Channel-dynamic Control on the Establishment of Riparian Trees after Large Floods in Northwestern California1

Large floods in northwestern California in the past two decades have mobilized extensive areas of valley floors, removed streamside trees, and widened channels. Channel cross sections were surveyed to illustrate an hypothesis on the linkage between sediment transport, colonization of channel margins by trees, and streambank recovery. Riparian trees, e.g., white alder (Alnus rhombifolia) , colonize the water's edge at low flow to receive adequate moisture during the dry season. Such stands can endure annual high flows only after the flood-enhanced sediment load declines and the width of the annually mobile bed contracts to the low-flow width. Streambank formation along the low-flow margin can then proceed by deposition of fine sediment and organic debris. A series of large floods from 1950 to 1975 (Harden and others 1978) greatly altered riparian ecosystems in north coastal California from the Eel River basin northward. Channel aggradation as much as several meters, channel widening as much as 100 percent, and extensive destruction of trees by flood waters widened the zone of active bed sediments at the expense of riparian corridors along many streams (Hickey 1969; Kelsey 1980; Lisle 1981). Although most aggraded channels have degraded to stable bed elevations as excess sediment has been transported downstream, many remain widened (Lisle 1981). If probability prevails and such large floods do not recur, how will riparian stands and associated streambanks reover over the next few decades? c This paper presents a hypothesis on a relation between colonization of streamside trees and channel dynamics that may govern the recovery of riparian stands and reconstruction of streambanks. During large floods, extensive areas of streambeds and floodplains can be mobilized by high shear stresses and new inputs of sediment. Riparian species of willow and alder that have low tolerance to moisture stress tend to colonize the water's edge during summer low-flow periods. Because these trees also require a stable substrate, they can establish only after the zone of annually mobilized bed material contracts to a small fraction of the width mobilized by the last large flood. Once established, the trees can trap fine sediment and organic debris, add root strength to bed material, reduce local shear stress, and thereby induce formation of new streambanks. Widened channels thus may recover when new streambanks form inside flood-eroded banks at a spacing dictated by the zone of annually mobilized bed sediment. Trees Along Mobile Bed Margins Channel cross sections showing substrate and vegetation of three creeks affected by recent floods (1964, 1972, and 1975) in north coastal California illustrate colonization and growth of riparian trees along mobile bed margins. All examples were surveyed across reaches where bed and banks were composed of alluvium. The first example (Prairie Creek) presents the hypothesis in detail and illustrates bank formation along a channel transporting abundant fine sediment. The second (Hurdygurdy Creek) shows contrasts with a channel transporting little fines. The third (Willow Creek) details plant species occupying micro-habitats within the channel.