Journal Articles

Posted: February 15, 2017
Two pumiceous tephra layers, widespread in meadow topsoils of the southern Sierra Nevada, are correlated on the basis of radiocarbon dates and trace-element analyses with two eruptive centers at the northern and southern ends of the Mono Craters--Inyo craters volcanic chain in eastern California. Pumice and obsidian that were erupted in the northern part of the chain are uniform in trace-element content, whereas those erupted from the southern part are nonuniform and distinctly different, particularly in Sr content. Similar differences are recognized in the two most recent and widespread tephra layers originating from these sites. These tephra layers are the deposits of the most recent explosive eruptions of magma from the Mono Craters and the Inyo craters. Tephra 1, characterized by sanidine microphenocrysts and a Sr content of about 215 ppm, was erupted 720 {+/-} 60 yr B.P. Its distribution defines a south-trending lobe extending over the Sierra Nevada from the upper San Joaquin drainage area to the Little Kern drainage area. Sr, Rb, and Zr contents of the ash are similar to those of a tephra-ringed obsidian dome at the south end of the Inyo craters. Tephra 2, characterized by a lack of microphenocrysts and a Sr content of less than 20 ppm, was erupted 1190 {+/-} 80 yr B.P. It is encountered as a fine ash layer in the Sierra Nevada from northernmost Yosemite to Kings Canyon. Its low Sr content indicates geochemical affinity with the Mono Craters. Panum Crater, a tephra-ringed dome at the north end of the chain, appears to be its most likely source vent.

Posted: February 15, 2017
Study Region: We analyzed the effects of groundwater pumping on a mountain wetland complex, Yosemite National Park, California, USA. Study Focus: Groundwater pumping from mountain meadows is common in many regions of the world. However, few quantitative analyses exist of the hydrologic or ecological effects of pumping. New Hydrological Insights for the Region: Daily hydraulic head and water table variations at sampling locations within 100. m of the pumping well were strongly correlated with the timing and duration of pumping. The effect of pumping varied by distance from the pumping well, depth of the water table when the pumping started, and that water year's snow water equivalent (SWE). Pumping in years with below average SWE and/or early melting snow pack, resulted in a water table decline to the base of the fen peat body by mid summer. Pumping in years with higher SWE and later melting snowpack, resulted in much less water level drawdown from the same pumping schedule. Predictive modeling scenarios showed that, even in a dry water year like 2004, distinct increases in fen water table elevation can be achieved with reductions in pumping. A high water table during summers following low snowpack water years had a more significant influence on vegetation composition than depth of water table in wet years or peat thickness, highlighting the impact of water level drawdown on vegetation.

Posted: February 13, 2017
A broad sample of 79 montane fens in the Sierra Nevada revealed that underlying geology and topography exert strong control over the distribution and vegetation of these ecosystems. Distinct granodiorite, metamorphic, volcanic, carbonate and serpentine bedrock geology resulted in very different water chemistry, which had significant effects on the particular plant species found at each site. Wide-ranging values of pH (4.28–8.00) and dissolved cation concentrations (1.6–62.0 mg L-1) spanned the categories of transitional poor– rich to extremely rich fens. The vegetation of a pair of fens on carbonate bedrock and two floating mat fens was markedly different from the vegetation recorded at any other study sites. Once these outlier fens were removed from the analyses, the environmental variables that correlated most closely with the vegetation data were pH, altitude, presence of volcanic bedrock and fen slope. The measured environmental parameters explained 9.7 % of the variability in the vegetation data. Species richness was primarily (and negatively) correlated with altitude. Peat thickness (15–253 cm) was constrained in smaller catchments and on steeper slopes, and was positively correlated with soil organic matter content (16–92 %). Of the four typical fen landforms (bedrock contact, slope, spring mound and basin), sloping fens were the most common (63 % of the 79-fen sample).

Posted: February 15, 2017
Valley-fill deposits, exposed by Twentieth-Century dissection of a number of meadows on the west slope of the southern Sierra Nevada, contain a stratigraphic record strongly affected by secular variations in watershed hydrology during the Holocence. Meadows are situated in low gradient reaches, adequately supported by seepage water, where fine textured materials accumulate under present hydrologic conditions. Meadows do not necessarily owe their origin to glacial modification of drainage. Many meadows have formed in both glaciated and unglaciated valleys by a water table rise in valley-fill deposits. Ground water in any meadow drainage basin is annually recharged by snowmelt. Significant evapotranspiration by meadow plants causes diurnal fluctuations of growing-season water tables on the order of 0.2 to 0.5 ft and seasonal fluctuations of 2 to 4 ft. Growing-season water-table depths are characteristically different for the two major plant communities, being usually shallower that 2 feet for meadows, and deeper than 4 ft for conifer forests. This relationship and a ground-water model are used to interpret paleohydrologic variations recorded in valley-fill stratigraphy. Stratigraphy, radiocarbon dating, and tephrachronology indicate the following sequence in upper tributary valleys of the montane belt. Pre-Holocene cobbly alluvium rests upon bedrock. A paleosol developed upon this alluvium between 10,200 and 8700 radiocarbon years B.P., records an early post-glacial climatic interval that established forests in the present upper montane belt. The overlying sequence of coarse loamy materials associated with in situ conifer stumps indicates on or more intervals of good soil drainage and dry valley-bottom conditions between 8700 and 1200 years B.P. At some sites there is an abrupt change from forest soils to overlying wet-meadow deposits dated 2500 years at some sites and 1200 years at others, suggesting many meadows originated coincidentally with neoglaciation in the Sierras. A water-table rise of a few feet, resulting from late melting snows, could cause the change from forest to meadow conditions. Meadow deposits are composed of organic-rich, sandy-loam, topsoil layers intercalcated with sheets of well-sorted sandy gravels deposited by flood flows with recurrence intervals greater than 50 years. A plot of upstream catchment area and valley gradient for dissected and undissected meadows indicates the geomorphic domain of unstable meadows subject to gully erosion under present hydrologic conditions on the Sierra west slope. Two pumiceous tephra layers, widespread in meadow topsoils of the southern Sierra, are radiocarbon dated and attributed to tephra-ringed eruptive centers at opposite ends of the Mono-Inyo Crater chain of eastern California. Tephra 1, characterized by sanidine microphenocrysts and Sr content of 215 ppm, erupted 720 years B.P. Distribution of this tephra in confined to a south trending lobe extending 120 miles over the Sierra from the upper San Joaquin drainage to the Little Kern drainage. Trace element analysis of tephra 1 best match those of the tephra-ringed obsidian flow just south of Deadman Creek in the Inyo Craters. Tephra 2, characterized by a lack of microphenocrysts and Sr contents less than 20 ppm erupted from on the northern Mono Craters eruptive centers. These two tephras appear to represent the most recent explosive eruptions of magma from this 40-km long chain of Holocene volcanoes.

Posted: February 10, 2016
Measurements of groundwater–surface water exchange at three wetland stream sites were related to patterns in benthic productivity as part of the US Geological Survey’s Northern Temperate Lakes–Water, Energy and Biogeochemical Budgets (NTL–WEBB) project. The three sites included one high groundwater discharge (HGD) site, one weak groundwater discharge (WGD) site, and one groundwater recharge (GR) site. Large upward vertical gradients at the HGD site were associated with smallest variation in head below the stream and fewest gradient reversals between the stream and the groundwater beneath the stream, and the stream and the adjacent streambank. The WGD site had the highest number of gradient reversals reflecting the average condition being closest to zero vertical gradient. The duration of groundwater discharge events was related to the amount of discharge, where the HGD site had the longest strong-gradient durations for both horizontal and vertical groundwater flow. Strong groundwater discharge also controlled transient temperature and chemical hyporheic conditions by limiting the infiltration of surface water. Groundwater–surface water interactions were related to highly significant patterns in benthic invertebrate abundance, taxonomic richness, and periphyton respiration. The HGD site abundance was 35% greater than in the WGD site and 53% greater than the GR site; richness and periphyton respiration were also significantly greater (p%0.001, 31 and 44%, respectively) in the HGD site than in the GR site. The WGD site had greater abundance (27%), richness (19%) and periphyton respiration (39%) than the GR site. This work suggests groundwater–surface water interactions can strongly influence benthic productivity, thus emphasizing the importance of quantitative hydrology for management of wetland-stream ecosystems in the northern temperate regions.