ALLUVIAL SEDIMENTS IN THE VALLEYS OF CHANNEL ISLANDS NATIONAL PARK CHRONICLE FLUVIAL SYSTEM RESPONSES TO CHANGING CLIMATE, SEA-LEVEL, AND HISTORIC LAND USE

Many of the streams on Santa Rosa and Santa Cruz Islands, Channel Islands National Park and The Nature Conservancy, CA, occupy broad valleys that have been filled with alluvium and later incised to form steep-walled arroyos, leaving a relict floodplain as much as 12-14 m above the present channel. These thick alluvial sequences host some of the most significant archaeological and paleontological resources in North America, including Arlington Man, Chumash occupation sites, and fossil remains of Columbian and pygmy mammoths. The morphology and sedimentology of these deposits describe the natural response of the landscape to climate and sea-level changes, and in the historic period, fluvial system responses to livestock stress and extreme weather events. As sea level fell following the last interglacial (after ~80 ka), streams eroded the valleys, largely clearing them of older sediment from previous glacial/interglacial cycles. Following the last glacial period (~25-20 ka), rising sea level caused the streams to progressively lose power, and alluvium began to accumulate on the valley floors, progressing from the mouths of valleys upstream toward the headwaters. Influences of local channel gradient and morphology, short-term climate variations, and intrinsic processes are reflected in the thickness and spacing of the episodic alluvial sequences. Floodplain aggradation slowed as sea level approached its present level. Massive hillslope and channel erosion occurred during the mid- to late 1800s, and stream channels continue to deepen and widen today. Sedimentary and historical evidence points to overgrazing and drought, followed by catastrophic flooding, as factors that may have accelerated and enhanced hillslope erosion and arroyo formation. However, incision would have occurred eventually as a natural response of the river systems to the halting of sea-level rise, even in the absence of the untimely combination of intense livestock stress and unusual weather events in the latter half of the 19^th century.