Geology topics

Research about Eklutna Lake

Source-linked reports with geographic coverage including Eklutna Lake.

5 recordsLinked to original sources

USGS2020

Turbidite stratigraphy in proglacial lakes: Deciphering trigger mechanisms using a statistical approach

Turbidites embedded in lacustrine sediment sequences are commonly used to reconstruct regional flood or earthquake histories. A critical step for this method to be successful is that turbidites and their trigger mechanisms are determined unambiguously. The latter is particularly challenging for prehistoric proglacial lake records in high-seismicity settings where both earthquake-generated and flood-generated turbidites interrupt the background varved sedimentation. This calls for a new method to allow efficient and objective identification and classification of turbidites. This study examined turbidites in five long (9 to 17 m) sediment cores from Eklutna Lake, a proglacial lake in south-central Alaska, using standard core logging and grain-size data. A novel statistical approach is presented, in which varve-thickness distributions were first analyzed to objectively identify the thickest turbidites and distinguish them from background sedimentation. For each turbidite, a selection of variables were then measured, including: basal grain-size, thickness, magnetic susceptibility and spectrophotometric variables. Triggering mechanisms were discriminated by a combination of principal component analysis and clustering, and by calibration with historical events. Using this approach, a 2250 year long lake-wide event stratigraphy was constructed, with 94 prehistoric events, including 24 earthquake and 70 flood events. Basal grain-size and thickness variables turn out to be the most effective proxies for discrimination. This statistical approach is a powerful and new method to identify turbidites and their triggering mechanisms in long prehistoric sediment records. It opens up new prospects for palaeoseismological, palaeohydrological and palaeoclimate studies in proglacial lakes worldwide.

Alaska↗

USGS2019

The sedimentary record of the 2018 Anchorage Earthquake in Eklutna Lake, Alaska: Calibrating the lacustrine seismograph

The 30 November 2018 M w "> M M w "> w 7.1 Anchorage earthquake caused modified Mercalli intensities of V¼ to V½ at Eklutna Lake (south central Alaska). A few hours after the earthquake, a “dirt streak” was observed on the lake surface, followed by a peak in sediment turbidity values ( ⁠ ∼ 80 "> ∼ 80 times normal) at a drinking water facility, which receives water from the lake through a pipe. These observations hint toward turbidity currents triggered by the earthquake in Eklutna Lake. Here, we study 32 short sediment cores retrieved from across Eklutna Lake and observe a millimeter‐to‐centimeter scale turbidite that can be confidently attributed to the 2018 earthquake in all coring locations. X‐ray computed tomography, grain‐size, and color‐spectral analyses of the turbidite show that it shares physical characteristics with the turbidite generated by the 1964 M w "> M w 9.2 Great Alaska earthquake, while it is considerably different from turbidites caused by historical floods. The 2018 turbidite reaches its largest thickness in the inflow‐proximal basin, but when compared to the 1964 turbidite and thereby canceling out local site effects, it is relatively thick in the inflow‐distal sub‐basin. The latter was exposed to stronger shaking during the 2018 earthquake, and this relative thickness trend may therefore be attributed to shaking intensity and gives an indication of the location of the earthquake epicenter relative to the basin axis. Furthermore, in contrast to the 1964 turbidite, which was sourced from both deltas and hemipelagic slopes, the 2018 turbidite was sourced from deltas only, as evidenced by its distribution. These results confirm that while it is generally accepted that shaking intensities of ≥ VI "> ≥ VI are needed to trigger turbidity currents from hemipelagic slopes, intensities as low as V¼ can be sufficient to trigger turbidity currents from deltaic slopes. Our results show that proglacial lakes can sensitively record differences in shaking intensity and that investigating deposits from recent earthquakes is crucial to calibrate the lacustrine seismograph.

Alaska↗

USGS2019

New approach to assessing age uncertainties – The 2300-year varve chronology from Eklutna Lake, Alaska (USA)

Developing robust chronological frameworks of lacustrine sediment is central to reconstructing past environmental changes. We present varve chronologies from five sites extending back 2300 years from Eklutna Lake, in the Chugach Mountains of south-central Alaska. The chronologies are built from image analysis of high-resolution photographs and CT scans of sediment cores. The age uncertainty of each record is tested by three methods. We first present varve chronologies from individual sites and reconcile the difference in varve delimitation from two observers. The varve chronologies from each site are then compared to each other using a series of marker beds that can be traced across the lake basin. Finally, using a new Bayesian probabilistic model, we develop age models that incorporate information regarding age uncertainty from the multiple-observer method and the age distribution of marker layers from multiple cores. To evaluate the accuracy of the Bayesian model output, we used seven radiocarbon ages from terrestrial macrofossils and four tephra layers traceable across the core sites. The major-element geochemistry of the tephra layers and their ages are presented here for the first time. The Bayesian age model offers a new approach to quantifying age uncertainty in inter-correlated cores of varved sediment.

Alaska↗

USGS1948

Preliminary report on the geology along the route of a proposed tunnel to develop hydroelectric power from Eklutna Lake, Alaska

A preliminary investigation of the geology of the ridge north- west of the lower end of Eklutna Lake was made in the period June 11-18, 1947, by the writer, assisted by L. A. Hale. The purpose of this study was to obtain geological information bearing on the feasibility of constructing a tunnel through this ridge as part of a hydroelectric power development under consideration by the city of Anchorage. Several traverses were made up each side of the ridge in a belt judged to include the most logical routes for the proposed tunnel. For the purpose of localizing the study two tentative routes were selected, of which one would require the shortest possible length of tunnel, and tho other would be somewhat longer but would place the lower portal of the tunnel at a location considered to be the most favorable for the construction of the penstock and power plant. The first route extends northwestward from tho most westerly embayment on the north shore of the lake to the canyon about half a wile east of Pioneer Creek (see map) . The second route extends from tho same point in the lake to the nearest point on the steep slope southwest of the mouth of Goat Creek. The distance along the first route from the lake shore (elevation 862 feet) to the 800-foot contour on the north side of the ridge is 4.3 miles. The distance between corresponding points along the second route is 4.5 miles. The length of the tunnel would exceed these distances by amounts depending on the elevation and grade at which the tunnel is constructed.

Alaska↗

USGS1947

Reconnaissance report on geology of Eklutna Lake dam site and conduit route near Anchorage, Alaska

Summary and Recommendations 1. Eklutna Lake and Eklutna Creek lie in a wide, deep, glaciated, trough-like valley. Downstream from the lake, this valley is partially filled with unconsolidated glacial and alluvial deposits. 2. Eklutna Lake dam site, located about 400 feet below the lake outlet, is suitable for a low dam of flexible, earth-embankment type. Adequate control of the stream can be obtained by raising the lake level about 50 feet to altitude 910 feet, which will provide hold-over storage from wet years to dry years. Such a structure will have a crest length of about 1,950 feet. A. Bedrock is probably 200 to 475 feet below stream bed at the proposed axis. The dam will rest on glacial deposits of till, clay, sand and gravel, and on deposits of lake-shore and alluvial fan gravels. B. Geologic conditions in the area of the right abutment, as yet imperfectly known, may make necessary a long, deep cutoff extending for an unknown distance beyond the north end of the dam. C. A foundation exploration program is recommended that includes deepening test pit No. 1 and drill hole No. 2, and drilling 11 new holes. It is suggested that one drill hold near the center of the valley be taken to bedrock to give a complete picture of the fill materials underlying the foundation. 3. Delivery of water from the forebay of the reservoir to the powerhouse eight miles downvalley by means of a conduit is regarded as infeasible because: difficult terrain of the route will require earthwork more extensive than the volume of the dam; the route is subject to land slides, and will require expensive maintenance; it is more or less completely exposed to adverse winter conditions that may engender icing conditions; and it is easily subject to sabotage. It is recommended that the water be taken to the powerhouse through a rock tunnel.

Alaska↗