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

TUOLUMNE RIVER TECHNICAL ADVISORY COMMITTEEHABITAT RESTORATION PLAN FOR THE LOWER TUOLUMNE RIVER CORRIDORCHAPTER 3from being delivered directly to the TuolumneRiver. Tributaries that deliver fine sediment directlyto the Tuolumne River are few (Table 3-5).To estimate particle size delivered by tributariesand gullies, bulk samples were collected ofdeposited material at a road cut gully at RM 35.3and at the Gasburg Creek delta at RM 50.3 (Table3-5). Sandy loam soils along the Tuolumne Rivercorridor, typical of the road cut gully sample, had100 percent of particles finer than 2 mm but only18 percent finer than 0.125 mm. Therefore, soilerosion from hillslopes is predominately sandsizedrather than silt-sized, indicating thathillslope and orchard erosion on uplands candeliver a large proportion of sand to the river. TheGasburg Creek delta sample was coarser, but alsohad a low percentage of the sample finer than0.125 mm. The D 50(median grain size) was 14mm, 35% of the sample was finer than 4 mm, andonly 1% of the sample was finer than 0.25 mm.The 1997 flood accessed the New Don PedroDam spillway for the first time in its history.Approximately 45,000 cfs flowed down TwinGulch, causing 25 to 50 feet of incision down tothe underlying bedrock, generating a tremendousvolume of coarse and fine sediment. Most coarsesediment was deposited in La Grange Reservoir;however, much of the sand was transportedthrough La Grange Reservoir and deposited in theTuolumne River channel downstream. Threetransects were surveyed through Twin Gulch toestimate the volume of topsoil washed downstream,mapping soil/bedrock contact lines andextrapolating this contact across the gulch. Weestimate over 200,000 yd 3 of topsoil alone wasexcavated and delivered to La Grange Reservoir,with much of these finer sediments transportedover the dam into the lower Tuolumne River. Thisfine sediment was primarily sand, which depositedonto floodplain surfaces downstream. Sanddeposition on the "floodplains" between LaGrange Dam and Basso Bridge (RM 47.5) was upto three feet deep (estimated average depth wasapproximately 0.5 feet).The Lower Dominici Creek delta and GasburgCreek delta were measured to estimate theportion of fine sediment delivered to theTuolumne River during the January 1997 floods.The existing tributary deltas were assumed to havedeposited on the receding limb of the flood.Recent deposition, approximating recedinghydrograph limb deposition, was approximately100 yd 3 for both Lower Dominici Creek andGasburg Creek. The Gasburg Creek tributary deltawas difficult to discern because the tributarysediment deposits were intermixed with sanddeposits from prior depositional events. Therefore,these values represented a minimum volumeof fine sediment delivery; the actual value isprobably much larger. For a rough yardstick, 100yd 3 is enough fine sediment to cover 325 linearfeet of the Tuolumne River low water channelsurface to a depth of 1 inch. Actual fine sedimentdelivery to the Tuolumne River was most likelymuch larger because the tributary floods precededthe large (60,000 cfs) spill into the TuolumneRiver, which most likely eroded much of any deltamaterial deposited in the preceding days.Rivers naturally sort much of their fine sedimentby deposition onto floodplains during high flows.However, in regulated rivers such as theTuolumne River, sand is often delivered to themainstem channel by tributaries when the mainstemis not flooding the floodplains. This sandaccumulates in the low water channel rather thanbeing deposited on the floodplain. Therefore,chinook salmon habitat is quickly degraded by inchannelsand deposition.Non-point sourcesNon-point sand sources are those that supply sandover an extended area, rather than from a singlepoint. The two primary non-point sand sourcesare bank erosion and sheet/surface erosion. Thenumber of these non-point sources along theTuolumne River upstream of Waterford (RM 32)are few. The natural banks along the TuolumneRiver are composed of a variety of sedimentclasses, ranging from clay hardpan to cobbles tosandy loam. Bank erosion is nearly non-existentin the study reach based on sequences of aerialphotographs and observations during our riverreconnaissance. Channel migration has notoccurred since New Don Pedro Dam was built,except near the 7/11 Materials plant site (RM37.8) and from eroding topsoil dikes throughoutthe gravel mining reach (RM 35.7 to 39.0). Theriver was migrating into a cultivated field at the 7/11 Materials plant site, and was rip-rapped in1997. This rip-rap eliminated this source of finesediment, but it also eliminated an importantsource of gravel, which supported extensivelyused spawning riffles immediately downstream.Bioengineering approaches should be used in thefuture in place of rip-rap if bank erosion must be90