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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 >10 ⁴ 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.

Over the past decades, a number of water sciences and management programs have been developed to better understand and manage the water cycles at multiple temporal and spatial scales for various purposes, such as ecohydrology, global hydrology, sociohydrology, supply management, demand management, and integrated water resources management (IWRM). At the same time, rapid advancements have also been taking place in tracing, mapping, remote sensing, machine learning, and modelling technologies in hydrological research. Despite those programs and advancements, a water crisis is intensifying globally. The missing link is effective interactions between the hydrological research and water resource management to support implementation of the UN Sustainable Development Goals (SDGs) at multiple spatial scales. Since the watershed is the natural unit for water resources management, watershed science offers the potential to bridge this missing link. This study first reviews the advances in hydrological research and water resources management, and then discusses issues and challenges facing the global water community. Subsequently, it describes the core components of watershed science: (1) hydrological analysis; (2) water-operation policies; (3) governance; (4) management and feedback. The framework takes into account water availability, water uses, and water quality; explicitly focuses on the storage, fluxes, and quality of the hydrological cycle; defines appropriate local water resource thresholds through incorporating the planetary boundary framework; and identifies specific actionable measures for water resources management. It provides a complementary approach to the existing water management programs in addressing the current global water crisis and achieving the UN SDGs.

Spatial scales and methods for dealing with scale have been widely discussed in the water resources literature. Different spatial processes operate at different scales so interpretations based on data from one scale may not apply to another. Understanding the behavior of phenomena at multiple-scales of data aggregation is thus imperative to accurate integrations of data and models at different geographic resolutions. This study tests theoretical concepts of scale by presenting empirical results of multiscale GIS and statistical analyses on gridded water-availability, water use and population data for the Danube Basin in Europe, with results corroborated by similar tests in the Ganges (South Asia) and Missouri (North America) Basins. Fine-resolution datasets were aggregated to coarser grid sizes and standard statistical measures of spatial variability were computed. Statistical analysis of spatial variability demonstrated two distinctly different cases for unscaled and scaled variables. Results show that variance (and standard deviation) in unscaled variables like freshwater supply, use and population increases at coarser scales—contrary to the common assumption of decreasing variability as grid-cell size increases. On the other hand, a decreasing trend in variability with scale is noted for variables scaled to area or population (like population density, water availability per capita etc.). Moreover, relationships between variability and scale show strong non-linear trends. No mention of these relationships has been found in the water resources or socio-economic literature for scale and variability. Regression analyses suggest that power functions are the most appropriate model to fit trends in increasing variability at multiple scales. These results can be applied to interpretations of water-stress and water scarcity data and their locations relative to water sources or topographic barriers.

Hydraulic gold mining in the Sierra Nevada, California (1853–1884) displaced ~1.1 billion m3 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 × 106 m3 of erosion and 5.8 × 106 m3 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.

The extreme rainfall of October 2015 in South Carolina generated numerous dam failures and spawned the flood of record at most U.S. Geological Survey stream gauges. Detailed field sampling and systematic image analysis are used to document the immediate and sustained geomorphic adjustments at four failed dams within the urbanized Gills Creek watershed. That urban focus is augmented with a similar analysis at five failed dams in more rural settings where less urban infrastructure exists. We also document the magnitude and type of geomorphic adjustments throughout the Gills Creek watershed unrelated to dam failures. Despite the extreme rainfall and associated flooding, the geomorphic effects were limited and localized, manifesting primarily at dam failures but not progressing significantly downstream, especially along Gills Creek, where the combination of intact dams, frequent urban roughness elements, thick floodplain vegetation, and numerous wetlands explains the lack of significant morphologic adjustments or major deposition. Dam failures in rural settings showed greater overbank deposition but the geomorphic effects were still limited to dam proximal locations because of the thick vegetal cover below the dam. Because these dams in rural and urban settings were constructed to maximize waterfront property, low gradient sites were selected; therefore, the geomorphic effects were unlike planned dam removals where massive headcutting, knickpoint migration, and sediment evacuation tend to occur. The most significant effect of the dam failures was the shift from an initial braided channel to a single thread channel within the emptied reservoirs, which occurred quickly, often within the first few months.

Much of the local and national news coverage of the October 2015 Columbia floods focused on high water along Gills Creek, a small urban watershed on the east side of Columbia that drains into the Congaree River floodplain about five miles below the canal. [...]as climate warming increases temperature and pressure gradients between the equator and the poles, global circulation patterns will become increasingly meridional as opposed to westerly, and this and other changes will increase the variability of weather systems.

The 1993 flood on the Upper Mississippi River was a rare, large‐magnitude hydrological event. Field and aerial survey analyses and Landsat 5 Thematic Mapper data were used to appraise the thickness of overbank deposits on leveed and unleveed reaches. Results indicate that minimal (<5 mm) overbank sedimentation occurred, except in the immediate vicinity of a levee break. Unleveed sections also lacked overbank sedimentation. Little geomorphological or sedimentological evidence of this extreme event is likely to be preserved. This raises questions about the completeness of the stratigraphic record: in situations where wide floodplains with cohesive soils provide effective resistance and dissipate energy so that erosion is minimized, and/or sediment supply is limited by event timing or sequencing, a large flood may leave little or no substantive evidence of its occurrence.

Large deposits of hydraulic gold mining sediment remain in main channels of the Bear River more than 100 years after the cessation of mining. This study examines these deposits and reevaluates Gilbert's (1917) classic model of sediment transport in a symmetrical wave that is based on hydraulic mining sediment primarily in the Yuba Basin. Sustained storage and transport of hydraulic mining sediment in the Bear Basin are documented and a revised model of sediment transport is proposed. In the lower Bear Basin, subsurface coring indicates that about 106 million cubic meters of mining sediment remain stored. This volume is more than double previous estimates of storage in the lower basin, and is greater than any previous estimate of storage in the entire basin, even though it excludes a large volume of mining sediment remaining in the mountains. More than 90 percent of the lower basin deposit remains in storage. In the upper basin, surface dimensions of the deposit, measured from cross-sections, reveal depths of erosion and the storage of an extensive but unknown volume of mining sediment. Field and historical evidence is presented of the continued reworking of mining sediment. Frequent flows are competent to move channel-bed material derived from mining sediment. Sediment erosion, transport, redeposition, and lack of dilution document sustained remobilization of mining sediment. Sediment loads are now greater than pre-mining values, which were constrained by bedrock-dominated channels. This sustained transport is in contrast to Gilbert's symmetrical wave model that predicts a rapid return of sediment loads to pre-mining levels. The empirical foundation of the symmetrical wave model is biased. Channel incision in the Sacramento Valley has been promoted by several factors in addition to decreased sediment loads and does not confirm the return of sediment loads to pre-mining levels. A revised, skewed sediment wave model is proposed for basins with a large component of long-term channel storage. This conceptual model is in better harmony with growing evidence of the importance of sediment storage in and near channels to long-term sediment loads. The persistence of anthropogenic sediment in fluvial systems may be much greater than implied by Gilbert's model.

Automated channel headwater mapping methods are reviewed and three methods are used to map a small watershed in the South Carolina upper Piedmont based on high-resolution LiDAR data. First, channels were mapped manually using crenulations on 0.6-m contours generated from the LiDAR data. Field verifications indicate the LiDAR-generated contour map is far superior to the 1:24,000 scale topographic quadrangle and the resulting network map is treated as the reference for assessing network simulations. Second, channel networks were generated using standard flow accumulation methods with three critical area thresholds representing drainage areas of 480, 800, and 1600 m². Third, channel networks were generated using a slope-area product proportional to stream power (Ω i,j ∝ A i,j S i,j). This multivariate method that includes hill-slope gradients improves the identification of small upland channels in steep terrain but may also introduce spurious gully sidewall channels. While all of the methods are capable of simulating fairly accurate channel networks with an appropriate drainage density, errors of omission and commission are greater with the conventional accumulation method and have an association with slope.

[...] the application and testing of conventional fluvial geomorphology theory to small watersheds is at a critical stage. Highresolution remote sensing methods, based on light detection and ranging (LiDAR) data, can generate high-resolution topographic data beneath forest canopy, and standard automated methods, based on a critical accumulation threshold (drainage area), can be used to create maps of channel networks.