The Lingering Loess Islands

Description

Many people may not know that the quaint city of Cheney is situated on the side of a loess island. In fact, where the water tower stands brings into perspective the overwhelming size of the floods and can be used as a reference point to try to comprehend just how large the Missoula floods actually were. At its peak, the flood is said to have reached approximately 762 meters (2500 ft.) compared to the height of the top of the hill at a close 774 meters or 2,539 ft. (Patton & Baker, 1978). If the floods had gotten any higher than this, it is possible that the turbulence could have removed the loess hill entirely. Figure 1 shows the relationship between the competent bottom velocity and the mean flow velocity, the larger the velocity the larger the amounts of sediment it can carry, potentially causing macro-turbulent boulder movement. This equation puts numbers behind the idea and represents the scale to which the flood was comparable (Baker, 1973).
Even more intriguing than the water tower is what it is actually founded upon, a loess hill. The word loess itself comes from the German word for “loose” which is a great description of the loosely packed, fine-grained properties of loess, often deposited by wind.
We can identify the specific type of loess island that we are looking at by examining how they have been carved out by the floods. There are 3 different types of loess islands in the Cheney-Palouse Scabland tract, subfluvial, partially submerged, and unsubmerged. By looking at Cheney from an aerial view and comparing it to the loess island types (from Patton and Baker 1978), the area identifies best with the sketch of the unsubmerged loess island, though a few channels may have been flooded, such as along Highway 904 between I-90 and Cheney.

Images Show

Left is an aerial image showing a large loess island with Cheney located on the southeast side of the streamlined hill. To the right is a slope map, the darker the color the steeper the slope. The slope map illustrates the changes made to the landscape by the Missoula Floods. | Creator: Flood Explorer Team [View Additional File Details]

Figure (15) from Baker (1973) showing the relationship between competent bottom velocity and mean flow velocity. Y-axis is not to scale. [View Additional File Details]

Three types of loess islands

Three types of loess islands

This figure illustrates three types of loess islands that were carved by the Missoula Floods, each island type if formed by different water depths, the deeper the water over the loess, the higher the veolocity of the water and the more loess was removed. | Creator: Judy McMillian modified from Patton and Baker (1978) [View Additional File Details]

Subjects

Cite this Page

Tessa Kainu, “The Lingering Loess Islands,” Ice Age Floods Explorer, accessed June 27, 2017, http://floodexplorer.org/items/show/46.
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