HOLOCENE ARROYO CYCLES AND CONTRIBUTING ENVIRONMENTAL FACTORS, SOUTH-CENTRAL WASHINGTON

Ephemeral streams have cut both modern and paleo-arroyos into alluvial and eolian-derived sediments in multiple watersheds within the Yakima Training Center military reservation, south-central Washington. 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. This incision exposed evidence of at least one previous episode of arroyo incision and refilling in the middle reach of Selah Creek. Additional factors commonly accepted as associated with arroyo incision, such as climate fluctuations, base-level change, and anthropogenic land use, may also have contributed to both the recent and pre-historical episodes.

Samples were collected for tephra, Optically Stimulated Luminescence (OSL), and ¹⁴C chronologies. Charcoal collected at the Fan Site where a paleo-arroyo is estimated to be 4-5 m deep and 20+ m wide yielded the most valuable results. A sample collected near the base of the paleo-arroyo fill 5 cm above a distinct contact with the incised unit yielded an age of 1435±95 cal yr BP, providing a minimum constraint on the age of the arroyo incision. Additional ¹⁴C dates from the Fan and Bank Swallow Sites indicate that filling of the paleo-arroyo ceased between 570±80 cal yr BP and 191±90 cal yr BP, before the deposition of a distinct 0.5-1 m thick, finely-bedded silt unit that caps the fill. This unit may have aggraded as Selah Creek spread onto the floodplain forming a wide, shallow channel as documented in 1908 photographs taken prior to the dam failures.

Changes in sediment supply, paleoclimatic conditions, flooding characteristics, or a combination may have contributed to paleo-arroyo cycle(s) within the Selah Creek basin. Further investigation will examine the timing of paleo-arroyo incision on similar stream channels within the local region to determine whether there is a regional coherence in the cut and fill cycles. This could also lead to initial results indicating the possibility of a negative temporal correlation with similar alluvial cycles in the southwestern U.S., which currently experiences opposite interannual climatic variations to those in the Pacific Northwest.