The Middle Fork Salmon River watershed spans high-elevation mixed-conifer forests to lower-elevation shrub-steppe. In recent decades, runoff from severely burned hillslopes has generated large debris flows in steep tributary drainages. These flows incised alluvial fans along the mainstem river, where charcoal-rich debris-flow and sheetflood deposits preserve a record of latest Pleistocene to Holocene fires and geomorphic response. Through deposit sedimentology and 14C dating of charcoal, we evaluate the processes and timing of fire-related sedimentation and the role of climate and vegetation change. Fire-related deposits compose ~66% of the total measured fan deposit thickness in more densely forested upper basins versus ~33% in shrub-steppe-dominated lower basins. Fires during the middle Holocene (~8000 - 5000 cal yr BP) mostly resulted in sheetflood deposition, similar to modern events in lower basins. Decreased vegetation density during this generally warmer and drier period likely resulted in lower-severity fires and more frequent but smaller fire-related sedimentation events. In contrast, thick fire-related debris-flow deposits of latest Pleistocene-early Holocene (~13,500-8000 cal yr BP) and late Holocene (u3c 4000 cal yr BP) age are inferred to represent higher-severity fires, though data in the former period are limited. Widespread fires occurred in both upper and lower basins within the Medieval Climatic Anomaly (1050-650 cal yr BP) and the early Little Ice Age ca. 550 cal yr BP. We conclude that a generally cooler late Holocene climate and a shift to denser lodgepole pine forests in upper basins by ~2500 cal yr BP provided fuel for severe fires during episodic droughts.
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机译:中叉鲑鱼河流域横跨高海拔的混合针叶林到低海拔的灌木草原。近几十年来,严重烧毁的山坡产生的径流在陡峭的支流排水道中产生了大量的泥石流。这些水流沿着主干河切开了冲积扇,那里富含木炭的泥石流和片状洪水沉积物保留了最新的更新世对全新世大火和地貌反应的记录。通过沉积物沉积学和木炭的14 C定年,我们评估了与火有关的沉积的过程和时机,以及气候和植被变化的作用。与火灾有关的沉积物占森林密布的上部盆地中测得的扇形沉积物总厚度的约66%,而以灌木草原为主的下部盆地中约占33%。中全新世(约8000-5000 cal yr BP)期间的大火主要导致表层洪水沉积,类似于下盆地的现代事件。在通常较暖和较干燥的时期内,植被密度的降低可能导致严重度较低的火灾以及与火灾相关的更频繁但较小的沉积事件。相比之下,据推断,最新的更新世-早全新世(〜13,500-8000 cal BP)和晚期全新世( u3c 4000 yr BP)年龄的与火相关的厚泥石流沉积物代表着较高烈度的火灾,尽管前期是有限的。中世纪气候异常(1050-650 cal yr BP)和小冰河时代早期的上下盆地都发生了大火。 BP 550 yr yr。我们得出的结论是,新世晚期气候总体较冷,到2500年BP左右,上流域向密集的黑松林转变,为偶发干旱期间的严重火灾提供了燃料。
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