Geology topics

Research about Idaho, Oregon

Source-linked reports with geographic coverage including Idaho, Oregon.

At least 19 recordsLinked to original sources

USGS2026

Long-term intermittent connection between the western Snake River Plain and Columbia basin: A two-phased incision history of Hells Canyon

For more than a century, researchers have debated the route of the Snake River across the northern Cordillera and U.S. Pacific Northwest, including the associated incision history of Hells Canyon. Here, we use detrital zircon U-Pb provenance analysis of Miocene strata upstream and downstream of Hells Canyon to constrain the evolution in fluvial pathways over time. Downstream of Hells Canyon, we show that the ca. 10−6 Ma Clarkston Heights gravel was dominantly sourced from the nearby Salmon and Clearwater rivers but also with substantial input from drainages that now flow into the western Snake River Plain. These same western Snake River Plain drainages were contributing to time-equivalent Lake Idaho strata, upstream of Hells Canyon, indicating a Hells Canyon fluvial connection between the western Snake River Plain and Columbia basin before 6 Ma. Results from Lake Idaho strata suggest a two-phased lacustrine history in the western Snake River Plain: Before ca. 6.0 Ma, lake strata were derived from local tributaries, suggesting that the western Snake River Plain was isolated from the modern upper Snake River system. Between ca. 4.3 Ma and 2.2 Ma, the source area expanded to include eastern tributaries in conjunction with migration of high-standing topography of the Yellowstone hotspot. Our results challenge the long-held hypothesis of “capture” of the Snake River through Hells Canyon. Instead, we infer that Hells Canyon was a long-established route for outflow of lakes occupying the western Snake River Plain, possibly intermittently, during a ca. 10−2 Ma phase of slow regional incision, followed by rapid incision in Hells Canyon starting ca. 2 Ma in conjunction with erosion of the topographic barrier impounding Lake Idaho.

Idaho, Oregon↗

USGS2025

Contrasting magnitude and timing of pulsed aqueous methylmercury bioaccumulation across a reservoir food web

Water column hypoxia is a key process influencing methylmercury (MeHg) production and availability in waterbodies worldwide. During seasonal destratification, large, short-lived pulses of aqueous MeHg may be released into the subsequently mixed water column, but little is known about the fate of these pulses, particularly whether there are concomitant increases in MeHg uptake into aquatic food webs. We examined the magnitude and timing of MeHg uptake across several trophic guilds relative to the reservoir stratification status using biweekly mercury data from water, zooplankton, and fish (Bluegill, Lepomis macrochirus and Smallmouth Bass, Micropterus dolomieu ). Zooplankton MeHg concentrations increased by up to 250% during destratification, concurrent with increases in aqueous MeHg concentrations. Zooplankton and filter-passing MeHg concentrations were positively correlated when the reservoir was mixed ( R 2 = 0.95) and destratifying ( R 2 = 0.57) but not while the reservoir was stratified ( R 2 = 0.21). Mercury concentrations in adult bluegill and juveniles of both fish species increased 20–70% following destratification, with responses lagging 4–8 weeks behind those in water and zooplankton MeHg. Mercury concentrations in piscivorous adult bass varied little over the course of the study. Our findings demonstrate the responsiveness of reservoir food webs to pulses in MeHg availability, suggesting that these pulses could play an important role in biotic MeHg exposure within and downstream of reservoirs.

Idaho, Oregon↗

USGS2024

Longitudinal and seasonal changes of organic matter sources through a semi-arid river-reservoir system

The quality and quantity of organic matter (OM) in a river system directly affects ecosystem health; thus, managers benefit from an in-depth understanding of the drivers and sources of OM. In the Snake River, a highly altered river-reservoir system in the semi-arid western United States, OM production and loading are key drivers of reservoir anoxia, which leads to several deleterious processes such as mercury methylation. However, sources and quantities of OM to the Snake River, and the effects of impoundment on OM moving through the river-reservoir system, are not well understood. Particulate organic carbon (POC), dissolved organic carbon (DOC), particulate nitrogen (PN), chlorophyll a (chl- a ), and δ 15 N–PN and δ 13 C–POC isotopic ratios were measured bi-weekly for over 2 years at four locations through the Snake River Hells Canyon Reservoir Complex to determine spatial and temporal patterns of OM quantities and sources. POC concentrations increased through the riverine zone upstream of the reservoirs, likely due to in situ primary production and/or inputs from tributaries and agricultural drains; then decreased through the most upstream reservoir likely due to particle settling. Isotopic ratios and other OM source indicators (δ 15 N–PN, δ 13 C–POC, POC:PN, chl- a :POC) show that the dominant source of particulate OM was phytoplankton with seasonal terrestrial/macrophytic inputs. Results highlight the effects of major tributary and agricultural drain inputs, primary production, and impoundment on OM composition and concentration through a large river-reservoir system and may inform water quality management efforts in this and similar systems.

Idaho, Oregon↗

USGS2024

Riparian methylmercury production increases riverine mercury flux and food web concentrations

The production and uptake of toxic methylmercury (MeHg) impacts aquatic ecosystems globally. Rivers can be dynamic and difficult systems to study for MeHg production and bioaccumulation, hence identifying sources of MeHg to these systems is both challenging and important for resource management within rivers and main-stem reservoirs. Riparian zones, which are known biogeochemical hotspots for MeHg production, are understudied as potential sources of MeHg to rivers. Here, we present a comprehensive quantification of the hydrologic and biogeochemical processes governing MeHg concentrations, loads, and bioaccumulation at 16 locations along 164 km of the agriculturally intensive Snake River (Idaho, Oregon USA) during summer baseflow conditions, with emphasis on riparian production of MeHg. Approximately one-third of the MeHg load of the Snake River could not be attributed to inflowing waters (upgradient, tributaries, or irrigation drains). Across the study reach, increases in MeHg loads in surface waters were significantly correlated with MeHg concentrations in riparian porewaters, suggesting riparian zones were likely an important source of MeHg to the Snake River. Across all locations, MeHg concentrations in surface waters positively correlated with MeHg concentrations in benthic snails and clams, supporting that riparian produced MeHg was assimilated into local aquatic food webs. This study contributes new insights into riparian MeHg production within rivers which can inform mitigation efforts to reduce MeHg bioaccumulation in fish.

Idaho, Oregon↗

USGS2024

Vegetation, fuels, and fire-behavior responses to linear fuel-break treatments in and around burned sagebrush steppe: Are we breaking the grass-fire cycle?

Background Linear fuel breaks are being implemented to moderate fire behavior and improve wildfire containment in semiarid landscapes such as the sagebrush steppe of North America, where extensive losses in perennial vegetation and ecosystem functioning are resulting from invasion by exotic annual grasses (EAGs) that foster large and recurrent wildfires. However, fuel-break construction can also pose EAG invasion risks, which must be weighed against the intended fire-moderation benefits of the treatments. We investigated how shrub reductions (mowing, cutting), pre-emergent EAG-herbicides, and/or drill seedings of fire-resistant perennial bunchgrasses (PBGs) recently applied to create a large fuel-break system affected native and exotic plant abundances and their associated fuel loading and predicted fire behavior. Results In heavily EAG-invaded areas, herbicides reduced EAG and total herbaceous cover without affecting PBGs for 2–3 years and reduced predicted fire behavior for 1 year (from the Fuel Characteristic Classification System). However, surviving post-herbicide EAG cover was still > 30%, which was sufficient fuel to exceed the conventional 1.2-m-flame length (FL) threshold for attempting wildfire suppression with hand tools. In less invaded shrubland, shrub reduction treatments largely reduced shrub cover and height by ~ half without increasing EAGs, but then redistributed the wood to ground level and increased total herbaceous cover. Herbicides and/or drill seeding after shrub reductions did not affect EAG cover, although drill seedings increased PBG cover and exotic forbs (e.g., Russian thistle). Fire behavior was predicted to be moderated in only one of the many yearly observations of the various shrub-reduction treatment combinations. Over all treatments and years, FLs were predicted to exceed 1.2 m in 13% of simulations under average (11 km h −1 ) or high (47 km h −1 ) wind speed conditions and exceed the 3.4-m threshold for uncontrollable fire in 11% of simulations under high-wind speeds only. Conclusions Predicted fire-moderation benefits over the first 4 years of fuel break implementation were modest and variable, but, generally, increases in EAGs and their associated fire risks were not observed. Nonetheless, ancillary evidence from shrublands would suggest that treatment-induced shifts from shrub to herbaceous fuel dominance are expected to improve conditions for active fire suppression in ways not readily represented in available fire models.

Idaho, Oregon↗

USGS2024

Variability in weather and site properties affect fuel and fire behavior following fuel treatments in semiarid sagebrush-steppe.

Fuel-treatments targeting shrubs and fire-prone exotic annual grasses (EAGs) are increasingly used to mitigate increased wildfire risks in arid and semiarid environments, and understanding their response to natural factors is needed for effective landscape management. Using field-data collected over four years from fuel-break treatments in semiarid sagebrush-steppe, we asked 1) how the outcomes of EAG and sagebrush fuel treatments varied with site biophysical properties, climate, and weather, and 2) how predictions of fire behavior using the Fuel Characteristic Classification System fire model related to land-management objectives of maintaining fire behavior expected of low-load, dry-climate grasslands. Generalized linear mixed effect modeling with build-up model selection was used to determine best-fit models, and marginal effects plots to assess responses for each fuel type. EAG cover decreased as antecedent-fall precipitation increased and increased as antecedent-spring temperatures and surface soil clay contents increased. Herbicides targeting EAGs were less effective where pre-treatment EAG cover was >40 % and antecedent spring temperatures were >9.5 °C. Sagebrush cover was inversely related to soil clay content, especially where clay contents were >17 %. Predicted fire behavior exceeded management objectives under 1) average fire weather conditions when EAG or sagebrush cover was >50 % or >26 %, respectively, or 2) extreme fire weather conditions when EAG or sagebrush cover was >10 % or >8 %, respectively. Consideration of the strong effects of natural variability in site properties and antecedent weather can help in justifying, planning and implementing fuel-treatments.

Idaho, Oregon↗

USGS2023

Satellite-derived plant cover maps vary in performance depending on version and product

Understanding the accuracy and appropriate application scale of satellite-derived maps of vegetation cover is essential for effective management of the vast, remote rangelands of the world. However, the underlying models are updated frequently and may combine with rapidly changing vegetation conditions to cause variations in accuracy and precision over time. We sought to assess how model performance changed between different versions of satellite-derived cover products (Rangeland Analysis Platform, RAP, and Rangeland Condition Monitoring and Assessment Protocol, RCMAP) and how the performance of LandCart compared to RAP and RCMAP. Additionally, we asked how variability in agreement between LandCart and field-based models varied with scale. We utilized an intensive dataset of grid-point intercept functional group cover data collected between 2016 and 2020 across the ∼113 kHA 2015 Soda Wildfire to 1) evaluate r 2 agreement between versions of each satellite-derived product and plot-level field data and 2) assess relative standard error of agreement in cover between LandCart and continuous field-based Empirical Bayesian Kriging (EBK) regression models. Agreement between satellite- compared to field-plot values of cover (r 2 ) increased for RCMAP Version 5.0 compared to Version 2.0, but there were negligible changes between versions of RAP. Despite this, r 2 values of RCMAP and LandCart were nearly always less than RAP. Variability in agreement between EBK regression model cover and LandCart-derived cover decreased with the scale of consideration. Variability in agreement between satellite-derived cover products and field-based metrics is lowest at larger scale (mega-fire or regional) and varies from year to year and across versions, which could complicate detection of temporal changes in plant cover.

Idaho, Oregon↗

USGS2022

Modeling of fire spread in sagebrush steppe using FARSITE: An approach to improving input data and simulation accuracy

Background: Model simulations of wildfire spread and assessments of their accuracy are needed for understanding and managing altered fire regimes in semiarid regions. The accuracy of wildfire spread simulations can be evaluated from post hoc comparisons of simulated and actual wildfire perimeters, but this requires information on pre-fire vegetation fuels that is typically not available. We assessed the accuracy of the Fire-Area Simulator (FARSITE) model parameterized with maps of fire behavior fuel models (FBFMs) obtained from the widely used LANDFIRE, as well as alternative means which utilized the classification of Rangeland Analysis Platform (RAP) satellite-derived vegetation cover maps to create FBFM maps. We focused on the 2015 Soda wildfire, which burned 113,000 ha of sagebrush steppe in the western USA, and then assessed the transferability of our RAP-to-FBFM selection process, which produced the most accurate reconstruction of the Soda wildfire, on the nearby 2016 Cherry Road wildfire. Results: Parameterizing FARSITE with maps of FBFMs from LANDFIRE resulted in low levels of agreement between simulated and observed area burned, with maximum Sorensen’s coefficient (SC) and Cohen’s kappa ( K ) values of 0.38 and 0.36, respectively. In contrast, maps of FBFMs derived from unsupervised classification of RAP vegetation cover maps led to much greater simulated-to-observed burned area agreement (SC = 0.70, K = 0.68). The FBFM map that generated the greatest simulated-to-observed burned area agreement for the Soda wildfire was then used to crosswalk FBFMs to another nearby wildfire (2016 Cherry Road), and this FBFM selection led to high FARSITE simulated-to-observed burned area agreement (SC = 0.80, K = 0.79). Conclusions: Using RAP to inform pre-fire FBFM selection increased the accuracy of FARSITE simulations compared to parameterization with the standard LANDFIRE FBFM maps, in sagebrush steppe. Additionally, the crosswalk method appeared to have regional generalizability. Flanking and backfires were the primary source of disagreements between simulated and observed fire spread in FARSITE, which are sources of error that may require modeling of lateral heterogeneity in fuels and fire processes at finer scales than used here.

Idaho, Oregon↗

USGS2022

In-reservoir physical processes modulate aqueous and biological methylmercury export from a seasonally anoxic reservoir

Anoxic conditions within reservoirs related to thermal stratification and oxygen depletion lead to methylmercury (MeHg) production, a key process governing the uptake of mercury in aquatic food webs. Once formed within a reservoir, the timing and magnitude of the biological uptake of MeHg and the relative importance of MeHg export in water versus biological compartments remain poorly understood. We examined the relations between the reservoir stratification state, anoxia, and the concentrations and export loads of MeHg in aqueous and biological compartments at the outflow locations of two reservoirs of the Hells Canyon Complex (Snake River, Idaho-Oregon). Results show that (1) MeHg concentrations in filter-passing water, zooplankton, suspended particles, and detritus increased in response to reservoir destratification; (2) zooplankton MeHg strongly correlated with MeHg in filter-passing water during destratification; (3) reservoir anoxia appeared to be a key control on MeHg export; and (4) biological MeHg, primarily in zooplankton, accounted for only 5% of total MeHg export from the reservoirs (the remainder being aqueous compartments). These results improve our understanding of the role of biological incorporation of MeHg and the subsequent downstream release from seasonally stratified reservoirs and demonstrate that in-reservoir physical processes strongly influence MeHg incorporation at the base of the aquatic food web.

Idaho, Oregon↗

USGS2022

How do accuracy and model agreement vary with versioning, scale, and landscape heterogeneity for satellite-derived vegetation maps in sagebrush steppe?

Maps of the distribution and abundance of dominant plants derived from satellite data are essential for ecological research and management, particularly in the vast semiarid shrub-steppe. Appropriate application of these maps requires an understanding of model accuracy and precision, and how it might vary across space, time, and different vegetation types. For a 113 k Ha burn area, we compared modeled maps of different vegetation cover types created from satellite data to ‘benchmark” models based on intensive field sampling (~1500-2000 plots resampled annually for 5 years) for three new satellite-derived models: USDA Rangeland Analysis Platform (RAP), the USGS Rangeland Condition Monitoring Assessment and Projection (RCMAP), and USGS fractional estimate of exotic annual grass cover (USGS-fractional-EAG). We assessed out-of-sample point accuracy and asked if and how accuracy changed each year due to vegetation shifts, new images, and model improvements (i.e. model versions). We also assessed how map agreement between satellite-based and field-based models changed with scale of application, topography, and time since fire. Accuracy and map agreement varied considerably among the vegetation types and across time and space (r 2 ranging from 0 to 0.53), and some of the variability was predictable. All models tended to over or underestimate cover when field-measured cover was relatively low or high, respectively, i.e. a “false moderating effect”. Accuracy was greater and improved with newer versions of RAP (+0.05 to 0.29 r 2 ) compared to RCMAP and USGS fractional model estimates, and in some cases was greater than field-based models. Variability in map agreement tended to decrease with larger areas sampled (particularly in areas >12 km), and this scale dependency was more evident in RAP and USGS-fractional-EAG models. Creating a “fair” basis for comparison of spatial models of low-statured semiarid vegetation derived from satellite compared to field data is not trivial because scaling the field data to the scale of large satellite pixels (or downscaling satellite-based models to field scale) requires modeling and associated model uncertainty. Accuracy can vary considerably and understanding the variation can help guide application of the models to the appropriate time, place, and variables.

Idaho, Oregon↗

USGS2022

Monitoring for adaptive management of burned sagebrush-steppe rangelands: addressing variability and uncertainty on the 2015 Soda Megafire

• Use of adaptive management supported by robust monitoring is vital to solving severe rangeland problems, such as the exotic annual grass invasion and fire cycle in sagebrush-steppe rangelands. • Uncertainty in post-fire plant-community composition and plant response to treatments poses a challenge to land management and research but can be addressed with a high density of observations over short time frames. • The monitoring for adaptive management of the 2015 Soda Megafire area (113,000 Ha) sampled up to 2000 observation plots in each of five post-fire years, and provided important insights on challenges, solutions, and insights that can be applied to monitoring future burned areas.

Idaho, Oregon↗

USGS2022

Mapping the probability of freshwater algal blooms with various spectral indices and sources of training data

Algal blooms are pervasive in many freshwater environments and can pose risks to the health and safety of humans and other organisms. However, monitoring and tracking of potentially harmful blooms often relies on in-person observations by the public. Remote sensing has proven useful in augmenting in situ observations of algal concentration, but many hurdles hinder efficient application by end users. First, numerous approaches to estimate aquatic chlorophyll-a are available and can produce inconsistent results. Second, lack of quantitative in situ observations limits opportunities to train models for specific waterbodies, such that models developed for other systems must be used instead. We (1) implement univariate and multivariate logistic regression models to estimate the probability that aquatic chlorophyll-a concentrations exceed an accepted threshold beyond which harmful effects become likely and (2) evaluate the use of visually classified bloom/no-bloom satellite imagery to augment in situ training data. Using a binary classification of aquatic chlorophyll-a exceeding 10 μg / L, we found that (1) logistic regression models were ∼80 % accurate, (2) univariate models trained with visually classified data produce nearly the same accuracy (79%) as models trained with in situ observations (80%), and (3) augmenting in situ chlorophyll-a observations with visual classifications outperformed (82% accuracy) models trained on in situ observations alone (80% accuracy). These results provide a framework for evaluating multiple spectral indices in retrieving algal bloom presence or absence and illustrate that training data derived directly from satellite imagery can be useful in augmenting in situ observations.

Idaho, Oregon↗

USGS2021

Patterns of post-fire invasion of semiarid shrub-steppe reveals a diversity of invasion niches within an exotic annual grass community

Disturbances such as fire provide an opportunity for invasive plant species to exploit newly created niche space. Whether initial invaders facilitate, compete with, or do not affect later invaders is important to determine in communities affected by multiple invaders. This analysis focuses on the newer invaders Taeniatherum caput-medusae (medusahead) and Ventenata dubia (ventenata) in sagebrush-steppe communities previously invaded by Bromus tectorum (cheatgrass), during the first 5 years of recovery after wildfire . We combined probabilistic co-occurrence analysis and Getis-Ord spatial clustering analysis to assess relationships between different exotic annual grass species and native and introduced perennial bunchgrasses, then used Bayesian generalized linear models to determine if and how medusahead and ventenata differed in their environmental relationships and thus invasion niches. Medusahead presence was positively associated with both other exotic annual grasses, but ventenata presence was negatively associated with cheatgrass presence. Medusahead hotspots were more spatially similar to cheatgrass hotspots while ventenata hotspots were unique. Both invaders were negatively related to total perennial bunchgrass cover but disassociations between invaders and different perennial bunchgrasses were species-specific. Medusahead and ventenata occupied different niches; medusahead in low elevation, low precipitation areas and ventenata in higher elevation, higher precipitation areas. Despite seemingly similar ecology and growth requirements among these annual grasses and a tendency to be considered uniformly in both research and management, the species appeared to have different invasion niches.

Idaho, Oregon↗

USGS2020

Seasonal dynamics and interannual variability in mercury concentrations and loads through a three-reservoir complex

The Hells Canyon Complex (HCC) along the Snake River (Idaho-Oregon border, USA) encompasses three successive reservoirs that seasonally stratify, creating anoxic conditions in the hypolimnion that promote methylmercury (MeHg) production. This study quantified seasonal dynamics and interannual variability in mercury concentrations (inorganic divalent mercury (IHg) and MeHg) and loads at four reservoir inflow and outflow locations through the HCC (2014-2017). We observed (1) that the HCC is a net sink for both IHg and MeHg, (2) interannual variability in IHg and MeHg loads largely reflecting streamflow conditions, and (3) seasonal variability in particulate IHg loading at the inflow (greatest from February to April) and MeHg export from the outflow (greatest from September to December) of the HCC. Seasonal export of MeHg was evidenced by increases in monthly mean concentrations of unfiltered MeHg (approximately twofold) and the percentage of total mercury (THg) as MeHg (≥ fourfold) coincident with reservoir destratification. Despite evidence of seasonal export of MeHg from the HCC, annual loads indicate a 42% decrease in unfiltered MeHg from HCC inflow to outflow. Results from this study improve the understanding of seasonal variability in mercury transport through and transformation within a reservoir complex.

Idaho, Oregon↗

USGS2020

Impact of unburned remnant sagebrush versus outplants on post-fire landscape rehabilitation

Nearly half of the vast sagebrush steppe in the western United states has lost many or nearly all native plant species, largely due to the interaction of invasive species and increased wildfire. Re-establishing sagebrush, a keystone component of these ecosystems, has become a management focus in recent decades using aerial broadcast seeding or limited plantings. One promising avenue for improving the planning and assessment of post-fire seedings involves the spatial patchiness of burn patterns and in the recovery of sagebrush after fire. Unburned remnant or post-fire planted islands (or patches) of sagebrush could be valuable seed sources for species recovery in surrounding burned areas. Information on how much spatial expansion of unburned remnant patches is expected over time could help in the planning of post-fire treatments. However, previous research has indicated that sagebrush seeds do not disperse far, which would imply that unburned or created patches do not contribute much to sagebrush reestablishment effects. Our objective was to determine whether remnant/unburned sagebrush patches contribute to sagebrush recovery in the surrounding burned areas. We quantified seed rain and seedling establishment in relation to patches of sagebrush that were either unburned remnant or had been planted in the first year or so after wildfire. We conducted a seed trapping experiment across 6 different wildfires during two winters to determine seed transport distances. We paired this with a seedling recruitment study on the Soda wildfire where we mapped distances between remnants and seedlings. We found that although a few seeds did travel much farther than previously recorded (maximum of 26 m), seed dispersal was highly variable across sites and patches, and only a small portion of seeds dispersed farther than a few meters from sagebrush patches. Our seedling recruitment assessment confirmed a limited contribution of remnants to seedling recruitment. Specifically, a microsite was only marginally more likely to have a sagebrush seedling even if there was >50 neighbors within a 40 m radius. There were no differences in the quantity of seeds dispersed from remnant versus actively managed patches. Overall, we found that isolated sagebrush patches are unlikely to significantly contribute to landscape regeneration of sagebrush on large fires and that aerial seeding is likely needed to overcome seed limitations. We did detect substantial variation in site-level sagebrush seed production among years, including one site that did not produce any seed in one year. Variability in seed production in space and time appeared to be a potentially more important variable potentially affecting sagebrush seed availability than dispersal distances and is a topic that merits more investigation.

Idaho, Oregon↗

USGS2019

Hydrogeologic framework of the Treasure Valley and surrounding area, Idaho and Oregon

Most of the population of the Treasure Valley and the surrounding area of southwestern Idaho and easternmost Oregon depends on groundwater for domestic supply, either from domestic or municipal-supply wells. As of 2017, 41 percent of Idaho’s population was concentrated in Idaho’s portion of the Treasure Valley, and current and projected rapid population growth in the area has caused concern about the long-term sustainability of the groundwater resource. In 2016, the U.S. Geological Survey, in cooperation with the Idaho Water Resource Board and the Idaho Department of Water Resources, began a project to construct a numerical groundwater-flow model of the westernmost western Snake River Plain (WSRP) aquifer system. As part of this project, a three-dimensional hydrogeologic framework model (3D HFM) of the aquifer system was generated, primarily from lithologic data compiled from 291 well-driller reports. Four major hydrogeologic units are shown in the 3D HFM: Coarse-grained fluvial and alluvial deposits, Pliocene-Pleistocene and Miocene basalts, fine-grained lacustrine deposits, and granitic and rhyolitic bedrock. Generally, the 3D HFM is in agreement with the geologic history of the WSRP and hydrogeologic frameworks developed by previous authors. The resolution (voxel size) of the 3D HFM is sufficient for the construction of a regional groundwater-flow model. The major components of inflow (or recharge) to the WSRP aquifer system are seepage from irrigation canals, direct infiltration from precipitation and excess irrigation water, seepage from the Boise and Payette Rivers and Lake Lowell, and subsurface inflow from adjoining uplands. The major components of outflow (or discharge) from the aquifer system are discharge to surface water (rivers, agricultural drains, and streams), groundwater pumping, and direct evapotranspiration from groundwater.

Idaho, Oregon↗

USGS2018

Mercury concentrations in water and mercury and selenium concentrations in fish from Brownlee Reservoir and selected sites in the Boise and Snake Rivers, Idaho and Oregon, 2013-17

Mercury (Hg) analyses were conducted on samples of water and sport fish collected from selected sampling sites in the Boise and Snake Rivers and Brownlee Reservoir, in Idaho and Oregon, to meet National Pollution Discharge and Elimination System permit requirements for the City of Boise, Idaho, from 2013 to 2017. City of Boise personnel collected water samples from six sites in October and November of 2013, 2015 and 2017, and sampled one site in 2014 and 2016. Total Hg concentrations in unfiltered water samples ranged from 0.41 to 8.78 nanograms per liter (ng/L), with the highest value (8.78 ng/L) observed in Brownlee Reservoir in 2013. All samples were less than the U.S. Environmental Protection Agency aquatic life criterion of 12 ng/L. Individual fillets of mountain whitefish ( Prosopium williamsoni ), rainbow trout ( Oncorhynchus mykiss ), smallmouth bass ( Micropterus dolomieu ), and channel catfish ( Ictalurus punctatus ) were collected and analyzed for Hg. The tissue Hg concentrations were compared with regulatory or advisory values for wet-weight methylmercury in fish tissue. In this report, methylmercury concentrations in fish tissue are considered similar to total Hg in fish muscle tissue and are simply referred to as Hg. The 2013 average Hg concentration for smallmouth bass (0.32 mg/kg) collected at Brownlee Reservoir and for channel catfish (0.33 mg/kg) collected at the Boise River mouth, exceeded the Idaho water quality criterion (>0.3 mg/kg). The 2017 Hg concentrations in smallmouth bass from Brownlee Reservoir (geometric mean of 0.22 mg/kg) was at the Idaho Fish Consumption Advisory Program action level. Selenium (Se) interacts with Hg to reduce the health risks of Hg, such that tissues with Se-to-Hg molar ratios greater than 1 are considered to present less potential health risks for a given Hg concentration than are tissues with lower Se-to-Hg ratios. One composite fish tissue sample per site was analyzed for Se. Selenium-to-Hg molar ratios in the fish tissue samples ranged from 0.99 to 24.7.

Idaho, Oregon↗

USGS2016

Mercury cycling in the Hells Canyon Complex of the Snake River, Idaho and Oregon

Introduction The Hells Canyon Complex (HCC) is a hydroelectric project built and operated by the Idaho Power Company (IPC) that consists of three dams on the Snake River along the Oregon and Idaho border (fig. 1). The dams have resulted in the creation of Brownlee, Oxbow, and Hells Canyon Reservoirs, which have a combined storage capacity of more than 1.5 million acre-feet and span about 90 miles of the Snake River. The Snake River upstream of and through the HCC historically has been impaired by water-quality issues related to excessive contributions of nutrients, algae, sediment, and other pollutants. In addition, historical data collected since the 1960s from the Snake River and tributaries near the HCC have documented high concentrations of mercury in fish tissue and sediment (Harris and Beals, 2013). Data collected from more recent investigations within the HCC continue to indicate elevated concentrations of mercury and methylmercury in the water column, bottom sediments, and biota (Clark and Maret, 1998; Essig, 2010; Fosness and others, 2013). As a result, Brownlee and Hells Canyon Reservoirs are listed as impaired for mercury by the State of Idaho, and the Snake River from the Oregon and Idaho border through the HCC downstream to the Oregon and Washington border is listed as impaired for mercury by the State of Oregon.

Idaho, Oregon↗