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Ephrata Erratics Fan at Rocky Ford Creek

View on map:47.323291°N 119.435892°W

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Ephrata fan erratics
47.327796°N 119.473954°W

The Ephrata Erratics Fan is a depositional area south of where water from the Missoula flood poured out of the lower Grand Coulee.  It is called a fan because the deposit is like a fan or delta.  The Rocky Ford Creek has cut through the fan here. Stop 2 of Baker et al.

"The Ephrata fan (Fig. 20) is an immense accumulation of gravel and sand that resulted when megaflood waters from Crab Creek, Dry Coulee, the lower Grand Coulee (ending at Soap Lake), and smaller scabland channels entered the Quincy Basin. The deposit probably formed more in the manner of an immense expansion bar (Baker, 1973a), rather than a fluvial fan in which relatively small alluvial channels shift across the fan surface without ever inundating the entire surface at once. Local areas of surface scour occurred on the fan, the most prominent of which is Rocky Ford Creek. The spring-fed headwaters of Rocky Ford Creek can be seen at this stop. The scour probably developed during waning flood stages, when draining of the inundated Quincy Basin caused relatively steep water-surface gradients to occur over the depositional surfaces that had been constructional during the high stages of megaflooding (Baker, 1973a). The scour processes produced the lag concentration of boulders on the fan surface, many of which can be seen from this viewpoint. An alternative explanation for the morphology of the Ephrata fan is that it was progressively incised by a sequence of multiple floods of successively decreasing magnitudes (Waitt, 1994; Waitt et al., 2009). It may also be that a more complex combination of these mechanisms occurred." 

Description

The Missoula Floods (also known as the Spokane Floods or the Bretz Floods) refer to the cataclysmic floods that swept periodically across eastern Washington and down the Columbia River Gorge at the end of the last ice age. The glacial flood events have been researched since the 1920s. These glacial lake outburst floods were the result of periodic sudden ruptures of the ice dam on the Clark Fork River that created Glacial Lake Missoula. After each ice dam rupture, the waters of the lake would rush down the Clark Fork and the Columbia River, flooding much of eastern Washington and the Willamette Valley in western Oregon. After the rupture, the ice would reform, creating Glacial Lake Missoula again.

References

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