Climate-change versus landslide origin of fill terraces in a rapidly eroding bedrock landscape: San Gabriel River, California

Fill terraces along rivers represent the legacy of aggradation periods that are most commonly attributed to climate change. In the North Fork of the San Gabriel River, an arid bedrock landscape in the San Gabriel Mountains, CA, a series of prominent fill terraces were previously related to climate-change-induced pulses of hillslope sediment supply that temporarily and repeatedly overwhelmed river transport capacity during the Quaternary. Based on field observations, digital topographic analysis, and dating of Quaternary deposits, we suggest instead that valley aggradation was spatially confined to the North Fork San Gabriel Canyon and a consequence of the sudden supply of unconsolidated material to upstream reaches by one of the largest known landslides in the San Gabriel Mountains. New 10 24 Be-derived surface exposure ages from the landslide deposits, previously assumed to be early to middle Pleistocene in age, indicate at least 27.11.16 Scherler et al. - Manuscript 2/55 three Holocene events at ~8-9 ka, ~4-5 ka, and ~0.5-1 ka. The oldest and presumably most extensive landslide predates the valley aggradation period, which is constrained by existing 14 28 C ages and new luminescence ages to ~7-8 ka. The spatial distribution, morphology, and sedimentology of the river terraces are consistent with deposition from far-travelling debris flows that originated within and mined the landslide deposits. Valley aggradation in the North Fork San Gabriel Canyon therefore resulted from locally enhanced sediment supply that temporarily overwhelmed river transport capacity, but the lack of similar deposits in other parts of the San Gabriel Mountains argues against a regional climatic signal. Our study highlights the potential for valley aggradation by debris flows in arid bedrock landscapes, provided sufficient supply of loose material, downstream of landslides that occupy headwater areas.