At both Cienega Amarilla and Cottonwood Canyon, stratum VI consists of a 0.2–1.3-m-thick surface layer of bedded, fluvial sand locally with a very weak soil profile (Figures 3 and 4; Table 3). No radiocarbon dates or plant macrofossils are associated with stratum VI, but radiocarbon dating of underlying stratum V alluvium indicates that stratum VI post-dates ~300 cal BP. Stratum VI alluvium appears to represent arroyo fan deposits associated with historic period arroyo cutting. Accounts from ranchers indicate that arroyo-cutting in this area occurred in the late 1800s or early 1900s, consistent with widespread arroyo formation across the southwest U.S. at this time (Waters and Haynes, 2001).
Geomorphic History and Paleoclimatic Context
During the late Holocene, the Cienega Amarilla and nearby Cottonwood Canyon floodplains underwent dramatic geomorphic changes, experiencing periods of rapid high-energy deposition, slow low-energy deposition, stability and soil formation, and deep channel entrenchment and subsequent infilling. Many of these shifts likely represent geomorphic responses of hillslope and valley subsystems to climate change and variability. Cienega Amarilla became a groundwater discharge dominated system from ~2.3–1.6 ka, and then transitioned back to a runoff-dominated system by ~1 ka. In this section, I examine possible linkages between these hydrological and geomorphic changes and paleoclimatic variation.
Even though summer precipitation accounts