LATE QUATERNARY VALLEY FILL AND SUBSEQUENT ARROYO 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. While common factors such as climate fluctuations, base-level change, and anthropogenic land degradation may have contributed to the initiation of the most recent channel incision episode, incision of sediments 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 varying in depth and width along a ~10 km reach beginning 3 km downstream of the dam. This incision exposed evidence of at least one previous episode of arroyo incision and refilling, which had cut a channel through ~6 m of massive silty and sandy alluvial sediments, before filling with fine to coarse bedded sands. The silt in the incised units was possibly reworked from glacial loess that mantled the ridges above the valley floor.

Initial ages from 14C, optically stimulated luminescence (OSL), and tephra chronologies from two sites bracket the most recent paleo-arroyo incision between 3140±40 BP and 1000±40 BP. The actual timing of incision could have occurred in the middle of this range, as ~3 m of valley fill had accumulated after 3140±40 BP and at least 2 m of sediment had already filled the paleo-arroyo by 1000±40 BP. Filling of the paleo-arroyo ceased between 520±60 BP and 110±40 BP where a distinct 0.5-1 m thick, finely-bedded silt unit caps the fill. This unit could be associated with discharge from the first dam break before incision initiated.

Changes in sediment supply, paleoclimatic conditions, flooding characteristics, or a combination may have contributed to paleo-arroyo cycle(s) within the Selah Creek watershed. Additional dating will allow further investigation of the possibilities of synchronous timing of paleo-arroyo incision on multiple channels within the local region, as well as initial indications of negative correlations with similar alluvial cycles in the southwestern US, which often experiences opposite climatic trends to those in the Northwest.