HOLOCENE ARROYO CUT AND FILL CYCLES, SOUTH CENTRAL WASHINGTON

Ephemeral streams have cut deeply incised arroyos into alluvial sediments in multiple watersheds within the Yakima Training Center (YTC) military reservation, south central Washington. Initiation of the most recent channel incision episode along Selah Creek is primarily attributed to multiple failures of an irrigation reservoir dam during the winters of 1909 and 1910. Floods from these dam breaks resulted in significant channel incision that varies in depth and width along an ~10 km reach beginning 3 km downstream of the dam. The creek cut through massive silty and sandy alluvial and eolian sediments, forming the modern entrenched channel. The abundant silt within the valley fill is probably reworked from the glacial loess that mantles the adjacent ridges.

The modern arroyo exposed evidence of at least one previous episode of arroyo incision to a depth of ~4 m between 3140±40 cal yr BP and 1000±40 cal yr BP. This paleo-arroyo cut through a tephra layer from Mt. Mazama (~7700 cal yr BP) at multiple locations. Filling of this paleo-arroyo with fine to coarse cross-bedded sand ceased between 520±60 cal yr BP and 110±40 cal yr BP. A distinct 0.5-1 m thick, finely-bedded silt unit caps the fill, which may have been deposited as Selah Creek spread onto the floodplain forming the shallow wide channel depicted in photographs taken prior to the dam ruptures.

Changes in sediment supply, paleoclimatic conditions, flooding characteristics, or a combination may have contributed to paleo-arroyo cycle(s) within the Selah Creek basin. Additional dating will allow further investigation into the possibilities of synchronous arroyo incision on multiple channels within the local region and initial indications of negative correlations with similar alluvial cycles in the southwestern United States.