Aeromagnetic map of Mt. St. Helens, Washington: Post May 1980 eruption
No abstract available.
Washington↗
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Juvenile salmonid monitoring in the White Salmon River, Washington, post-Condit Dam removal, 2016
Condit Dam, at river kilometer 5.3 on the White Salmon River, Washington, was breached in 2011 and removed completely in 2012, allowing anadromous salmonids access to habitat that had been blocked for nearly 100 years. A multi-agency workgroup concluded that the preferred salmonid restoration alternative was natural recolonization with monitoring to assess efficacy, followed by a management evaluation 5 years after dam removal. Limited monitoring of salmon and steelhead spawning has occurred since 2011, but no monitoring of juveniles occurred until 2016. During 2016, we operated a rotary screw trap at river kilometer 2.3 (3 kilometers downstream of the former dam site) from late March through May and used backpack electrofishing during summer to assess juvenile salmonid distribution and abundance. The screw trap captured primarily steelhead ( Oncorhynchus mykiss ; smolts, parr, and fry) and coho salmon ( O. kisutch ; smolts and fry). We estimated the number of steelhead smolts at 3,851 (standard error = 1,454) and coho smolts at 1,093 (standard error = 412). In this document, we refer to O. mykiss caught at the screw trap as steelhead because they were actively migrating, but because we did not know migratory status of O. mykiss caught in electrofishing surveys, we simply refer to them as O. mykiss or steelhead/rainbow trout. Steelhead and coho smolts tagged with passive integrated transponder tags were subsequently detected downstream at Bonneville Dam on the Columbia River. Few Chinook salmon ( O. tshawytscha ) fry were captured, possibly as a result of trap location or effects of a December 2015 flood. Sampling in Mill, Buck, and Rattlesnake Creeks (all upstream of the former dam site) showed that juvenile coho were present in Mill and Buck Creeks, suggesting spawning had occurred there. We compared O. mykiss abundance data in sites on Buck and Rattlesnake Creeks to pre-dam removal data. During 2016, age-0 O. mykiss were more abundant in Buck Creek than in 2009 or 2010, though age-1 and older O. mykiss abundance was similar. In Rattlesnake Creek, age-0 O. mykiss abundance during 2016 slightly exceeded the mean abundance from 2001 through 2005, although age-1 and older O. mykiss abundance was lower than from 2001 through 2005. These sampling efforts also provided the opportunity to collect genetic samples to investigate parental and stock origin, although funding to analyze the samples was not part of this grant. Juvenile salmonid sampling efforts during 2016 have shown that natural spawning produced steelhead and coho smolts and that coho were colonizing some tributaries. The 2016 efforts also provided the first post-dam juvenile abundance estimates. We hope to continue monitoring to better understand abundance trends, distribution, and life history patterns of recolonizing salmonids in the White Salmon River to assess efficacy of natural recolonization and to inform management decisions.
Washington↗
Approach, passage, and survival of juvenile salmonids at Little Goose Dam, Washington: Post-construction evaluation of a temporary spillway weir, 2009
This report describes a study of dam passage and survival of radio-tagged juvenile salmonids after installation of a temporary spillway weir (TSW) at Little Goose Dam, Washington, in 2009. The purpose of the study was to document fish passage and survival when the dam was operated with the TSW in place. Spillway weirs are one of several methods used to improve downstream passage of juvenile salmonids. Each spillway weir design is based on the concept of providing an overflow weir with a depth more similar to the natural migration depth of juvenile salmonids than conventional spill bays. Little Goose Dam was the last of the four lower Snake River dams to have a spillway weir installed. This was the first year that some form of surface passage device was operating at all Snake River and Columbia River dams between Lewiston, Idaho, and the Columbia River estuary. The study design stipulated that a total of 30 percent of the river discharge would continuously be passed over the TSW and the conventional spill bays, and this percentage was achieved. The TSW also was to be operated at the 'low crest' elevation during the spring and the 'high crest' elevation during the summer, but the TSW was only operated at the low crest elevation during this study. Behavior, passage, and survival of spring and summer juvenile salmonid migrants passing through Little Goose Dam were examined using radio telemetry. Survival was estimated using the Route Specific Survival Model (RSSM) by releasing tagged fish near Central Ferry State Park 21 kilometers upstream of the dam and in the tailrace approximately 0.5 kilometer downstream of the dam. From April 18 to May 21, 2009, 1,520 yearling Chinook salmon (Oncorhynchus tshawytscha) and 1,517 juvenile steelhead (O. mykiss) were radio tagged and released. From June 6 to July 5, 2009, 4,251 subyearling Chinook salmon (O. tshawytscha) were radio tagged and released. Release dates of subyearling Chinook salmon were selected to avoid 'reservoir-type' fish that cease to migrate around July. Detection sites were installed in the forebay 2 kilometers upstream of the dam, on the dam, and at several sites downstream. Detection equipment was operated from April 18 to June 5, 2009, and from June 6 to July 6, 2009, hereinafter referred to as the study periods. We describe passage behaviors through the forebay, main passage routes, and tailrace, survival probabilities through the pool (release to the forebay) and forebay and passage and survival probabilities through the main passage routes (TSW, conventional spill bays, turbines, juvenile bypass), and survival passing the concrete (the dam itself) and the dam (concrete plus the forebay).
Open-File Report↗
Approach, passage, and survival of juvenile salmonids at Little Goose Dam, Washington: Post-construction evaluation of a temporary spillway weir, 2009
This report describes a study of dam passage and survival of radio-tagged juvenile salmonids after installation of a temporary spillway weir (TSW) at Little Goose Dam, Washington, in 2009. The purpose of the study was to document fish passage and survival when the dam was operated with the TSW in place. Spillway weirs are one of several methods used to improve downstream passage of juvenile salmonids. Each spillway weir design is based on the concept of providing an overflow weir with a depth more similar to the natural migration depth of juvenile salmonids than conventional spill bays. Little Goose Dam was the last of the four lower Snake River dams to have a spillway weir installed. This was the first year that some form of surface passage device was operating at all Snake River and Columbia River dams between Lewiston, Idaho, and the Columbia River estuary. The study design stipulated that a total of 30 percent of the river discharge would continuously be passed over the TSW and the conventional spill bays, and this percentage was achieved. The TSW also was to be operated at the ?low crest? elevation during the spring and the ?high crest? elevation during the summer, but the TSW was only operated at the low crest elevation during this study. Behavior, passage, and survival of spring and summer juvenile salmonid migrants passing through Little Goose Dam were examined using radio telemetry. Survival was estimated using the Route Specific Survival Model (RSSM) by releasing tagged fish near Central Ferry State Park 21 kilometers upstream of the dam and in the tailrace approximately 0.5 kilometer downstream of the dam. From April 18 to May 21, 2009, 1,520 yearling Chinook salmon (Oncorhynchus tshawytscha) and 1,517 juvenile steelhead (O. mykiss) were radio tagged and released. From June 6 to July 5, 2009, 4,251 subyearling Chinook salmon (O. tshawytscha) were radio tagged and released. Release dates of subyearling Chinook salmon were selected to avoid ?reservoir-type? fish that cease to migrate around July. Detection sites were installed in the forebay 2 kilometers upstream of the dam, on the dam, and at several sites downstream. Detection equipment was operated from April 18 to June 5, 2009, and from June 6 to July 6, 2009, hereinafter referred to as the study periods. We describe passage behaviors through the forebay, main passage routes, and tailrace, survival probabilities through the pool (release to the forebay) and forebay and passage and survival probabilities through the main passage routes (TSW, conventional spill bays, turbines, juvenile bypass), and survival passing the concrete (the dam itself) and the dam (concrete plus the forebay).
Scientific Investigations Report↗
The fellow speaks: Sometimes you get only one chance
I am grateful to AGU for selecting me as one of the five recipient of the 2014 Ambassador Award, which also includes election as a Union Fellow. I thank my colleague Steve Ingebritsen for nominating me. As Steve’s citation mentions my work on the Deepwater Horizon oil spill response, I would like to reflect on this experience. The Deepwater Horizon oil spill is well documented in the report of the National Commission on the BP Deepwater Horizon Oil Spill and Offshore Drilling (2011). Washington Post writer John Achenbach’s (2011) book gives a behind-the-scene portrayal of the crisis and explains oil drilling technology in layman’s terms. A special feature in the Proceedings of the National Academy of Sciences (v. 109, no. 50, December 11, 2012) presents 4 perspectives and 11 research articles on “Science Applications in the Deepwater Horizon Oil Spill.” Here, I will simply share my personal perspective.
AGU Hydrology Section Newsletter↗
Living fossils: You haven't changed a bit: Tuatara
Explore the source record for details and available documents.
Washington Post↗
Pomona Member of the Columbia River Basalt Group: an intracanyon flow in the Columbia River Gorge, Oregon.
The Pomona Member of the Saddle Mountains Basalt (Columbia River Basalt Group) occurs as an intracanyon flow greater than 75m (250ft) thick along the S side of the Columbia River Gorge between Mitchell Point and Shellrock Mountain, Oregon. Best exposures are at Mitchell Point, where this flow caps more than 70m (230ft) of cobble conglomerate that partially fills a canyon cut into flows of the underlying Frenchman Springs Member. These exposures provide a necessary link between outcrops of the Pomona Member in the Columbia Plateau and western Washington. Post-Frenchman Springs, pre-Pomona canyon cutting implies deformation in the ancestral Cascade Range between about 14.5 and 12Ma ago.-Author
Oregon Geology↗
Post 12 m.y. rotation of southwest Washington
Paleomagnetic field directions from the basalt of Pack Sack Lookout are compared to those from the Pomona Member of the Saddle Mountains Basalt of the Columbia River Basalt Group. The Pomona crops out over a wide region on the Columbia Plateau east of the Cascade Range, and the basalt of Pack Sack Lookout crops out well to the west of the Cascades about 30 to 60 km east of the Washington coast. Our paleomagnetic results support existing geologic and geochemical evidence that both these units are part of a single 12‐m.y.‐old flow that erupted in western Idaho and flowed to the Washington coast. The magnetic data further indicate that southwest Washington has undergone clockwise rotation with respect to the Columbia Plateau and stable North America. The data suggest that both a large‐scale regional rotation of ∼16° and locally complex small‐scale rotations exist, the two being present to different degrees in the eastern and western regions of southwest Washington. The Cascade Range appears to coincide with the tectonic boundary, separating rotated and unrotated regions of Washington state. Rotation of southwest Washington appears to have been associated with the rotation of large pieces of western Oregon and southern Washington, possibly as a result of either a ridge‐push force from the Basin and Range province or shear along the Pacific‐North America plate boundary.
Washington↗
Inventory of Glaciers in the North Cascades, Washington
Perennial bodies of ice in the North Cascades having areas of at least 0.1 km2 (square kilometer) are tabulated and classified. The inventory, a contribution to the International Hydrological Decade, includes 756 glaciers, covering 267 km2, about half of the glacier area in the United States south of Alaska. Listings include each glacier's location, drainage basin, area, length, orientation, altitude, and classification as to form, source, surface, nature of terminus, and activity. These glaciers contribute annually about 800 million cubic meters of water to streamflow in the State of Washington.
Professional Paper↗
Emergency assessment of Mount St. Helens post-eruption flood hazards, Toutle and Cowlitz rivers, Washington
During several periods of volcanic-ash eruption at Mount St. Helens, Wash., (March 30, May 25-26, May 30-June 2, and June 12-13, 1980) strong winds from the north occurred at high altitudes. As a result, the volcanic ash fell some 50 miles to the south in the Bull Run watershed, the principal water-supply source for the metropolitan area of Portland, Oreg. Water samples collected from three stream sites within the watershed were compared with samples collected during the same season in previous years. No detectable changes were noted in chemical characteristics. Precipitation samples collected immediately after the June 12-13 ash fall ranged in specific conductance from 20 to 41 micromhos per centimeter at 25C and in pH from 4.0 to 4.3 pH units. Stream samples collected during the May-June period ranged in specific conductance from 18 to 28 micromhos per centimeter at 25C and in pH from 6.7 to 7.5 pH units. Volcanic-ash samples were collected and analyzed for particle size, chemical composition, and weight. Significant differences in particle size of ash were found in samples from two separate eruptions. (USGS)
Circular↗
Geochemical characterization and dating of R tephra, a post-glacial marker bed in Mount Rainier National Park, Washington, U.S.A.
The oldest postglacial lapilli–ash tephra recognized in sedimentary records surrounding Mount Rainier (Washington State, USA) is R tephra, a very early Holocene deposit that acts as an important stratigraphic and geochronologic marker bed. This multidisciplinary study incorporates tephrostratigraphy, radiocarbon dating, petrography, and electron microprobe analysis to characterize R tephra. Tephra samples were collected from Tipsoo Lake and a stream-cut exposure in the Cowlitz Divide area of Mount Rainier National Park. Field evidence from 25 new sites suggests that R tephra locally contains internal bedding and has a wider distribution than previously reported. Herein, we provide the first robust suite of geochemical data that characterize the tephra. Glass compositions are heterogeneous, predominantly ranging from andesite to rhyolite in ash- to lapilli-sized clasts. The mineral assemblage consists of plagioclase, orthopyroxene, clinopyroxene, and magnetite with trace apatite and ilmenite. Subaerial R tephra deposits appear more weathered in hand sample than subaqueous deposits, but weathering indices suggest negligible chemical weathering in both deposits. Statistical analysis of radiocarbon ages provides a median age for R tephra of ∼10 050 cal years BP, and a 2σ error range between 9960 and 10 130 cal years BP.
Washington↗
Five years (2000-2004) of post-reconstruction monitoring of freshwater tidal wetlands in the urban Anacostia River, Washington, D.C. USA
The Anacostia River in Washington, D.C. USA consisted of over 809 hectares (2000 acres) of freshwater tidal wetlands before mandatory dredging removed most of them in the first half of the 20th century. Much of this13 kilometer (8 mile) reach was transferred to the National Park Service (NPS). Planning processes in the 1980's envisioned a restoration (rejuvenation) of some wetlands for habitat, aesthetics, water quality and interpretative purposes. Subsequently, the U.S. Army Corps of Engineers in a cost share agreement with the District of Columbia reconstructed wetlands on NPS lands at Kenilworth - 12.5 hectares (1993), Kingman 27 hectares (2000), a Fringe Marsh - 6.5 hectares (2003) and is currently constructing Heritage Marsh - 2.5 hectares (2005/2006). The USGS Patuxent Wildlife Research Center in conjunction with the University of Maryland Biological Engineering Department was contracted to conduct post-reconstruction monitoring (2000-2004) to document the relative success and progress of the Kingman Marsh reconstruction primarily based on vegetative response but also in conjunction with seed bank and soil characteristics. Results from Kingman were compared to Kenilworth Marsh (reconstructed 7 years prior), Dueling Creek Marsh (last best remaining freshwater tidal wetland bench in the urbanized Anacostia watershed) and Patuxent River Marsh (in a more natural adjacent watershed). Vegetation establishment was initially strong at Kingman, but declined rapidly as measured by cover, richness, diversity, etc. under grazing pressure from resident Canada geese and associated reduction in sediment levels. This decline did not occur at the other wetlands. The decline occurred despite a substantial seed bank that was sustained primarily be water born propagules. Soil development, as true for most juvenile wetlands, was slow with almost no organic matter accumulation. By 2004 only two of 7 planted species remained (mostly Peltandra virginica) at Kingman which did provide almost 50% of the approximately 1/3 total vegetation cover remaining.
Book chapter↗
Effectiveness of post-fire seeding at the Fitzner-Eberhardt Arid Land Ecology Reserve, Washington
In August 2007, the Milepost 17 and Wautoma fires burned a combined total of 77,349 acres (31,302 hectares) of the Fitzner-Eberhardt Arid Land Ecology Reserve (ALE), part of the Hanford Reach National Monument administered by the U.S. Fish and Wildlife Service (USFWS) Mid-Columbia National Wildlife Refuge. In 2009, the USFWS implemented a series of seeding and herbicide treatments to mitigate potential negative consequences of these fires, including mortality of native vegetation, invasion of Bromus tectorum (cheatgrass), and soil erosion. Treatments included combinations of seeding (drill and aerial), herbicides, and one of six different mixtures of species. Artemisia tridentata ssp. wyomingensis (Wyoming big sagebrush) also was planted by hand in a small area in the southern end of the fire perimeter. Due to differences in plant communities prior to the fire and the multiple treatments applied, treatments were grouped into five treatment associations including mid-elevation aerial seedings, low-elevation aerial seedings, low-elevation drill seedings, high-elevation drill seeding, and no seeding treatments. Data collected at the mid-elevation aerial seedings indicate that the seeding did not appear to increase the density of seedlings compared to the non-seeded area in 2010. At the low-elevation aerial seedings, there were significantly more seedlings at seeded areas as compared to non-seeded areas. Low densities of existing perennial plants probably fostered a low-competition environment enabling seeds to germinate and emerge in 2010 during adequate moisture. Low-elevation drill seedings resulted in significant emergence of seeded grasses in 2009 and 2010 and forbs in 2010. This was likely due to adequate precipitation and that the drill seeding assured soil-to-seed contact. At the high-elevation drill seeding, which was implemented in 2009, there were a high number of seedlings in 2010. Transplanting of A. tridentata following the fires resulted in variable survival rates that warrant further testing; however, transplants located closer to washes tended to have the highest survival rates. Overall, the low-elevation aerial and drill seedings, and the high-elevation drill seedings resulted in significant numbers of seedlings. Further research is needed on methods that provide land managers with critical information about whether or not to seed post-fire areas including status of pre-fire vegetation and estimates of plant mortality due to fire.
Open-File Report↗
Post-glacial Mw 7.0-7.5 earthquakes on the North Olympic fault zone, Washington
Holocene crustal faulting in the northern Olympic Peninsula of Washington State manifests in a zone of west‐northwest‐striking crustal faults herein named the North Olympic fault zone, which extends for ∼ 80    km "> ∼ 80 km ∼80 km along strike and includes the Lake Creek–Boundary Creek fault to the east and the Sadie Creek fault and newly discovered scarps to the west. This study focuses on the Sadie Creek fault, which extends for > 14    km "> > 14 km >14 km west‐northwest from Lake Crescent. Airborne light detection and ranging (lidar) imagery reveals the trace of the Sadie Creek fault and offset postglacial landforms showing a history of Holocene surface‐rupturing earthquakes dominated by dextral displacement along a steeply dipping fault zone. Paleoseismic trenches at two sites on the Sadie Creek fault reveal till and outwash overlain by progressively buried forest and wetland soils developed on scarp‐derived colluvial wedges. Trench exposures of complex faulting with subhorizontal slickenlines indicate dextral displacement with lesser dip slip. Correlation of broadly constrained time intervals for earthquakes at the Sadie Creek sites and those to the east along the Lake Creek–Boundary Creek fault is consistent with rupture of much of the length of the North Olympic fault zone three to four times: at about 11, 7, 3, and 1 ka, with a shorter rupture at about 8.5 ka. Dated ruptures from trenches only partially coincide with coseismic landslides and megaturbidites in Lake Crescent, indicating that some earthquakes did not trigger megaturbidites, and some turbidites were unrelated to local fault rupture. Landform mapping suggests single‐event dextral displacement of 4 ± 1    m "> 4 ± 1 m 4±1 m on the Sadie Creek fault. Inferred maximum rupture length and single‐event slip imply earthquake magnitudes M w "> M w Mw 7.0–7.5. Dextral slip rates of 1.3 – 2.3    mm / yr "> 1.3 – 2.3 mm / yr 1.3–2.3 mm/yr and the ∼ 11 , 000    yr "> ∼ 11 , 000 yr ∼11,000 yr slip history suggest that the North Olympic fault zone is a prominent contributor to permanent strain in the northern Cascadia fore‐arc.
Washington↗
Pre- and post-eruptive investigations of gas and water samples from Mount St. Helens, Washington, 2002 to 2005
Samples of gas and water from thermal springs in Loowit and Step canyons and creeks that drain the crater at Mount St. Helens have been collected since October 2004 to monitor the flux of dissolved magmatic volatiles in the hydrologic system. The changing composition of the waters highlights a trend that began as early as 1994 and includes decreasing SO 4 and Cl concentrations and large increases in HCO 3 . Geochemical models indicate that mineral sources and sinks are not the main controls on the changing water chemistry, and carbon and helium isotopes indicate that their sources in the gases and waters have remained unchanged during this time. The present-day molar ratios of C, S, and Cl in the springs approximate ratios measured in plume emissions in August 2005 and provide supporting evidence that changes in water chemistry most likely reflect changes in the release rates of sulfur gases, HCl, and CO 2 from the magma and a varying degree of efficiency of gas scrubbing by the overlying water. Results from coupled chemical analyses and discharge measurements on the creeks yield an estimate of the dissolved flux of magmatic HCl, SO 2 , and CO 2 of around 5.2, 4.7, and 22 metric tons per day, respectively.
Washington↗
Surface-water/ground-water interaction of the Spokane River and the Spokane Valley/Rathdrum Prairie aquifer, Idaho and Washington
Historical mining in the Coeur d’Alene River Basin of northern Idaho has resulted in elevated concentrations of some trace metals (particularly cadmium, lead, and zinc) in water and sediment of Coeur d’Alene Lake and downstream in the Spokane River in Idaho and Washington. These elevated trace-metal concentrations in the Spokane River have raised concerns about potential contamination of ground water in the underlying Spokane Valley/Rathdrum Prairie aquifer, the primary source of drinking water for the city of Spokane and surrounding areas. A study conducted as part of the U.S. Geological Survey’s National Water-Quality Assessment Program examined the interaction of the river and aquifer using hydrologic and chemical data along a losing reach of the Spokane River. The river and ground water were extensively monitored over a range of hydrologic conditions at a streamflow-gaging station and 25 monitoring wells situated from 40 to 3,500 feet from the river. River stage, ground-water levels, water temperature, and specific conductance were measured hourly to biweekly. Water samples were collected on nearly a monthly basis between 1999 and 2001 from the Spokane River and were collected up to nine times between June 2000 and August 2001 from the monitoring wells. Hydrologic and chemical data indicate that the Spokane River recharges the Spokane Valley/ Rathdrum Prairie aquifer along a 17-mile reach between Post Falls, Idaho, and Spokane, Washington. Ground-water levels in the near-river aquifer (less than 300 feet from the river) indicate variably saturated conditions below the river and a ground-water flow gradient away from the losing reach of the river. Calculated monthly mean losses, during water years 2000 and 2001 along a nearly 7-mile reach between two gages, ranged from near 69 to 810 cubic feet per second. Losses generally increased with increased streamflow. However, late summer warm water temperatures also appear to be a factor as losses increased due to lower viscosity as water temperatures increased. Chemical data indicated that river recharge may influence ground-water chemistry as far as 3,000 feet from the river, but ground water within a few hundred feet of the river is most affected. Major-ion concentrations, stable isotopes, and temperature of the river and ground water from near-river wells were similar and exhibited similar temporal trends, whereas ground water from wells located farther from the river generally had higher major-ion concentrations and more stable temperatures and chemistry. Although trace-element concentrations sometimes exceeded aquatic-life criteria in the water of the Spokane River and were elevated above national median values in the bed sediment, trace-element concentrations of all river and ground-water samples were at levels less than U.S. Environmental Protection Agency drinking-water standards. The Spokane River appears to be a source of cadmium, copper, zinc, and possibly lead in the near-river ground water. Dissolved cadmium, copper, and lead concentrations generally were less than 1 microgram per liter (µg/L) in the river water and ground water. During water year 2001, dissolved zinc concentrations were similar in water from near-river wells (17-71 µg/L) and the river water (22-66 µg/L), but were less than detection levels in wells farther from the river. Arsenic, found to be elevated in ground water in parts of the aquifer, does not appear to have a river source. Although the river does influence the ground-water chemistry in proximity to the river, it does not appear to adversely affect the ground-water quality to a level of human-health concern.
Idaho;Washington↗
Rise and fall of endrin usage in Washington state fruit orchards: Effects on wildlife
A study of the effects of endrin on wildlife was conducted from 1981 to 1983 in fruit orchards in central Washington State. The single post-harvest application of endrin as a rodenticide resulted in both acute and chronic toxicity to a variety of avian species. Of 194 birds found dead, brains of 125 were analysed; endrin toxicosis accounted for >24% of the total and 37% of those analysed. Most mortality occurred soon after application, but several raptors died during the spring and summer. Most wildlife sampled in the orchard system contained residues of endrin. There was no evidence that endrin depressed reproductive success. Use of endrin abruptly declined during this study and its use is currently limited to emergency situations. Wildlife mortality from endrin also decreased; only six endrin-related mortalities were detected the last year of the study and there have been no reports of die-offs since the study ended.
Washington↗
Post-glacial stratigraphy and late Holocene record of great Cascadia earthquakes in Ozette Lake, Washington, USA
Ozette Lake is an ~100-m-deep coastal lake located along the outer coast of the Olympic Peninsula (Washington, USA); it is situated above the locked portion of the northern Cascadia megathrust but also relatively isolated from active crustal faults and intraslab earthquakes. Here we present a suite of geophysical and geological evidence for earthquake-triggered mass transport deposits (MTDs) and related turbidite deposition in Ozette Lake since ca. 14 ka. Comprehensive high-resolution bathymetry data, seismic reflection profiles, and sediment cores are used to characterize the post-glacial stratigraphic framework and examine paleoseismic evidence in the lacustrine sediments. Stacked sequences of MTDs along the steep eastern flanks of the lake appear to grade basin-ward from thick, chaotic, blocky masses to thin, parallel-bedded turbidite beds. The discrete turbidite event layers are separated by fine-grained (silt and clay) lake sedimentation. The event layers are observed throughout the lake, but the physical characteristics of the deposits vary considerably depending on proximity to primary depocenters, steep slopes, and subaqueous deltas. A total of 30–34 event deposits are observed in the post-glacial record. Radiometric dating was used to reconstruct a detailed sedimentation history over the last ~5.5 k.y., develop an age model, and estimate the recurrence (365–405 yr) for the most recent 12 event layers. Based on sedimentological characteristics, temporal overlap with other regional paleoseismic chronologies, and recurrence estimates, at least 10 of the dated event layers appear to be sourced from slope failures triggered by intense shaking during megathrust ruptures; the recurrence interval for these 10 events is 440–560 yr. Thus, Ozette Lake contains one of the longest and most robust geological records of repeated shaking along the northern Cascadia subduction zone.
Washington↗