MEASURING LATE QUATERNARY GEOECOLOGICAL RELATIONSHIPS IN THE LOWER SALMON RIVER CANYON, IDAHO

Studies of late Quaternary-age deposits in the lower Salmon River canyon of western Idaho reveal how complex interactions between geomorphic events, subsequent geomorphic processes, and associated biological populations give rise to different geoecological states. Addressing this problem provides a means of elucidating the ecological context of prehistoric hunter-gatherer use of canyons in the Columbia River plateau. Abiotic and biotic components of the canyon's paleoenvironmental context were measured by assembling data from a battery of proxy indicators, including alluvial and aeolian sedimentation sequences, opal phytoliths, and stable carbon and oxygen isotope records derived from sedimentary organic matter, pedogenic carbonates, and freshwater mussel shell carbonate, all organized into a stratigraphic framework. Correlated and compared across diachronic and synchronic time scales, these proxy indicators form the basis for making interpretations of geoecological interactions during the last 12,000 radiocarbon years. Emplacement of landslide debris into the bottom of the canyon occurred during the middle Pleistocene ca. 450-350 ka. This landslide event provided a historical contingency causing the Salmon River to aggrade fine sediments in a low-energy floodplain from ca. 12,000 to 2,000 BP. Although isotope records indicate synchroneity between levels of C4 vegetation on canyon slopes and increasing aridity during the late Pleistocene to middle Holocene, isotope records from contemporaneous alluvial facies are dominated by C3 plants. At 2,000 BP, normal displacement along a local fault apparently caused the Salmon River to abruptly incise through the canyon's alluvial fill. This adjustment established the fluvial characteristics of the modern lower Salmon River complete with its lower productivity riparian ecosystem. The results of this study illustrate the importance of considering the manner and timing of geoecological interactions in studying the environmental histories of alluvial canyon systems. The implications of this study are far reaching and provide contextual perspectives on the late appearance of semi-sedentary hunter-gatherer settlements in the Salmon River canyon and may help explain the timing of this lifeway elsewhere in the interior Pacific Northwest.