Tuolumne River Report - U.S. Fish and Wildlife Service

TUOLUMNE RIVER TECHNICAL ADVISORY COMMITTEEHABITAT RESTORATION PLAN FOR THE LOWER TUOLUMNE RIVER CORRIDORCHAPTER 2In gravel-bedded reaches, channel avulsion andfloodplain disturbance typically occurred duringlarge floods (>5 to 10-year recurrences on theannual maximum series). Channel relocationappears to have been rapid, perhaps occurring inmany locations within a single flood event.Additionally, larger floods scoured and redepositedsediment around trees, logs, and shrubson floodplains and low terraces, which createdsloping, undulating, floodplain surfaces. Thisfloodplain morphology led to considerablevariation in soil and drainage conditions, andprovided diverse plant species (each with differentphysiological tolerances) a broad range ofmicrohabitats in which to survive (Strahan 1984).However, these larger floods also caused mortalityto the riparian community, primarily in thesteeper, more confined gravel-bedded reach.Typically, the closer to the low water channel ariparian plant initiated, the higher the risk ofscour mortality. Conversely, the farther from thelow water channel, the higher the risk of desiccationmortality. Channelbed scouring floodshistorically prevented riparian hardwood encroachmentinto the active channel and kept point barsrelatively free of riparian vegetation (Figures 2-17to 2-19). Seedlings initiating close to the summerlow water edge have shallow roots and areparticularly susceptible to channelbed surfacescour, and moderate winter flood events typicallykill over 90% of the plants that survive the firstgrowing season (McBain and Trush 1997).Floods caused scour mortality not only on younggenerations of trees, but also on mature andsenescent trees. As floods progressively inundatedfloodplain surfaces with greater depths,woody debris in transport would accumulate onmature and senescent stands with force greatenough to topple trees. In some cases, a group oftrees may have been removed in a domino effect,leaving bare holes in a stand where new initiationcould occur. Historically, the large volume ofwoody debris derived from the upper watershedprior to dam blockage would have had significanteffects on riparian stand structure.Figure 2-28. Conceptual historical cross-section near Shiloh Bridge (RM 3.5). Riparian vegetation occurs acrossentire cross section, and is concentrated in oxbows (historic channel locations) and scour channels. Note tree sizenot to scale.50