South Fork of the Salmon River, Idaho
Considerations for Using Ecological
Restoration: Fine Sediment Loadings
The South Fork of the Salmon River (South Fork) is located in the forested, mountainous area of central Idaho. At lower elevations, the watershed is primarily forested with ponderosa pine and Douglas-fir. Lodgepole pine, grand fir, Engelmann spruce, and subalpine fir dominate as elevation increases. Meadows are found along the river, especially in its upper reaches.
The river and its tributaries flow on a granitic bedrock formation known as the Idaho Batholith, which is characterized by heavily dissected topography and highly erodible soils. Elevations range from 3,600 to 9,179 feet. Basin slopes are steep, with many over 70 percent. The South Fork, between its headwaters and the Secesh River confluence, drains 370 square miles.
Average annual precipitation varies with elevation from 20 to 60 inches per year. Summers are typically warm and dry, with warm-season precipitation occurring primarily during high-intensity thunderstorms. Winters are characterized by heavy snows and cold temperatures; most of the annual precipitation falls as snow. Long-duration, low-intensity storms are common in fall, winter, and spring; and winter and spring rain-on-snow events occur occasionally above 5,000 feet. The annual hydrograph reflects the winter precipitation pattern with snowpack accumulation and late spring snowmelt. The hydrograph, therefore, rises to a peak in mid to late May and gradually declines to base flows by early September. Base flows occur during the fall and winter.
The South Fork system supports populations of resident fish species, such as trout and char, and anadromous species, including salmon and steelhead. It is highly valued as a source of Chinook salmon and steelhead trout spawning and rearing habitat. The river once supported Idaho's largest run of summer Chinook salmon, estimated at approximately 10,000 returning adults; and runs of returning steelhead were estimated at 3,000 adults before logging began. These populations rely on just a few locations to spawn, however, since even under ideal conditions, spawning sites along the river are limited to the upper 35 miles and gradients along most of this channel length are too high to support required spawning conditions.
During its most recent statewide water quality assessment, the Idaho Division of Environmental Quality determined that three segments of the upper South Fork are water quality limited due to fine sediment, which has adversely affected salmonid spawning and the river's ability to support cold water biota. Nonpoint sources are responsible for most of this fine sediment. Reductions of sediment loadings from these sources should improve salmonid spawning habitat.
Stressors of Concern
Using a computer model and professional judgment, the Forest Service has estimated that sources in the South Fork basin above Glory Hole deliver 18,550 tons of sediment to the river each year (Table 6-3). Glory Hole is approximately 3 miles above the Secesh River confluence. Over 85 percent of the sediment delivered has been attributed to natural background sources (EPA 1992b). This is not surprising given the erodible soils that exist in the basin.
Above the Secesh River confluence, the South Fork basin is primarily within the Boise and Payette National Forests. Timber harvesting has been the primary land use activity. Activities in the South Fork drainage prior to 1940 included intensive mining and grazing. Mining activities were responsible for significant deposits of sediment and chemicals to the stream system, while uncontrolled grazing contributed to increased sediment loads and degradation of riparian areas. From 1945 to 1965, intensive logging activities resulted in dense road networks and other sources of accelerated sedimentation.
Grazing activities ceased because of the Forest Service's moratorium on ground-disturbing activities in the basin took effect in the mid-1960s. Because they are no longer profitable, mining activities have also ceased, but sediment from tailings piles continues to be delivered to the river. These loadings are considered minor compared to the amount of sediment originating from current forestry activities. Forestry roads appear to have been the major source of sediment from human activities. Early roads penetrated the South Fork basin during the 19th century; the South Fork Road was pioneered by the Civilian Conservation Corps during the 1930s; and road building associated with timber harvesting increased in the 1950s and early 1960s. Then, in the early 1960s, a large area of the canyon and adjacent slopes was burned by wildfire. As mitigation, the Forest Service benched (terraced) large areas of the burn, but during the winter of 1964-65 a series of rain-on-snow events in the basin caused road fills on unstable slopes and benched areas in the Poverty Burn to saturate and fail. This resulted in massive sedimentation of the river and inundated five primary critical salmonid spawning areas (Glory Hole, Krassel, Poverty Flats, Upper Stolle, and Lower Stolle Meadows) with coarse to fine sediments.
In recent years, the numbers of Chinook salmon and steelhead trout that are spawning on the South Fork have declined. This is partially because fine sediment has covered and infiltrated the larger bottom materials at spawning sites, which also function as areas of deposition because of their low gradient. The sediment may trap fry that are attempting to emerge; deplete intergravel oxygen levels, smothering eggs that have been laid; limit the aquatic invertebrate populations used as a food source by predatory fish in rearing areas; and fill the pools and pockets between rocks and boulders on which young fish depend to protect them from predators and to rest from swimming in fast currents.
In addition, some of the decline in salmonid population is due to the downstream influences of commercial and sport fishing, and the construction of eight mainstream hydroelectric dams on the Columbia and Snake Rivers. Dams can prevent salmonid smolts from safely leaving the river and can prevent adults from returning to spawn.
The Goals For Restoration
For salmonid spawning and cold water biota, no specific state numerical sediment criteria have been established. However, because of the problems associated with excess sediment in the South Fork, interim water quality criteria were set for the river and its tributaries by a consensus team composed of two hydrologists and one fisheries research scientist from the Intermountain Research Station, one forest hydrologist and one district fisheries biologist from Boise National Forest, one hydrologist and one district fisheries biologist from Payette National Forest, one representative from the Environmental Protection Agency (Region X), and two from the Idaho Division of Environmental Quality—one being from the Forest Service on a Interagency Personnel Agreement. The consensus team recognized that sediment input from human activities has to be reduced if full recovery of salmonid spawning potential and cold water biota uses of the South Fork can be expected.
The interim numeric sediment standards are as follows: the goal for cobble embeddedness, as measured by the Burns technique (Burns 1984), was set at a 5-year mean below 32 percent with no individual year above 37 percent. The goal for percent depth of fines, as measured with a McNeil core and percent of fines by weight analysis, was set at a 5-year mean of less than 27 percent with no individual year over 29 percent.
An interim objective is to provide habitat sufficient to support fishable populations of naturally spawning and rearing salmon and trout by 1997. This determination will be based on evaluation of fish populations, harvest of wild fish, cobble embeddedness, core sampling, photographs, and other pertinent data. Data must indicate that habitat is sufficient to sustain naturally producing populations of Chinook and steelhead tolerant of sustained recreational harvest. For now, the interim objective, which does t define fully restored habitat, is interpreted as follows:
2. A 5-year mean of less than percent and no individual year greater than percent must be observed in locations where cobble embeddedness now exceeds 32 percent. Other locations must exhibit no increased sediment deposition beyond natural variation.
3. A 5-year mean of less than percent and no individual year greater than percent must be observed in locations where the percentage of fine sediment now exceeds 27 percent. Other locations must exhibit no increased sediment deposition beyond natural var iation.
Annual project accomplishments and monitoring results are to be reported in the monitoring results documents prepared by the two National Forests. All sediment reduction projects will be completed by 1996. The interim goals for depth fines and cobble embeddedness are to be met by January 2001, or acceptable improving trends in other appropriate water quality parameters should be observed by then. Targeting and Prioritizing: As a result of public interest in restoring the salmonid fishery, the State identified the South Fork as a priority for development of a total maximum daily load for sediment.
Monitoring and Data: The South Fork and its tributaries have been monitored extensively since 1965. The South Fork Monitoring Committee, composed of soil, water, and aquatic specialists from the Boise and Payette National Forests and the Intermountain Research Station, collected data on sediment load, depth fines, and cobble embeddedness over several years. These monitoring tasks were assumed by the two forests as part of their monitoring plans after their forest plans were implemented (Boise National Forest 1990; Payette National Forest 1990).
Data indicate that sediment yield peaked above 20,000 m3/yr in the late 1960s (162 percent of natural), with approximately 2x106 m3 being delivered to the river channel. By 1989 sediment yield had declined to 3,000-4,000 m3/yr.
After its gravel bottom was completely inundated with fine sand, the river began to carry excess sediment downstream as bedload. Core samples and embeddedness measurements indicate that surface and depth fines decreased from the late 1960s until 1977, remaining constant since then except for a slight increase in the early 1980s. There has been some fluctuation in later years, but they represent neither an improving nor declining trend. Graphical analysis showed that the amount of fine sediment at the sampling stations decreased sharply between 1966 and 1970. This improvement was attributed to the moratorium on ground-disturbing activity that began at that time. The amount of fine sediment leveled off after the mid-1970s, indicating that it is necessary to reduce sediment loading below current levels if the spawning areas are to improve any further.
Surface fines currently are between 10 percent and 15 percent, while depth fines are between 20 percent and 36 percent. Cobble embeddedness data have been collected in separate locations and with varied techniques for a much shorter period. These values vary from 14 percent to 56 percent (Platts et al. 1989; Ries and Burn 1989; Boise National Forest 1990).
Long-term stream flow data have been monitored in the South Fork drainage near the Krassel Ranger station. Information from this site has been collected in conjunction with the U.S. Geological Survey (gage 133-10700).
Modeling: Sediment loading estimates for the South Fork road, presented in Table 6-3, were calculated using analysis procedures that were developed during detailed research on erosion and sediment delivery from roads in the Silver Creek watershed, a tributary of the Middle Fork of the Payette River, in Boise National Forest (Payette National Forest 1990).
All other sediment loading estimates were generated using the less rigorous BOISED model. The professional judgment of individuals having years of experience observing sedimentation processes in the river basin was invaluable in both cases (Megahan, personal communication).
BOISED is the operational sediment yield model that is used by the Boise and Payette National Forests to evaluate alternative land management scenarios. It is a local adaptation of the sediment yield model developed by the Northern and Intermountain Regions of the Forest Service for application on forested watersheds of approximately 1 to 50 square miles that are associated with the Idaho Batholith. To estimate cumulative average annual sediment yield using BOISED, the South Fork watershed was broken into land types, which are units of land with similar landform, geologic, soil, and vegetative characteristics. Dominant erosion processes, including surface and mass erosion, were then evaluated for each land type to estimate the sediment yield. When erosion and sediment yield data were missing, available research data were extrapolated to areas with similar characteristics to predict the effects of alternative watershed disturbances, including general road construction, timber harvest, and forest fire.
The model produced quantified estimates of average annual sediment yield for undisturbed conditions, past activities, and proposed future activities. While it was inappropriate to use the model as a highly reliable predictor of absolute quantities of sediment delivered to the river at a specific time, it was appropriate to use model results for comparison of alternative management scenarios within the watershed.
Determining the Load Allocation Scheme: The consensus team devised a strategy to accomplish project goals based on information from the models, fisheries research, and the best professional judgement of members of the team.
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