Stream channel stability is affected by peak flows rather than average annual water yield. Timber harvesting and other land management activities that contribute to soil compaction, vegetation removal, or increased drainage density can increase peak discharges and decrease the recurrence interval of bankfull discharges. Increased peak discharges can cause more frequent movement of large streambed materials, leading to more rapid stream channel change or instability. This study proposes a relationship between increased discharge and channel stability, and presents a methodology that can be used to evaluate stream channel stability thresholds on a stream reach basis. Detailed surveys of the channel cross section, water surface slope, streambed particle size distribution, and field identification of bankfull stage are used to estimate existing bankfull flow conditions. These site specific stream channel characteristics are used in bed load movement formulae to predict critical flow conditions for entrainment of coarse bed material (D84 size fraction). The “relative bed stability” index, defined as the ratio of critical flow condition to the existing condition at bankfull discharge, can predict whether increased peak discharges will exceed stream channel thresholds.

The Jack Creek watershed, a 133 km2 (51.5 mi2) drainage in southwestern Montana, was impacted by a mountain pine beetle (Dendroctonus ponderosae Hopkins) epidemic in 1975–1977 which killed an estimated 35 percent of its total timber. Analyses of USGS streamflow data for four years prior to and five years after mortality suggest a 15 percent post‐epidemic increase in annual water yield, a two‐to three‐week advance in the annual hydrograph, a 10 percent increase in low flows and little increase of peak runoff.

The Forest Service of the U.S. Department of Agriculture is dedicated to the principle of multiple use management of the Nation's forest resources for sustained yields of wood, water, forage, wildlife, and recreation. Through forestry research, cooperation with the States and private forest owners, and management of the national forests and national grasslands, it strives-as directed by Congress-to provide increasingly greater service to a growing Nation. The U.S. Department of Agriculture (USDA) prohibits discrimination in all its programs and activities on the basis of race, color, national origin, age, disability, and where applicable, sex, marital status, familial status, parental status, religion, sexual orientation, genetic information, political beliefs, reprisal, or because all or part of an individual's income is derived from any public assistance program. (Not all prohibited bases apply to all programs.) Persons with disabilities who require alternative means for communication of program information (Braille, large print, audiotape, etc.

Annual evapotranspiration (ET) and net photosynthesis (PSN) were estimated for a mountainous 28 x 55 km region of predominantly coniferous forests in western Montana. A simple geographic information system integrated topographic, soils, vegetation, and climate data at a 1.1—km scale defined by the satellite sensor pixel size. Leaf area index (LAI) of the forest was estimated with data from the NOAA (National Oceanic and Atmospheric Administration) Advanced Very High Resolution Radiometer (AVHRR). Daily microclimate of each cell was estimated from ground and satellite data and interpolated using MT—CLIM, a mountain microclimate simulator. A forest ecosystem simulation model, Forest—BGC, was used to calculate ET and PSN daily for each cell. Ranges of estimated LAI (4—15), ET (25—60 cm/yr), and PSN (9—20 Mg°ha—1°yr—1) across the landscape follow the trends expected in both magnitude and spatial pattern. These estimates compared well with field measurements of related variables, although absolute validation of these predictions is not possible at large spatial scales.

[1] Critical shear stress and unit discharge flow competence models were tested against coarse bed load data from Dupuyer Creek, Montana, United States. Maximum particle sizes sampled (D max ) and D 50 to D 90 percentiles in the bed load grain size distribution were well correlated with both shear stress and unit discharge. Bed load grain sizes became coarser with increasing flow strength. For the D max curve, Shields dimensionless parameter for the surface D 50 was estimated at 0.044, and the exponent for relative particle size (D i /D 50 ) was À0.59. In the unit discharge criterion the critical flow to entrain the surface D 50 was poorly predicted. Flow competence relationships based on D max are prone to the influence of outliers and sample mass variability. The mean of the three largest particles, D max (3), is more sensitive to changes in flow strength than the D 50 to D 90 bed load grain sizes, and may represent a good compromise.