Merced River at Happy Isles Bridge near Yosemite, California (Station 11264500)

Site Characteristics and Land Use

The Merced River HBN Basin is located on the western slope of the central Sierra Nevada, in the Sierra-Cascade Mountains physiographic province in central California (Figure 6. Map showing study area in the Merced River Basin and photograph showing the Echo Creek subbasin). The basin drains an area of 469 km² and ranges in elevation from 1,224 to 3,997 m. The landscape in the Merced River Basin is alpine-subalpine-montane and is characterized by broad glaciated valleys with steep-sided walls. Forests and meadows mainly cover the alluvial areas along the valley floors, and the ridges have broad expanses of exposed granite and thin patches of forest. Headwater

Echo Creek from Trail (A. Mast; 8/91)

cirques are alpine in character and have numerous glacial lakes and abundant talus and till deposits. The HBN station is located 3.0 km southeast of the town of Yosemite Village, Calif., at latitude 37°43'54'' and longitude 119°33'28''. The Merced River is a west-flowing tributary of the San Joaquin River that has a channel length of about 31.9 km upstream from the HBN station and an average stream gradient of 27.3 m/km. The annual hydrograph is driven by melting of a seasonal snowpack that accumulates between October and early April and melts during late April to June. Mean monthly discharge varies from 1.1 m³/s in October to 35.4 m³/s in May, and average annual runoff from the basin was 66.5 cm from 1916 through 1995 (Hayes and others, 1996). The area has a Mediterranean-type climate with cool, wet winters and warm, dry summers. Average daily air temperatures range from 3°C in January to 22°C in July (National Weather Service at URL http://www.wrcc.dri.edu/climsum.html, 1998). Average annual precipitation at the elevation of the HBN station is about 94 cm of which about 25 percent falls as snow between October and April. Precipitation amount increases with increasing elevation up to about 2,000 m and then stabilizes (Stephenson, 1988). Precipitation averages about 140 cm at treeline (~3,200 m) of which as much as 95 percent falls in the form of snow (Stephenson, 1988; Melack and Stoddard, 1991).

The basin is located in the Sierran Steppe-Mixed Forest-Coniferous Forest-Alpine Meadow ecoregion (Bailey and others, 1994) and vegetation is dominated by old-growth pine and fir vegetation types (Rundel and others, 1977). Less than one-half of the basin area is forested and the remainder is covered by lakes, wetlands, meadows, rock outcrops, and talus slopes (Rundel and others, 1977). In the montane zone, which ranges from about 1,500 to 2,700 m, the predominant trees are ponderosa pine, Jeffrey pine, Douglas-fir, sugar pine, white fir, California red fir, and incense cedar. In the subalpine zone (about 2,700 to 3,200 m), the predominant trees include mountain hemlock, California red fir, lodgepole pine, western white pine, and whitebark pine. In the alpine zone, above about 3,200 m, vegetation consists of low-lying tundra plants and alpine meadow vegetation. Soils in the basin are Inceptisols, which mostly developed on alluvial deposits and glacial tills derived from the local bedrock. The soils are usually shallow, coarse textured, dark colored, and acidic and have variable organic-matter content. Most of the soils are highly permeable, except for meadow soils, which are poorly drained. Cryumbrepts form predominantly in granitic colluvium on ledges, on ridge slopes, and in joint systems on large rock outcrops (Huntington and Akison, 1987). These soils usually have a thick, dark-colored surface layer overlying lighter colored substrata that rest on coarse boulders or slightly weathered granite rock at depths ranging from 50 to more than 150 cm. Cryochrepts and Cryorthents are developed on hillslopes

Upper Merced (A. Mast; 8/91)

above the valley floors, where Cryoboralfs dominate. A typical forest-soil profile has a 10-cm-thick organic-rich surface layer, a 30-cm-thick brown cobbly silty loam, and a 30-cm-thick yellowish-brown cobbly sandy loam sublayer that is underlain by massive granite boulders.

The Sierra Nevada are formed by Cretaceous-age granitic rocks of the Sierra Nevada Batholith (Huber, 1987; Bateman, 1992). Most of the rocks in the upper Merced River Basin are granites and granodiorites that are part of the Tuolumne Intrusive Suite, a group of four concentrically arranged plutons (Huber, 1987). The oldest and most mafic unit forms the margin of the suite and is present in the southwest part of the basin. The rocks are progressively younger and more felsic toward the core of the suite, which underlies the basin near the northern boundary. The predominant minerals in the granodiorites are plagioclase, biotite, and hornblende; the predominant minerals in the granites are plagioclase, quartz, potassium feldspar, biotite, and hornblende. Small outcrops of hornfels and other metavolcanic rocks were probably derived from pyroclastic rocks (tuff and breccia) that underlie the southeastern part of the basin. Limited exposures of metasedimentary rocks and diorite crop out along the eastern margin of the basin. About 20 percent of the basin is underlain by surficial deposits, which primarily include glacial till that covers the valley bottoms as lateral and recessional moraines. Till is particularly extensive in the drainage basin of Illilouette Creek (fig. 6). Talus and colluvium form substantial deposits in high-elevation cirques, especially along the Sierran Divide, which forms the eastern boundary of the basin.

The Merced River HBN Basin drains the eastern parts of Mariposa and Madera Counties in California and is entirely within the boundaries of Yosemite National Park. The only road in the basin follows the western margin of the park, providing access to a scenic overlook. An extensive trail system provides access to much of the basin during the summer, but deep snowpacks limit accessibility during the winter. The trail system and three large campgrounds (capacity of more than 50 people per night) are heavily used during the summer. The campgrounds are operated by concessionaires as semipermanent installations that have showers and water-treatment facilities. The basin is managed solely for wilderness and recreation because of its location in a national park. Forest fires, floods, and landslides are the main natural disturbances in the basin. Possible human effects primarily are trail erosion caused by people and pack animals, which may result in siltation of surface water.

Historical Water-Quality Data and Time-Series Trends

The data set for the Merced River HBN station analyzed for this report includes 212 water-quality samples that were collected from February 1968 through September 1995. Sampling frequency ranged from bimonthly to biweekly from 1969 to 1982, then was decreased to quarterly from 1983 to 1995. Samples from the early part of the period of record probably were analyzed at a USGS district laboratory in Sacramento, Calif. (Durum, 1978). After establishment of the central laboratory system, samples were analyzed at the Salt Lake City, Utah, laboratory from 1973 to 1975 and at the NWQL in Arvada, Colo., from 1976 through 1995. Daily discharge records for the Merced River (station 11264500) are available beginning in August 1915. Daily water temperature at the station was measured from October 1965 to September 1977 and October 1978 to September 1993.

Calculated ion balances for 202 samples that have complete major-ion analyses are shown in figures 7a and 7b. Graphs showing temporal variation of discharge, field pH, major-ion concentrations, and ion balance in the Merced River, California. Ion balances ranged from -61 to +42 percent, and only 60 percent of the samples had values within the ±10-percent range. This wide range of ion balances reflects the difficulty in making precise analytical measurements at the low solute concentrations typical of this station. The average ion balance was -6.3 percent, and more than 65 percent of the samples had negative ion balances, indicating an excess of measured anions over cations in

North fork of Lyell Fork (A. Mast; 8/91)

solution. Natural water commonly has an excess of cations in solution because of the presence of unmeasured organic anions. An anion excess is more difficult to explain than a cation excess and may indicate a bias in the analytical measurements. The alkalinity determination is often the largest single source of error in major-ion analyses, particularly in dilute water (Fishman and Friedman, 1989). Laboratory alkalinity in HBN samples was determined by a fixed-endpoint titration to pH 4.5. For waters that have alkalinities less than 100 meq/L, however, the endpoint pH is closer to 5.0, and titration to pH 4.5 may overestimate alkalinity by as much as 25 meq/L (Barnes, 1964). Because the average alkalinity and anion excess for the Merced River HBN station were 130 meq/L and 27 meq/L, respectively, a bias in the laboratory alkalinity seems to be a reasonable explanation for the negative bias in the ion balance. Time-series plots of the major dissolved constituents were inspected for evidence of other method-related effects (fig. 7). The most notable pattern was in sulfate concentrations, which decreased considerably in scatter beginning in 1983 and had a period of low concentrations in the 1990's. Although natural variations in stream-water chemistry cannot be completely ruled out, this pattern probably was caused by several changes in the analytical technique for sulfate, including a change to a turbidimetric technique in 1983 (Office of Water Quality Technical Memorandum No. 83.07, Analytical methods: Sulfate determinations, issued February 25, 1983, at URL http://water.usgs.gov/admin/memo/) and another method change to ion chromatography in 1990 (Fishman and others, 1994). The period of elevated and uniform sulfate concentrations in 1982 and 1983 is related to an increase in the analytical reporting limit for sulfate to 5 mg/L (104 meq/L) during those 2 years. The temporal pattern in concentrations of nitrite plus nitrate at this station also was affected by changes in the analytical reporting limit for this constituent, as indicated by the stepped pattern in figure 7. High chloride concentrations were measured in several samples collected in late 1977 and early 1978; however, this pattern probably reflects natural variation in stream-water chemistry because the samples were collected during a period of extremely low streamflows.

Table 10. Minimum, first quartile, median, third quartile, and maximum values of physical properties and major dissolved constituents measured in water-quality samples from the Merced River, California, March 1968 through September 1995, and volume-weighted mean concentrations in wet precipitation collected in Yosemite National Park, California

[Concentrations in units of microequivalents per liter, discharge in cubic meters per second, specific conductance in microsiemens per centimeter at 25 degrees Celsius, pH in standard units, and silica in micromoles per liter; n, number of stream samples; VMW, volume-weighted mean; inst., instantaneous; spec. cond., specific conductance; <, less than; --, not reported]

^a Values are volume-weighted mean concentrations for 1982-95.

^b Laboratory pH.

^c Nitrate only.

The median concentrations and ranges of major dissolved constituents in stream water collected at the HBN station and VWM concentrations in wet-only precipitation measured at the Yosemite National Park NADP station are presented in table 10. Precipitation chemistry at the NADP station, which is about 25 km northwest of the HBN station at an elevation of 1,408 m, is very dilute and slightly acidic and has a VWM pH of 5.4 for 14 years of record. The predominant cation in precipitation was ammonium, which contributed 39 percent of the total cation charge, and the predominant anion was nitrate, which accounted for 44 percent of the total anions. The predominance of these ions in precipitation may reflect the influence of agricultural activities in the San Joaquin Valley on the western side of the Sierra Nevada.

Stream water in the Merced River is dilute and weakly buffered; specific conductance ranged from 3.0 to 65 mS/cm, and alkalinity ranged from 20 to 360 meq/L (table 10). The major cations in stream water were calcium and sodium

Lyell Fork (A. Mast; 8/91)

and the major anion was bicarbonate. The predominance of these solutes in stream water is attributed to the weathering of plagioclase and hornblende minerals in the granites and granodiorites of the Sierra Nevada Batholith (Clow and others, 1996). The median chloride concentration (68 meq/L) in stream water was substantially greater than the VWM concentration of chloride (4.1 meq/L) in precipitation, indicating that most stream-water chloride is derived from sources in the basin. In a detailed study of stream-water chemistry in the basin, Clow and others (1996) hypothesized that spring water that had high chloride concentrations and that was discharging from joints and fractures in the granitic bedrock was the only plausible source of chloride to surface water other than precipitation. The median concentration of sulfate in stream water was 23 meq/L compared to the VWM concentration of 5.9 meq/L in precipitation, indicating that some sulfate also seems to be derived from sources other than atmospheric deposition. In contrast, Clow and others (1996) reported a median sulfate concentration of 10 meq/L for surface water in the basin, which was accounted for by atmospheric inputs and evapotranspiration. One possible explanation for this discrepancy is that the median sulfate concentration in table 10 may be overestimated because of positive bias introduced by analytical techniques used in the past. Concentrations of inorganic nitrogen species in stream water were less than the VWM concentrations in precipitation, indicating that most atmospheric nitrogen is retained by vegetation and soils in the basin.

The solute composition of stream water was further evaluated by analyzing correlations between solutes and stream discharge (table 11). Most weathering-derived solutes had strong inverse correlations with stream discharge, particularly calcium (rho = -0.806) and sodium (rho = -0.796). These results are consistent with a hydrologic system where weathering-enriched base flow is diluted by water from shallow or surficial sources during periods of increased discharge, particularly spring snowmelt. Chloride also had a strong inverse correlation with discharge (rho = -0.842), providing additional evidence for the effect of chloride-rich ground-water springs on the chloride concentration of the stream (Clow and others, 1996). For the solutes, the strongest correlations were between calcium and sodium (rho = 0.886), calcium and chloride (rho = 0.901), and sodium and chloride (rho = 0.893). These strong correlations may indicate a common source of these solutes, such as saline springs discharging from joints and fractures in the bedrock. Calcium and sodium also had strong positive correlations with silica, which is consistent with the weathering stoichiometry of plagioclase in the granitic bedrock.

Table 11. Spearman rank correlation coefficients (rho values) showing the relation among discharge, pH, and major dissolved constituents, Merced River, California, 1968 through 1995

[Q, discharge; Ca, calcium; Mg, magnesium; Na, sodium; K, potassium; Alk, alkalinity; SO4, sulfate; Cl, chloride; Si, silica]

Table 12. Results of the seasonal Kendall test for trends in discharge and unadjusted and flow-adjusted pH and major dissolved constituents, Merced River, California, March 1968 through September 1995

[Trends in units of microequivalents per liter per year, except for discharge in cubic meters per second per year, pH in standard units per year, aluminum, iron, and silica in micromoles per liter per year; <, less than; --, not calculated]

^a Criteria used in selection of sampling sites: BG = bedrock geology, MC = main channel, LU = land use.