Probing – 15 Eco-soundings

Probing the Depths of Crater Lake: A Century of Scientific Research by Douglas Larson


Hans Nelson lowers a Petersen-type dredge into Crater Lake on August 12, 1960. Photo courtesy of the National Park Service

During the summer of 1959, R. E. Williams and other scientists from the U.S. Coast and Geodetic Survey sounded the lake with more than four thousand echo-soundings to determine its maximum depth and to map the topography of the lake bottom. This followed previous attempts to sound the lake, first by William Steel and Clarence Dutton in 1886 and later by park naturalist John Doerr in 1938, 1939, and 1940.  Williams converted the soundings to fathoms, and a bathymetric map based on ten-fathom contour intervals was developed to illustrate the significant geomorphic features across the lake bottom, including a submerged lava-flow extending eastward from Wizard Island and Merriam Cone. The map and the original bathymetric information for Crater Lake (area, average and maximum depths, volume, and shoreline length) were published in 1965 by John Byrne, a professor in the Department of Oceanography at Oregon State University and later the president of that university. Byrne’s map later served several purposes, such as locating sampling stations, estimating hydrologic and chemical balances for the lake, and computing the lake’s seasonal heat budgets, which represent the quantity of heat assimilated by the lake to warm its waters from the lowest winter temperature to the highest summer temperature.

Guided by the Survey’s bathymetric map, ranger-naturalist Carlton Hans Nelson collected sediment samples from 730 locations on Crater Lake during the summer of 1960 as part of his thesis research at the University of Minnesota. Using Ekman- and Petersen-type dredges, Nelson hauled up sediment samples from as deep as eighteen hundred feet. He obtained deeper samples with a Phleger coring device, basically a heavily weighted pipe or tube several inches in diameter that was allowed to free-fall vertically to the bottom at the rate of about six hundred feet per minute. The coring device penetrated lake-bottom sediments to a depth of one to two feet. After retrieving the tube and extracting its sediment core, Nelson could analyze the core for mineral composition, sediment texture (clay, silt, and sand), organic content, acidity and alkalinity, and the presence of biological materials such as pollen and plankton remains. The core was also examined stratigraphically to determine the lake’s sedimentary and environmental history Nelson reported that accumulations of diatomaceous and moss ooze covered portions of the lake’s deep bottom as well as the summits of submerged volcanic cones. The diatomaceous materials represented untold billions of planktonic diatoms that had slowly settled to the bottom of the lake over hundreds if not thousands of years. Nelson also discovered that landslides from the caldera’s walls frequently entered the lake, producing powerful turbidity currents that carried sand across the caldera floor for miles to the lake center. He speculated that clay, deposited in the lake and widely dispersed, helped to seal the caldera floor, thus allowing the lake to increase in depth. An abundance of tree pollen and pumice dust was found in lake sediments, delivered there by water running off the caldera walls and strong winds sweeping up the flanks of Mount Mazama and into the caldera.

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