USING TREE-RING ANATOMY TO DATE FLOODPLAIN DEPOSITION

Determination of sediment deposition rates from recent stratigraphy is typically limited by a scarcity of chronological information. We present a method for precise dating of sedimentary beds based on the change in anatomy of tree rings upon burial. When stems of tamarisk (Tamarix ramosissima) and sandbar willow (Salix exigua) are buried, subsequent annual rings in the buried portions become narrower, xylem vessel diameter increases and ray width increases. Observation of these changes can be combined with tree-ring counts to determine the year of deposition of sedimentary beds that are at least 20 cm thick. Using a backhoe we dug trenches across the flood plain at three locations along the arroyo of the Rio Puerco, New Mexico, USA. At each cross section we prepared a detailed stratigraphic description and excavated several tamarisks to depths as great as 5 meters. From each excavated tree we cut and sanded 15-50 slabs for tree-ring analysis. We cross-dated slabs within and between plants and used the burial signature in the tree rings to date all sedimentary beds in the vertical sequence near each plant. We tied vertical sequences into cross sections using trench stratigraphy, and extended results between cross sections using topographic surveys, GIS analysis of historic aerial photos, and repeat surveys of historic cross sections. Using this method we quantified sediment transport by 9 floods between 1936 and 1999. Along the Rio Puerco, arroyo cutting, widening, and filling have all migrated upstream over time. Introduction of tamarisk in 1926 occurred just prior to the beginning of channel narrowing and arroyo filling. Thus the tamarisks record a process of channel change to which they may have contributed.