Geology topics

Lisa Ely

Publications and source records attributed to Lisa Ely.

4 recordsLinked to original sources

USGS2025

Decadal-scale effects of a dam removal on channel geomorphology, sediment and large wood on the Elwha River, Washington, USA

The removal of Glines Canyon Dam on the Elwha River in western Washington, USA, from 2011 to 2014 introduced a 20-Mt pulse of stored sediment and logs into the downstream channel. We used terrestrial laser scanning, high-resolution orthoimages, and surveys of large wood (LW) and sediment grain-size distribution to quantify changes to the channel and LW in four different geomorphic settings spanning a 16-year period before, during, and after dam removal. The results provide insights into the role of site-specific geomorphology on the interplay among sediment size and supply, wood, and channel form in the aftermath of a dam removal. Sediment-size distribution, braiding index, and number of log jams rapidly reached new steady states. Other factors, such as channel sinuosity and log jam area, were still evolving six years after the dam was removed. The rate and type of river response was partially dictated by the geomorphic setting and the accumulation of LW. Complex reaches trapped more sediment and LW, initiating immediate changes in the channel count, position and lateral migration that continued to evolve through positive feedbacks. Single-channel sites experienced less initial erosion and deposition, but channel migration continued for years once it was underway. The post-dam sediment composition progressively shifted to a mixed size distribution midway between the armored cobbles when the dam was in place and the influx of primarily sand and fine gravel during dam removal. Reworking of sediment was most rapid in the first year after dam removal, especially at the site with the greatest channel complexity. The relation between log jams and channel divisions fundamentally changed. There were 11 log jams in the middle reach of the Elwha River downstream of Glines Canyon Dam, and all log jams associated with channel divisions occurred at the heads of stable, vegetated islands. During dam removal, the number of log jams rapidly increased to 86 and stabilized near that level in the post-dam period. While log jams on stable islands persisted, more were added at divisions around transient sediment bars, scattered across gravel bars, or outside of the active channel. Following a brief spike during dam removal, there was a net long-term increase of ∼10 % in the number of channel divisions associated with log jams. The sediment deposits, LW, vegetation, channel morphology and river discharge continued to cause adjustments within the fluvial system a decade after the start of dam removal. This state of greater variability could be the new equilibrium for years to come.

Washington↗

USGS2020

Holocene relative sea-level change along the tectonically active Chilean coast

We present a comprehensive relative sea-level (RSL) database for north, central, and south-central Chile (18.5°S – 43.6°S) using a consistent, systematic, and internationally comparable approach. Despite its latitudinal extent, this coastline has received little rigorous or systematic attention and details of its RSL history remain largely unexplored. To address this knowledge gap, we re-evaluate the geological context and age of previously published sea-level indicators, providing 78 index points and 84 marine or terrestrial limiting points spanning from 11 ka to the present day. Many data points were originally collected for research in other fields and have not previously been examined for the information they provide on sea-level change. Additionally, we describe new sea-level data from four sites located between the Gulf of Arauco and Valdivia. By compiling RSL histories for 11 different regions, we summarise current knowledge of Chilean RSL. These histories indicate mid Holocene sea levels above present in all regions, but at highly contrasting elevations from ∼30 m to <5 m. We compare the spatiotemporal distribution of sea-level data points with a suite of glacial isostatic adjustment models and place first-order constraints on the influence of tectonic processes over 10 3 –10 4 year timescales. While seven regions indicate uplift rates <1 m ka −1 , the remaining regions may experience substantially higher rates. In addition to enabling discussion of the factors driving sea-level change, our compilation provides a resource to assist attempts to understand the distribution of archaeological, palaeoclimatic, and palaeoseismic evidence in the coastal zone and highlights directions for future sea-level research in Chile.

Quaternary Science Reviews↗

USGS2015

Vertical deformation through a complete seismic cycle at Isla Santa María, Chile

Individual great earthquakes are posited to release the elastic strain energy that has accumulated over centuries by the gradual movement of tectonic plates 1 , 2 . However, knowledge of plate deformation during a complete seismic cycle—two successive great earthquakes and the intervening interseismic period—remains incomplete 3 . A complete seismic cycle began in south-central Chile in 1835 with an earthquake of about magnitude 8.5 (refs 4 , 5 ) and ended in 2010 with a magnitude 8.8 earthquake 6 . During the first earthquake, an uplift of Isla Santa María by 2.4 to 3 m was documented 4 , 5 . In the second earthquake, the island was uplifted 7 by 1.8 m. Here we use nautical surveys made in 1804, after the earthquake in 1835 and in 1886, together with modern echo sounder surveys and GPS measurements made immediately before and after the 2010 earthquake, to quantify vertical deformation through the complete seismic cycle. We find that in the period between the two earthquakes, Isla Santa María subsided by about 1.4 m. We simulate the patterns of vertical deformation with a finite-element model and find that they agree broadly with predictions from elastic rebound theory 2 . However, comparison with geomorphic and geologic records of millennial coastline emergence 8 , 9 reveal that 10–20% of the vertical uplift could be permanent.

Isla Santa María↗

USGS2015

Coastal evidence for Holocene subduction-zone earthquakes and tsunamis in central Chile

The &sim;500-year historical record of seismicity along the central Chile coast (30–34°S) is characterized by a series of &sim;M 8.0–8.5 earthquakes followed by low tsunamis (<4 m) occurring on the megathrust about every 80 years. One exception is the AD 1730 great earthquake (M 9.0–9.5) and high tsunami (>10 m), but the frequency of such large events is unknown. We extend the seismic history of central Chile through a study of a lowland stratigraphic sequence along the metropolitan coast north of Valparaíso (33°S). At this site, higher relative sea level during the mid Holocene created a tidal marsh and the accommodation space necessary for sediment that preserves earthquake and tsunami evidence. Within this 2600-yr-long sequence, we traced six laterally continuous sand beds probably deposited by high tsunamis. Plant remains that underlie the sand beds were radiocarbon dated to 6200, 5600, 5000, 4400, 3800, and 3700 cal yr BP. Sediment properties and diatom assemblages of the sand beds—for example, anomalous marine planktonic diatoms and upward fining of silt-sized diatom valves—point to a marine sediment source and high-energy deposition. Grain-size analysis shows a strong similarity between inferred tsunami deposits and modern coastal sediment. Upward fining sequences characteristic of suspension deposition are present in five of the six sand beds. Despite the lack of significant lithologic changes between the sedimentary units under- and overlying tsunami deposits, we infer that the increase in freshwater siliceous microfossils in overlying units records coseismic uplift concurrent with the deposition of five of the sand beds. During our mid-Holocene window of evidence preservation, the mean recurrence interval of earthquakes and tsunamis is &sim;500 years. Our findings imply that the frequency of historical earthquakes in central Chile is not representative of the greatest earthquakes and tsunamis that the central Chilean subduction zone has produced.

Quaternary Science Reviews↗