U.S. Dept. of Agriculture
	USDA Forest Service
	Forest Service Research
	Rocky Mountain Research Station
	RMRS Labs

 

Silver Creek Study
General Study Information

The Silver Creek administrative-research study, was designed and initiated in the mid-1960's to determine feasible methods to provide access to and harvest the timber resource on granitic soils in mountainous terrain. The evaluation of erosion and hydrologic consequences of various forest management activities was conducted by the Rocky Mountain Research Station and the Intermountain Region of the U.S. Forest Service. The study area is underlain with intrusive granites of the Idaho batholith. Much of the 40,000 km2 Idaho batholith has cohesionless coarse-grained soils that are highly erodible, especially when disturbed. The combination of erodible soils, relatively high climatic stresses from summer convective rainstorms and rapid spring snowmelt runoff, and mountainous terrain creates a high potential for increases in erosion and stream sedimentation following ground-disturbing activities.

 

Publications

 

 

Location

The study area is located in the mountains of central Idaho in the Silver Creek drainage on the Boise National Forest. Silver Creek is a tributary to the Middle Fork of the Payette River. The approximate center of the study area is 44° 25' N latitude and 115° 45' W longitude. Elevations on the study watersheds range from about 1389 m to 2067 m.

 

Physiography

Physiographic characteristics of these headwater drainages are typical of mid-elevation fluvial landscapes of the Idaho batholith, with V-shaped valleys and steep slopes. Side slopes are steep with slopes ranging from 25 to 75 percent. Additional watershed characteristics are provided in the following table.

 

Watershed Name	Watershed Number	Area (ha)	General Aspect (o)	Maximum Elevation (m)	Minimum Elevation (m)	Channel Length km	Drainage Density (km/km2)	Relief Ratio (m/m)
C	SC1	186	125	2073	1490	3.7	2.0	0.25
D	SC2	117	121	2067	1477	3.1	2.6	0.25
Eggers	SC3	128	127	2006	1444	3.5	2.7	0.24
Ditch	SC4	101	149	1839	1426	2.4	2.4	0.24
Cabin	SC5	109	136	1745	1389	2.9	2.7	0.19
Control	SC6	161	142	1778	1414	5.0	3.0	0.19
K1	SC7	22	291	1820	1418	0.8	3.5	0.36

 

Forest Vegetation

Timber types vary from nearly pure stands of ponderosa pine (Pinus ponderosa Laws.) in the lower elevations and south-facing slopes to mixed ponderosa pine and Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco.) at the middle elevations and stands of Douglas-fir and subalpine fir (Abies lasiocarpa (Hook.) Nutt.) in the upper reaches. There are limited areas of lodgepole pine (Pinus contorta Dougl. Ex Loud.) on the flat and poorly drained areas along Silver Creek. The two principal vegetation habitat types (Steele et al., 1981) are Douglas fir/white spirea, ponderosa pine phase and Douglas-fir/ninebark, ponderosa phase. Much of this timber is mature to overmature. There had been no previous timber harvest program on National Forest lands in this drainage at the initiation of this study.

 

Geology

The watersheds are located within the southern Idaho batholith section of the Northern Rocky Mountain Province (Fenneman, 1931). Bedrock is a uniform, medium to coarse-grained biotite granodiorite that is moderately to well weathered (Clayton et al., 1979; Clayton, 1986).
 

Soils

Soils are generally weakly developed with A horizons ranging from 5 to 25 cm thick overlying moderately weathered granitic parent material. Soil textures are loamy sands to sandy loams and depth to bedrock is usually less than 1.0 m. Soils less than 20 cm deep are common on ridges and south slopes. Scattered outcrops of granitic bedrock are found in the upper elevations of the watersheds. Four predominant types of soils are found on the watersheds. Sandy-skeletal, mixed Typic Xerorthents predominate on south slopes. Sandy-skeletal, mixed Typic Cryorthents, sandy-skeletal mixed Typic Cryoborolls, and mixed Alfic Cryopsamments are found at other locations (Clayton and Kennedy, 1985).

 

Climate

Average annual precipitation on these watersheds is approximately 1020 mm. Summers are hot and dry with occasional convective storms that are often quite localized. Snowfall accounts for about 65% of the annual precipitation and the average maximum snowpack water equivalent accounts for slightly over half the average annual precipitation. The spring snowmelt dominates the annual hydrograph with peak streamflows typically occurring in March through May.

 

References

Clayton, J.L., W.F. Megahan, and D. Hampton. 1979. Soil and Bedrock Properties - Weathering and Alteration Products and Processes in the Idaho Batholith. USDA For. Serv. Res. Paper. INT-237, 35 pp.

 

Clayton, J.L. 1986. An Estimate of Plagioclase Weathering Rate in the Idaho Batholith Based      upon Geochemical Transport Rates. In: Coleman, S.M.; Dethier, D.P., editors. Rates of      chemical weathering of rocks and minerals. Orlando, FL: Academic Press: 1986: 453-466.

 

Clayton, J.L. and D.A. Kennedy. 1985. Nutrient Losses from Timer Harvest in the Idaho Batholith. Soil Sci. Soc. Amer. J. 49:1041-1049.

 

Fenneman, N.M. 1931. Physiography of Western United States. McGraw-Hill Book Company, Inc. New York. 534p.

Steele, R., R.D. Pfister, R.A. Ryker and J.A. Kittams. 1981. Forest Habitat Types of Central Idaho. USDA, For. Serv. Gen. Tech. Rep. INT-114.

 

Climatic Data

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