2015 Yosemite National Park Looking Downstream Project report

The interdisciplinary Yosemite National Park (NPS) Looking Downstream Project is designed to better understand the physical processes and ecology of the Tuolumne River between O’Shaughnessy Dam and the western boundary of Yosemite National Park. The project goal is provide information the SFPUC can use to manage O’Shaughnessy Dam environmental water releases to more closely replicate natural hydrology for the benefit of water-dependent ecosystems downstream.

The 2014-2015 water year was dry: precipitation was approximately 58% of average. April 1st and May 1st snow water equivalent, however, were at record lows (9% and 2% of average April 1st, respectively) continuing a historically unprecedented drought in central California.  Hydrology studies in spring 2015 focused on monitoring low flow conditions.  These low flows precluded any experimental releases from O'Shaughnessy Dam.

Vegetation-related fieldwork in Poopenaut Valley documented the rapid spread of non-native common mullein within and adjacent to the seasonal pond on the north side of the river. Although the 2013 Rim Fire may have contributed to the spread of common mullein in this area, it is most likely related to the nearly continuously dry conditions in the pond over the past few years.

Spring bird surveys in Poopenaut Valley consisted of area searches, point counts, spot mapping, nest searches, and color banding. A cumulative total of 240 individual birds from 38 species were observed. Spot mapping determined breeding territories of the four target riparian focal species (Warbling Vireo, Yellow Warbler, Song Sparrow, and Black-headed Grosbeak) to elucidate their habitat-use patterns. Data collected during spot mapping and nest searching in 2011–2015 were used to calculate arrival, initiation of breeding, and fledging dates for the target riparian focal species. In 2015, NPS found 68 nests, which is substantially more than in previous year. Nest failure rate in Poopenaut Valley seemed unusually high; of the 68 nests found, 60% failed and 25% had unknown fates.

Bat studies have identified an impressive biodiversity of bat species inhabiting Poopenaut Valley from spring 2011 through late summer 2015. Bat detection frequencies decreased in 2015 to levels comparable to pre-2014 levels. The only species with marked increases were western mastiff bat and big brown bat at the south site. Species richness decreased slightly in 2015 at both sites and was fairly consistent with pre-2014 numbers. Seasonal patterns in species richness were also consistent with previous years at both sites.

The 2015 Yosemite National Park Looking Downstream Project report can be accessed here.

Revised fisheries monitoring approach and 2014 results

Since 2007 the SFPUC has conducted fisheries monitoring in the Hetch Hetchy Reach of the upper Tuolumne River.  In 2014, the SFPUC asked Stillwater Sciences (in collaboration with McBain Associates) to take a fresh look at our existing approach and provide recommendations that would increase sampling efficiency and statistical robustness.  Stillwater's recommendations included sampling shorter sites with an increased number of passes, thus providing a slight increase in efficiency, and over time, a more reliable statistical basis for understanding the relationship among environmental variables, O'Shaughnessy Dam releases, and trout populations.  The recommendations and their implementation during our fall 2014 snorkel survey effort are described in this report.