MAINTENANCE OF A SINGLE-THREADED CHANNEL WITHOUT PERSISTENT FLOODPLAIN VEGETATION IN A GRAVEL-BEDDED RIVER (Invited Presentation)

A number of studies have reported the need for persistent floodplain vegetation to maintain the single-threaded form of alluvial rivers. Without floodplain vegetation, and in the absence of cohesive riverbank materials, overbank flows can unravel the floodplain surface to create multi-threaded (braided or anastomosing) forms. Observations from the Lower Yuba River in the Central Valley of California suggest that alternative controls must be at play. The single-threaded, coarse bedded (median particle size of 97 mm) river has been heavily modified by years of gravel mining, resulting in recent efforts to restore what remains important habitat. Because of large dams, riverbed sediment supplies are limited to the recycling of materials within the river valley during large flow releases. Further, historical aerial photographs indicate that vegetation has only sparsely populated the riverbanks and floodplain. Still, colonization is evident during low-flow periods, as is a systematic variation in vegetation size, with the largest plants occupying the most distal portions of floodplain. The river is also absent of any artificial measures to prevent the mobilization of riverbank and floodplain materials. To assess how the Lower Yuba River has maintained its single-threaded form given the seemingly difficult conditions, we used three topo-bathymetric maps (1999, 2008, and 2014) to assess geomorphological changes that resulted from flow releases in the context of varying forms of floodplain vegetation. River discharges from 1999-2008 produced five floods (> 141.6 cms) and only one major flood (peak of 22 times bankfull discharge). In contrast, river discharges from 2008-2014 produced seven floods, of which five were at least double the bankfull discharge. This study quantified the lateral exchanges of sediment that took place between the channel margins and the floodplain during these quiescent (1999-2008) and disturbance (2008-2014) periods. We evaluated mechanisms by which these exchanges are responsible for maintaining the single-threaded form and for redistributing floodplain sediment during large floods. Although vegetation influenced patterns of aggradation during quiescent periods, large floods overwhelmed these effects in the lateral redistribution of sediment.