Minam River at Minam, Oregon (Station 13331500)

Site Characteristics and Land Use

The Minam River HBN Basin is located in the Columbia Plateaus physiographic province in the Wallowa Mountains in northeastern Oregon (Figure 20. Map showing the study area in the Minam River Basin and photograph showing the landscape in the headwaters). The 622-km² basin ranges in elevation from 774 to 2,780 m and is characterized by a long, U-shaped valley bounded by jagged ridges and alpine cirques. The HBN station is located 0.5 km west of the town of Minam, Oreg., at latitude 45°37'12'' and longitude 117°43'32''. The Minam River flows northwest into the Wallowa River, a tributary of the Snake River, and has a channel length of 73 km upstream from the HBN station and an average stream gradient of 13.0 m/km. The main channel is perennial, and mean monthly discharge varies from a minimum of 2.7 m³/s in October to a

Eagle Cap Wilderness (A. Mast; 8/91)

maximum of 44.6 m³/s in June during peak snowmelt. Average annual runoff from the basin was 65 cm from 1912 through 1995 (Hubbard and others, 1996). The climate is characterized by short, mild summers and long, cold winters (Cole, 1982). Average monthly air temperatures range from -7.2°C in January to 14.6°C in August (National Climatic Data Center, 1997). Precipitation varies strongly with elevation, ranging from about 100 to 150 cm annually, and is evenly distributed throughout the year, except for a short summer dry season (Cole, 1982). The average annual snowfall is 680 cm, most of which falls between October and April (National Climatic Data Center, 1997).

The basin is in the Southern Rocky Mountains ecoregion and has a high diversity of plant communities that vary with landform, elevation, and aspect (Bailey and others, 1994). Below 2,000 m, forests are dominated by Douglas-fir, although ponderosa pine, western larch, lodgepole pine, and grand fir may be locally abundant (Cole, 1982). Common understory species in this forest community are Grouse huckleberry, big huckleberry, pinegrass, Idaho fescue, bluegrass, and bluebunch wheatgrass. The subalpine fir community dominates most forested sites between 2,000 and 2,300 m (Cole, 1982). The predominant species are subalpine fir and Englemann spruce; the understory is grouse whortleberry. The highest elevations in the basin are dominated by stands of whitebark pine and subalpine fir interspersed with subalpine grasslands and meadows. Other predominant species in this zone include alpine sagebrush, sandwort, fleeceflower, needlegrass, lupine, elk sedge, paintbrush, yarrow, and mosses (Wallowa-Whitman National Forest, written commun., 1998). Little information is available on soil conditions and characteristics in the Eagle Cap Wilderness (U.S. Department of Agriculture, 1995). Most soils in the Wallowa Mountains contain volcanic ash from the eruption of Mount Mazama 6,850 years ago, that has been redeposited and mixed with colluvium and alluvium from local bedrock sources.

Bedrock in the basin consists of intrusive rocks of Cretaceous age that are overlain by basalts of the Columbia River Basalt Group of Tertiary age (Weis and others, 1976). Intrusive rocks include granites of the Wallowa Batholith that are extensively exposed in the upper part of the basin roughly south of Reds Horse Ranch. The granite is generally light gray and medium grained and consists of quartz, plagioclase, hornblende, and biotite. The granite is intruded by aplite, pegmatite, and basalt dikes that range from 8 to 90 cm in width and is cut by several widely spaced joint sets. The basalts of the Columbia River Basalt Group cover the flanks of the Wallowa Range and are exposed along the lower reaches of the river canyon. The Columbia River Basalt Group consists of dark-colored lava flows that range in thickness from 9 to 30 m. The Wallowa Mountains were extensively glaciated during the late Pleistocene, and glacial deposits are present on the walls and floors of the

Near Reds Horse Ranch (A. Mast; 8/91)

river valleys and on ridges in high-elevation cirques. Some pyrite- bearing veins are present in sheared and fractured granitic rock in the Eagle Cap Wilderness, although the potential for mineral deposits is low. No active mines exist in the basin, although several abandoned prospects and adits are mapped in the southwestern part of the basin (Weis and others, 1976).

The Minam River HBN Basin drains parts of Union and Wallowa Counties in Oregon. More than 90 percent of the basin is in the Eagle Cap Wilderness in the Wallowa-Whitman National Forest, and the remainder is privately owned. Almost all of the basin is roadless, except for the area north of the national forest boundary and a few areas along the western basin divide. The national forest maintains a 165-km trail system that parallels the entire length of the main channel of the Minam River, as well as several of the major tributaries. The Eagle Cap Wilderness was created in 1964 and is managed under a nondegradation principle, which seeks to maintain the wilderness in at least as wild a condition as it was at the time of classification (U.S. Department of Agriculture, 1990b). The upper 65 km of the Minam River was added to the National Wild and Scenic Rivers System in 1988 and has the distinction of being the only component of the system to be entirely within a wilderness area (U.S. Department of Agriculture, 1995). Recreation is the major human-related activity in the basin, including backpacking, horsepacking, hunting, and fishing.

The well-maintained trail system and ease of access make the Eagle Cap Wilderness one of the most heavily used wilderness areas in Oregon. About 800 people visit the Minam River corridor each year, of which about 30 percent are with commercial outfitters and guides (U.S. Department of Agriculture, 1995). The basin contains two backcountry lodges (Minam Lodge and Reds Horse Ranch), a landing strip located along the river in the center of the basin, and a homestead on the Little Minam River used as an administrative site by the Forest Service. The Minam Lodge is still privately owned and operated; however, Reds Horse Ranch was purchased by the Forest Service in 1994 and since then has been used as an administrative facility. Although recreation is currently the only land use, timber harvesting and grazing have been important in the past. Logging occurred from 1918 to 1924, and a splash dam was constructed 9.7 km upstream from Reds Horse Ranch. The dam was removed in 1947 (U.S. Department of Agriculture, 1995). A small amount of timber harvesting has occurred in the basin in upper Squaw Creek and along the western margin of the basin since the establishment of the HBN station. Until 1940, the river corridor was heavily grazed by sheep and cattle, which resulted in some plant community alterations in the basin. However, grazing allotments in the basin have not been stocked since 1985, and range conditions have improved steadily (U.S. Department of Agriculture, 1995).

Historical Water-Quality Data and Time-Series Trends

The data set for the Minam River HBN station that was analyzed for this report includes 204 water- quality samples that were collected from January 1966 through August 1995. Samples were collected 2 to 14 times per year from 1966 to 1982 and

Little Minam River (A. Mast; 8/91)

quarterly from 1983 to 1995. Water-quality samples from the early part of the period of record probably were analyzed at a USGS district laboratory in Portland, Oreg. (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 to 1995. Daily discharge records for the Minam River (station 13331500) are available beginning in September 1965, and daily water temperature was measured from 1966 to 1985. Daily precipitation totals were measured at the station from 1990 through 1995.

Calculated ion balances for 204 samples that have complete major-ion analyses are shown in figures 21a and 21b. Graphs showing temporal variation of discharge, field pH, major-ion concentrations, and ion balance in the Minam River, Oregon. Ion balances ranged from -31 to +16 percent, and 85 percent of samples had values in the ±10-percent range, indicating that the analytical measurements were of good quality. The average charge balance for all samples was -1.4 percent, and 60 percent of samples had slightly negative ion balances, indicating an excess of measured anions compared to cations. Natural water more commonly has an excess of cations 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 one of the analytical measurements. Time-series plots of the major dissolved constituents were inspected for evidence of other method-related effects (fig. 21). The most notable pattern was observed in sulfate concentrations, which were higher during the middle of the period of record, particularly during the 1980's. The high concentrations in the 1980's coincided with the period when sulfate was analyzed by the turbidimetric titration technique. In 1989, the NWQL determined that sulfate concentrations can be overestimated by this technique and changed the technique to ion chromatography in 1990 (Office of Water Quality Technical Memorandum No. 90.04, Turbidimetric sulfate method, issued December 21, 1989, at URL http://water.usgs.gov/admin/memo/). The bias was most pronounced in dilute waters, although it was not consistent among samples and seemed to be affected by other factors, such as color and turbidity (Schertz and others, 1994). In addition to the analytical bias, some of the elevated sulfate concentrations in 1982 and 1983 reflect an increase in the analytical reporting limit for sulfate from 21 meq/L (1.0 mg/L) to 104 meq/L (5.0 mg/L) during those 2 years of record.

Table 37. Minimum, first quartile, median, third quartile, and maximum values of physical properties and major dissolved constituents measured in water-quality samples from Minam River, Oregon, January 1966 through August 1995, and volume-weighted mean concentrations in precipitation collected at the Starky Experimental Station

[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 1984-95.

^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 Starky Experimental Station NADP site are presented in table 37. Precipitation chemistry at the NADP station, which is about 80 km southwest of the HBN station, is very dilute and slightly acidic and has a VWM pH of 5.3 for 12 years of record. The predominant cations in precipitation were hydrogen ion,

Upper basin (A. Mast; 8/91)

ammonium, and sodium, which contributed 34, 20, and 19 percent of the total cation charge, respectively. The predominant anions were sulfate and nitrate, which accounted for 41 and 43 percent of the total anions, respectively. These results indicate that precipitation is a dilute mixture of strong acid anions derived from industrial emissions of sulfur and nitrogen compounds and from salt derived from marine aerosols.

Stream water in the Minam River is fairly dilute and moderately buffered; specific conductance ranged from 19 to 137 mS/cm and alkalinity ranged from 180 to 720 meq/L (table 37). The major cations in stream water were calcium, magnesium, and sodium, and the major anion was bicarbonate. The predominance of these solutes in stream water, in addition to the relatively high concentrations of silica, is consistent with the weathering stoichiometry of plagioclase, hornblende, and biotite in the granitic rocks and more mafic minerals in rocks of the Columbia River Basalt Group. The median concentration of chloride in stream water was 14 meq/L, which is about 8 times greater than the VWM concentration of 1.8 meq/L in precipitation. Based on the difference between precipitation and runoff, evapotranspiration can account for no more than a threefold increase in the chloride concentration of precipitation, indicating that some stream-water chloride is derived from sources other than wet precipitation. Because land-use activities in the basin are minimal, the only plausible sources of additional stream-water chloride are dust from arid valleys surrounding the Wallowa Mountains (Turk and Spahr, 1991) and weathering of amphibole minerals in the granitic and basaltic bedrock (Peters, 1991). The median concentration of sulfate in stream water was 33 meq/L compared to the VWM concentration of 4.6 meq/L and was slightly higher than was expected based solely on the effects of evapotranspiration. As with chloride, these results may indicate that there are additional sources of stream-water sulfate, such as bedrock weathering or eolian dust. Alternatively, stream-water sulfate may be high relative to concentrations in precipitation because of analytical bias in the historical records rather than because of natural sources. Most concentrations of inorganic nitrogen species in stream water were similar to VWM concentrations in precipitation, indicating that not all atmospheric nitrogen is retained by vegetation and soil in the basin.

The solute composition of stream water was further evaluated by analyzing correlations between solutes and stream discharge (table 38). Most weathering-derived solutes were negatively correlated with stream discharge, particularly calcium (rho = -0.649), sodium (rho = -0.643), and alkalinity (rho = -0.579). 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. For the solutes, the strongest correlations were found among base cations, alkalinity and silica, which is consistent with the weathering stoichiometry of silicate minerals in the granitic and basaltic bedrock. A strong correlation between silica and magnesium (rho = 0.866) is somewhat unusual and may reflect the weathering of magnesium silicate minerals, such as biotite and hornblende, in the granitic rocks or mafic minerals in the basalt. Sulfate and chloride were weakly correlated with discharge and with the major solutes, supporting the assumption that most sulfate and chloride are derived from atmospheric deposition.

Table 38. Spearman rank correlation coefficients (rho values) showing the relation among discharge, pH, and major dissolved constituents, Minam River, Oregon, 1966 through 1995

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

^a Flow model not significant at a = 0.10.

^b Insufficient data to calculate trend.

Bailey, R.G., Avers, P.E., King, T., McNab, W.H., eds., 1994, Ecoregions and subregions of the United States with supplementary table of map unit descriptions: Washington, D.C., U.S. Department of Agriculture, Forest Service, scale 1:7,500,000.

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Last updated August 3, 2000.