Poopenaut Valley: Studying hydrologic impacts from O'Shaughnessy Dam

Yosemite National Park Division of Resources Management and Science staff, in cooperation with scientists from McBain & Trush, Inc., propose to characterize impacts on hydrologic processes in Poopenaut Valley and the immediate vicinity (Fig. 1) downstream of O'Shaughnessy Dam. There is also a smaller component of the study that would focus on the area downstream of Eleanor Dam. Impoundment of Tuolumne River water behind these dams, and diversion of water from the reservoirs, may be adversely affecting hydrologic processes downstream of the dams. This, in turn, may be affecting the aquatic, meadow, and riparian ecosystems that depend on certain hydrologic conditions.
Poopenaut Valley has been identified as the most ecological sensitive area downstream of these dams, and is the main area of research. Quantifying the effects of O'Shaughnessy on hydrologic processes in Poopenaut Valley would be done by installing up to 20 groundwater monitoring wells along two transects across the meadow area in the downstream end of the valley. Soil profiles and vegetation plots established in association with the groundwater wells would help to delineate wetlands. In addition, river stage and water temperature loggers would be installed where each transect crosses the Tuolumne River; a similar instrument would also be installed in the seasonal pond on the north side of Poopenaut Valley to record the seasonal fluctuation of water surface levels and water temperature. One of the stage recorders would be a more sophisticated Campell Scientific datalogger, capable of measuring water turbidity, which requires a small solar panel. A staff plate would be installed near one of the river stage reocorders to measure river stage visually. Finally, two time lapse cameras would be installed in Poopenaut Valley to record changes in habitat availability (i.e., extent of seasonal flooding). All instruments would be located in designated wilderness or in a potential wilderness addition. The instruments would be tracked in a GIS and removed within five years; after removal there would be no trace of these installations. Yosemite RMS staff will coordinate with closely with RMS archeologists and landscape architects to avoid or minimize impacts to cultural resources from the proposed work.
Contractors McBain & Trush, Inc., also propose to study sediment transport immediately downstream of O'Shaughnessy and Lake Eleanor dams. Below O'Shaughnessy Dam these studies would not take place in wilderness, but below Eleanor dam they would. The sediment transport study would have two components: (1) placing tracer rocks of different diameters in the riverbed to determine critical thresholds for mobilizing sediment, and (2) installing scour cores to document river scour depth. Locally-derived metamorphic rocks, which can be easily distinguished from the granitic cobbles presently in the riverbed, would be used as tracer rocks. Scour cores would consist of six-inch diameter holes excavated into sediment in the riverbed and filled with quartzite gravel and surveyed to a vertical datum. Following a high flow event, the scour hole is excavated down to the quartzite gravel, providing the depth of maximum scour and subsequent fill.
Overall, hydrologic investigations would focus on reaches of the river bounded by meadows (such as those in Poopenaut Valley), as these are areas most sensitive to changes in hydrology and sediment flux. River stage recorders and flow data from an existing USGS gaging station below the dam permit the construction of stage-duration relationships and identification of bankfull discharge under current management of the reservoir. These relationships, coupled with detailed topographic surveying of the river channel, would allow comparison with modeled pre-dam flow regimes. Ultimately, results would be tied to biological and vegetation surveys in the adjacent habitats to investigate impacts to these ecosystems from the dams.