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

TUOLUMNE RIVER TECHNICAL ADVISORY COMMITTEEHABITAT RESTORATION PLAN FOR THE LOWER TUOLUMNE RIVER CORRIDORCHAPTER 2Ft.Figure 2-19. 1937 example alternate bar sequence in gravel-bedded reach, near 7/11 Materials plant site at RM 38.The effects of these changes are illustrated bycomparing 1937 photos with 1993 photos. In thegravel-bedded zone, the impact of aggregateextraction is well illustrated: historic floodplainswere mined to depths far below the river thalweg,and the resultant pits are separated from the riverby dikes constructed of either gravel or topsoil(Figure 2-21). In downstream reaches from RM35.0 to 24.0, in-channel aggregate extraction pitswithin the low water channel range up to 38 ftdeep and 400 ft wide. Reduction in the magnitudeof flood-peaks has caused the low water channellocation to remain nearly constant over time. Thepreviously dynamic channel and point bar on thenorth bank in Figure 2-21, and elsewhere, havenot changed since 1937 and have rarely beenmobilized since 1970 as a result of the reducedhigh flow regime. Leading edges of point barsthat were previously free of mature vegetaton, arenow encroached by a thick band of willows,buttonbush, and alders. In upstream reaches wheredredgers operated across the valley, the unnaturaland fragmented channel location has not changedappreciably since 1937 (Figure 2-22).Changes to the sand-bedded reaches are similar,but resulted from agricultural and urban encroachmentrather than aggregate extraction. Conversionof the riparian corridor to agriculture and urbanuses reduced floodway width, and introducedbank protection that functionally halted channelmigration (compare Figures 2-17 and 2-18 toFigures 2-23 and 2-24). Reduced peak flows alsoinhibit channel migration.2.3. ATTRIBUTES OF ALLUVIALRIVER ECOSYSTEM INTEGRITYAlthough the ecological integrity of the TuolumneRiver has suffered, there is considerable opportunityto improve the river corridor by re-establishingcritical fluvial geomorphic processes. Butdefining, let alone restoring, a riverine ecosystemis challenging. We hypothesize that the fundamentalAttributes of river ecosystem integrity aredefined by the physical processes that create andmaintain the ecosystem form or physical structure.Based on our interpretation of historicalconditions on the Tuolumne River (assumed to38