USING NEW HIGH-RESOLUTION DENDROGEOMORPHIC TOOLS TO RECONSTRUCT A FINE-SCALED HISTORY OF 20TH CENTURY FLOODPLAIN DEPOSITION AND CHANNEL NARROWING

During the 20^th century, river systems of semi-arid western North America experienced channel narrowing through floodplain expansion. This pattern of narrowing was coincident with climatic shifts, water and land development, and the introduction and spread of the invasive riparian shrub tamarisk (Tamarix spp.). Retrospective measurements of channel change have typically been restricted temporally by the multi-decadal gaps in aerial or ground-based photos or spatially by the locations of long-term gaging stations. Attempts to identify cause-and-effect relationships have been confounded by the close temporal proximity of natural and anthropogenic stressors at least partially because past geochronologic tools have not provided the temporal resolution necessary to obtain details of the rates and processes of floodplain building. Recently-developed dendrogeomorphic tools allow for fine-scale aging of floodplain sediments using characteristic shifts in the wood anatomy and annual growth rings of buried portions of tamarisk stems. When combined with detailed floodplain stratigraphy, these tools can produce a powerful retrospective picture of floodplain building at individual locations. We excavated several trenches in the floodplains of two rivers which have undergone channel narrowing, the upper Green River in Dinosaur National Monument and the Rio Grande River in Big Bend National Park. We applied these new, high-resolution dendrogeomorphic tools to obtain years of deposition on individual sedimentary units between the establishment elevation and the ground surface of buried tamarisk shrubs. These ages were combined and cross-referenced with additional bodies of evidence, and an annual to decadal-scale depositional history was reconstructed for each excavation site. Our results show that natural and human-induced hydrologic changes were the primary drivers of channel narrowing, and tamarisk establishment typically followed floodplain depositional events. Furthermore changes in the timing and magnitude of annual floods from upstream water development facilitated the establishment and proliferation of tamarisk in these river systems