisture retention capacity. These factors thus modulate the climatic influence on tree ring isotopes. Additionally, we identified growth signatures and tree ring isotope changes associated with recent restoration of minimum streamflows in the Rhône, which made new phreatic water sources available to some trees in otherwise dry years. KEY POINTS Water shifts due to climatic fluctuations between floodplain storage reservoirsAnthropogenic changes to hydrology directly impact water available to treesEcohydrologic approaches to integration of hydrology afford new possibilities.

Imprint of climate and climate change in alluvial riverbeds: Continental United States, 1950–2011

Alluvial riverbed elevation responds to the balance between sediment supply and transport capacity, which is largely dependent on climate and its translation into fl uvial discharge. We examine these relations using U.S. Geological Survey streamfl ow and channel measurements in conjunction with basin characteristics for 915 reference (“least disturbed”) measurement stations across the conterminous United States for the period A.D. 1950‐2011. We fi that (1) 68% of stations have bed elevation change (BEC) trends (p < 0.05) with median values of +0.5 cm/yr for aggradation and ‐0.6 cm/yr for degradation, with no obvious relation to drainage basin structure, physiography, or lithology; (2) BEC correlates with drainage basin area; (3) high-fl ow variability (Q 90 /Q 50 , where Q is discharge and 90 and 50 are annual fl ow percentiles) translates directly into the magnitude, though not the direction, of BEC, after accounting for the scale dependence; (4) Q 90 /Q 50 declines systematically from dry to wet climates, producing disproportionately high rates of BEC in drier regions; and (5) marked increases in precipitation and streamfl ow occurred disproportionately at dry sites, while streamfl ow declined disproportionately at wet sites. Climatic shifts in streamfl ow have the potential to increase/decrease sediment fl ux and thus affect riverbed elevation by altering fl ood frequency. These unforeseen responses of bed elevation to climate and climate change have important implications for sediment budgets, longitudinal profi les, ecology, and river management.

An empirical-stochastic, event-based program for simulating inflow from a tributary network: Framework and application to the Sacramento River basin, California

A stochastic streamflow program was developed to simulate inflow to a large river from a network of gauged tributaries. The program uses historical streamflow data from major tributary gauges near their confluence with the main stem and combines them stochastically to represent spatial and temporal patterns in flood events. It incorporates seasonality, event basis, and correlation in flood occurrence and flood peak magnitude between basins. The program produces synchronous tributary inflow hydrographs, which when combined and routed, reproduce observed main stem hydrograph characteristics, including peak, volume, shape, duration, and timing. Verification of the program is demonstrated using daily streamflow data from primary tributary and main stem gauges in the Sacramento River basin, California. The program is applied to simulating flow at ungauged main stem locations, assessing risk in fluvial systems, and detecting bed level change.

Sediment-adsorbed total mercury flux through Yolo Bypass, the primary floodway and wetland in the Sacramento Valley, California

The fate and transport of mercury are of critical concern in lowland floodplains and wetlands worldwide, especially those with a history of upstream mining that increases the mobility of both dissolved and sediment-bound Hg in watersheds. A mass budget of total mercury (THg) quantifies sources and storage for particular areas - knowledge that is required for understanding of management options in lowland floodplains. In order to assess contaminant risk in the largest flood-control bypass, prime wetland, and restoration target in the Sacramento River basin, we estimated empirical relationships between THg, suspended sediment concentration (SSC), and streamflow (Q) for each of the major inputs and outputs using data from various publicly available sources. These relationships were improved by incorporating statistical representations of the dynamics of seasonal and intra-flood exhaustion (hysteresis) of sediment and mercury. Using continuous records of Q to estimate SSC suspended sediment flux and SSC to estimate THg flux, we computed the net transfer of sediment-adsorbed mercury through the Yolo Bypass over a decade, 1993-2003. Flood control weirs spilling Sacramento River floodwaters into the bypass deliver ~75% of the water and ~50% of the rivers suspended sediment load, while one Coast Range tributary of the bypass, Cache Creek, contributes twice the THg load of the mainstem Sacramento. Although estimated sediment flux entering Yolo Bypass is balanced by efflux to the Sacramento/San Francisco Bay-Delta, there is much evidence of deposition and remobilization of sediment in Yolo Bypass during flooding. These factors point to the importance of the bypass as sedimentary reservoir and as an evolving substrate for biogeochemical processing of heavy metals. The estimates of mercury flux suggest net deposition of ~500 kg in the 24,000 ha floodway over a decade, dominated by two large floods, representing a storage reservoir for this important contaminant.

Remote sensing of suspended sediment concentration during turbid flood conditions on the Feather River, California—A modeling approach

Received 11 January 2011; revised 15 November 2011; accepted 22 November 2011; published 19 January 2012. [1] Direct measurements of suspended sediment concentration (SSC) in rivers are surprisingly sparse. We present an approach for measuring these concentrations from space, tailored to fit rivers with limited records of flood-level SSC. Our approach requires knowledge of a typical particle-size distribution of sediment suspended during floods, the dominant mineralogy, and a calibration consisting of above-water reflectance field spectra with known SSC. Surface SSC values were derived for two Landsat images covering 70 km of the Feather and portions of the Sacramento, Yuba, and Bear Rivers in California in order to capture conditions during a large flood event. Using optical theory and radiative transfer modeling we modeled remote-sensing reflectance (Rrs) for a number of three-component mixtures composed of color dissolved organic matter (CDOM), water, and montmorillonite particles. We then iteratively estimated CDOM by fitting modeled spectra for a range of absorption coefficients to field-measured spectra collected from the Sacramento River and matched to measured SSC values. Spectral mixture analysis with a two-end-member model yielded end-member fractions and SSC via a look-up table specific to the Landsat sensor. Model closure was within the error of measured SSC values, suggesting that this approach is promising for deriving SSC on rivers during flood conditions when empirical relationships established between low SSC values and Rrs are no longer valid.

Channel and Floodplain Change Analysis over a 100-Year Period: Lower Yuba River, California

Hydraulic gold mining in the Sierra Nevada, California (1853-1884) displaced ~1.1 billion m 3 of sediment from upland placer gravels that were deposited along piedmont rivers below dams where floods can remobilize them. This study uses topographic and planimetric data from detailed 1906 topographic maps, 1999 photogrammetric data, and pre- and post-flood aerial photographs to document historic sediment erosion and deposition along the lower Yuba River due to individual floods at the reach scale. Differencing of 3 × 3-m topographic data indicates substantial changes in channel morphology and documents 12.6 × 10 6 m 3 of erosion and 5.8 × 10 6 m 3 of deposition in these reaches since 1906. Planimetric and volumetric measurements document spatial and temporal variations of channel enlargement and lateral migration. Over the last century, channels incised up to ~13 m into mining sediments, which dramatically decreased local flood frequencies and increased flood conveyance. These adjustments were punctuated by event-scale geomorphic changes that redistributed sediment and associated contaminants to downstream lowlands.