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

LINKING PHYSICAL PROCESSES AND SALMON LIFE HISTORYbut occasionally reaching as long as 4,000 ft.Meanders typically had low amplitude (< 400 ft)and large radius of curvature (>1,500 ft), and thechannel form was a combination of single-threadand split channels (mild braiding) (Figure 2-19).Channel movement appears to have been dominatedby a combination of channel avulsion andchannel migration.2.2.3.2. Historical channel morphology insand-bedded zoneThe sharp transition from gravel-bedded to sandbeddedriver downstream of Geer Road (RM 24)caused a notable shift in channel morphology: inthe sand-bedded zone the channel was almostentirely single thread, and alternate bars approachedthe idealized pattern (Figure 2-18).Bankfull widths estimated from the 1937 airphotos were approximately 600 ft. Meanderwavelengths were much less variable than in thegravel-bedded zone, tending toward 3,000 ft. Theamplitude of meanders was much higher than thegravel-bedded zone, ranging from 600 ft near RM23 to 750 ft near present-day Shiloh Bridge (RM3.5). Channel movement in this zone appearstypical of Midwestern sand-bedded streams (e.g.,Mississippi River), where progressive channelmigration increased meander amplitude andsinuosity until the meander “pinched off” or “cutoff”, abandoning most of the meander and reinitiatingthe migrational pattern. This meandercutoff process created oxbow wetland and riparianhabitats that are typically highly productive areasfor many species. The lower sand-bedded sectionof the Tuolumne River is of great potential valueas a richly diverse and highly productive area tofish and wildlife production.2.2.3.3. Historical sediment supply andtransportSediment comprising the bed and banks of thelower Tuolumne River originated in the SierraNevada, and was transported downstream to theCentral Valley by winter and spring snowmeltfloods. Floods transported a wide range ofparticles, from cobbles and boulders that formedthe structure of gravel bars, to fine sands and siltsthat formed fertile floodplains. As slopedecreased from the Sierra Nevada toward theconfluence with the San Joaquin River, particlesize decreased from cobbles and boulders nearLa Grange (RM 50) to fine sand downstream ofDry Creek confluence (RM 16). Specific rates ofhistoric sediment transport were less importantthan the process itself: the critical process for thealluvial river reaches, including both sand- andgravel-bedded zones, was that sediment scouredand transported downstream from a particularlocation was replaced by sediment originatingfrom similar processes upstream. This functional“conveyor-belt” periodically transported sediment,scoured and rebuilt alluvial deposits, andover time, maintained equilibrium in the quantityand quality of in-channel storage depositsthroughout the river. This process in turn provideda consistent renewal and maintenance ofhigh quality aquatic and terrestrial habitat in thelower river.2.2.3.4. Evolution to present-day conditionsBeginning with the California Gold Rush in 1848and continuing to the present, Anglo-Europeanmanipulation of the channel morphology hascaused large-scale changes to the Tuolumne Rivercorridor (Table 2-6).After more than a century of cumulative impacts,the river has been transformed from a dynamic,alluvial river (capable of forming its own bed andbank morphology) to a river fossilized betweeneither man-made dikes, or agricultural fields, orfossilized within riparian vegetation that hasencroached into the low water channel. Riparianforests have been reduced in areal extent, andnatural regenerative processes have been inhibited.Excavation of stored bed material for goldand aggregate mining eliminated active floodplainsand terraces and left behind large inchanneland off-channel pits. Off-channel pits areseparated from the river by steep-banked dikesand dikes which confine the channel to anunnaturally narrow corridor. The loss of coarsesediment supply that historically providedessential sediment for the formation of alternatebar features and in-channel and floodplain habitatstructure, combined with the dramatic reductionin high flows, has prevented regenerative fluvialprocesses from promoting river recovery. Thesechanges are largely responsible for the currentlydegraded state of the river channel. Not only arethe ingredients for a healthy channel no longeravailable to the river (sediment supply), but theprocesses are handicapped or absent (high flowregime and natural variability within hydrographcomponents).CHAPTER 235