From the flank of Monument Peak on the California-Nevada state line, it is easy to see why Mark Twain said, “To obtain the air the angels breathe, you must go to Lake Tahoe.” The lake glitters indigo blue 3,500 ft. below the snowbound ridge, and its encircling mountains frame a landscape so vast and primal that to leave tracks in the snow here is an archetypal experience.
A person exalting in January’s fresh powder at Heavenly Ski Resort would never suspect that the resort retains a full-time summer staff whose sole purpose is to ensure that the mountain’s runoff is as pure as, well, the stuff angels drink. This is no paltry task. In summer, when the protective cover of snow fields melts, the barren soils of steep chutes, runs, and roads are vulnerable to erosion. Some parts of the resort drain to Lake Tahoe, whose quality is protected as a national treasure. To make matters worse, the mountain is granitic – one of the most highly erodible geologic parent materials in the West.
Nationwide, 146 ski areas such as Heavenly operate on federal lands. To understand the impact of existing and proposed ski-area runs, roads, and facilities on erosion and water quality, resort planners start by gathering information about the erosion potential of the region. This includes getting a handle on the weathering of local rock and the behavior of the resulting soils. Other factors include how elevation and growing season affect plant species and distribution and how weather and climate influence the energy, timing, and periodicity of the region’s erosional dynamics. In the Tahoe Basin, many of the very same environmental factors that create splendid scenery and year-round recreational opportunities also put the region at high risk for erosion.
Erosion Potential Begins With Rock
The “country” bedrock of the Tahoe Basin is granite: a pale, salt-and-pepper shaded rock composed of conspicuous crystals, or phenocrysts, of quartz, feldspar, and black mica. These large crystals denote slow cooling and solidification of molten magma about 5 mi. beneath the earth’s surface. The granites formed more than 100 million years ago, as magma intruded into older metamorphic rocks. As the huge mass of slowly churning, molten material rose up, the older rock uplifted and eroded, uncovering the granite beneath. The same faults that created the Sierra Nevada mountain range slowly lifted the granitic rocks. Major exposures of granitic batholith in the Western United States include the Sierra Nevada, California’s Klamath-Siskiyou Mountains and Southern California Batholith, the Wallowa Mountains of northeast Oregon, and others in Montana, northeast Washington, and western Idaho.
Anyone who has hiked, fished, packed, camped, or taken a sightseeing drive in the granitic Sierra Nevada can understand how John Muir dubbed it “The Range of Light.” The bright rock is radiant, and almost everywhere granite is present at latitudes and elevations that were glaciated during the Pleistocene. Large expanses of the freshly scoured rock allow rare views and explorations into Zen-like mountain landscapes not obscured by dense vegetation.
But despite its stunning beauty, granite is not stable at the earth’s surface. Weathering begins, often along fracture lines, which admit water, plant roots, and organic acids. Freeze/thaw helps physically break down the rock and chemical changes occur. As the minerals alter, the once-solid rock begins to disintegrate. Acids and water react with minerals to form clay. Quartz, which is more stable, becomes a sand grain in a matrix of the softer, decomposed materials. Grus, a lightweight, easily eroded granular material, is left behind.
No one knows this better than road crews and resource managers who work in the mountains of the West, where decomposed granite is known simply as “DG.” Just east of Heavenly on the Kingsbury Grade to the Sierra summit, crib walls hold back the rolling grus at the toes of slopes cut in weathered granite. Beyond the summit, a person pulling off onto the shoulder to admire the breathtaking plunge of the faulted Sierra into the high desert below can hear the trickling of a million grains of grus as they shift downslope under the relentless pull of gravity. These can be typical background conditions for many proposed development projects in the Tahoe Basin.
Elevation and Growing Season Play a Part
At timberline in this region – where annual snowfall can average 30 ft. or more, where the snow lingers late into the season, where summer nights are cold and the growing season is short – not much grows. Sparse vegetation may be typical of even the most undisturbed sites. Add steep slopes and well-drained, dry southern exposures, and there might be insufficient plant detritus to develop an organic soil layer. The absence of a humus layer to bind the soil, dissipate rainfall energy, and aid in infiltration makes granitic soils in alpine settings extremely susceptible to erosion and dry ravel. Where woody vegetation is sparse, there is no large organic material at the soil surface to hold back the dry ravel and water erosion of the grus. In some places the surfaces of entire interfluves might be on the move, transporting sediments from slopes to streams.
The northern end of the 360-mi. uplifted block of the Sierra is punctuated by horst-and-graben structural basins that hold pastoral montane valleys and natural lakes, the largest and deepest of which is Tahoe, cupped in a faulted basin at the north-easternmost precipice of the Sierra Nevada. Its waves lap a 72-mi. shoreline at 6,229 ft. in elevation, and daily evaporation from its 193 mi.2 of surface area exceeds the volume of the Truckee River, where it pours over the outlet each day. Powerful summer thunderstorms are born in this setting by the confluence of three weather systems. Moisture-laden air masses rise off the lake and are joined by air masses moving eastward from the Pacific. These masses cool and condense as they ascend eastward over the final pitch of the Sierra. In summer, differential heating of the earth’s surface in the nearby high desert sends hot air aloft into this mix. This can result in fierce and spatially unpredictable turbulence, which produces brief but intense localized thunderstorms.
During such events, individual raindrops strike the rounded grus and bounce them into the air. The particles fall back downslope, knocking others loose. Fine materials splashed about by the dashing rainfall clog the small openings between particles, and almost every drop of moisture hurled to the ground results in runoff. Rapid runoff on the steep slopes detaches the deeply weathered, loose, and friable granite bits. Rills result, which translocate sediments from hillslopes to footslopes and channels. Once in these channels, they head for the lake.
Heavenly Resort (www.skiheavenly.com) operates 4,800 ac. (7.5 mi.2) of skiable terrain in this dynamic setting. “It is a landscape that is steep and full of energy,” says Heavenly’s planning director, Andrew Strain. The high-energy summer events, he notes, can be “land-altering sediment movers and watershed unravelers – events that change the landscape.” Strain points out that highly erodible conditions are present in both disturbed and undisturbed areas. “It’s a tough situation to begin with.”
Heavenly opened in 1955, at a time when population was sparse in the Tahoe Basin and development was limited in scale and extent. The ski runs of the ’60s and ’70s were constructed by cutting trees and grading smooth swaths into the earth. In those days, the runs were “summer groomed.” When the runs were bare of snow, crews removed boulders, trees, shrubs, and down logs in order to extend the ski season. Where it was convenient, runoff was routed to drainage channels. This was a common practice everywhere at the time.
By the early ’70s, the declining quality of Lake Tahoe had become a basinwide issue. The lake was losing clarity at an alarming rate, and sediments from nonpoint erosion were among the pollutants targeted for control. One of the nation’s earliest best management practice (BMP) manuals was developed by basin stakeholders at this time. In 1974, Heavenly’s Summer Operations and Erosion Control Plan was developed by the Forest Service and was one of the first of its kind in the ski industry. At first it was a relatively simple document, but it expressed the resort’s stewardship commitment to public lands it used for skiing.
By the ’90s, impacts from development in the Tahoe Basin had cumulatively threatened the world-renowned clarity of the deep lake’s waters. All stakeholders were enlisted to develop and implement a regional water-quality improvement plan. Of the 16 ski areas that flourish in the dry-powder snow of the Tahoe Basin, Heavenly is among five possessing ski areas that drain directly to Lake Tahoe. The Forest Service, a major land manager in the basin and therefore a major stakeholder in the lake’s water quality, completed a cumulative watershed effects (CWE) analysis for Heavenly.
A team of 16 hydrologists, soil scientists, and other scientists from six national forests, the Natural Resources Conservation Service, the US Geologic Survey, and University of Nevada Cooperative Extension participated in the CWE analysis for Heavenly. They sought to identify a threshold of concern, expressed as equivalent roaded area, above which water-quality impacts to downstream beneficial uses could not be tolerated.
First the team identified beneficial uses at risk of degradation from sediments eroded from the ski area. These were the ultimate receiving waters of drainage from the resort: Lake Tahoe (a total maximum daily load water body), the Carson River, and their fisheries. Soil loss from roads and rills in a handful of sample plots was calculated using Agronomy Tech Note No. 6 from the Soil Conservation Service. This protocol involves laying out a linear transect 84 ft. long in each plot. The depth and width of each rill or gully along the transect are measured. The number of tons per acre of soil loss for the plot is derived by dividing the sum of square inches by 84.
Sediment production from all ski runs and road segments was calculated using a modified version of the Universal Soil Loss Equation. To estimate sediment delivery to streams from steep, forested lands, the team used an adaptation of this equation.
Rates of sediment delivery to defined channels were derived using a complex model for which the following data were gathered: acres of disturbed ground for each run and road prism (including cuts and fills); K-factors for soils; precipitation amounts for the various aspects and elevations on the mountain; slope gradients, lengths, and shapes; canopy and ground cover; surface roughness and soil texture; the available water to transport eroded material; the delivery distance measurement for overland flow traveling to defined channels; and the percentage of fine roots below the soil surface (assumed to be equal to the area occupied by vegetation aboveground). The model calculated that the highest rates of sediment delivery to channels came from runs and roads.
Each of the resort’s 10 subwatersheds was stratified to derive its sensitivity to disturbance. Four levels of sensitivity were selected based on (1) physical characteristics, including soils, topography, climate, vegetation, geology, and channel conditions; (2) ambient sediment yield; (3) location of the watershed relative to Lake Tahoe; and (4) a qualitative evaluation about the ease or hardship of revegetating disturbed ground in each sensitivity zone. Finally, disturbance coefficients for each sensitivity zone were derived.
The CWE analysis identified a threshold of concern above which development disturbance at the resort should be closely monitored, reduced, or mitigated. The threshold ranged from 4% to 7% equivalent roaded area. This created a challenging situation, states planner Strain, because the target condition was not even present in some completely undisturbed locations on the mountain. Hydrologist Sherry Hazelhurst of the Forest Service reports that the existing rates were between 0% and 18%. Six of the 10 resort subwatersheds exceeded their respective thresholds of concern. The average resort soil is 77% sandy DG, she points out. Areas that had been bladed and shaped for roads and runs had no organic soil horizon at all. What remained, says Hazelhurst, was basically sand.
So the resort had its erosion control planning cut out for it, and because of the unique rock, soils, weather, climate, slopes, aspects, elevation, and land use of the site, there were no models to draw on. The CWE report concluded with a generic list of erosion prevention BMPs, and the resort began implementing these (see Table 1). Remediation plans and schedules were developed for each subwatershed. A 10-year plan was put in place to restore stream zones, install BMPs on roads and ski runs and around existing structures, and retrofit impervious surfaces with BMPs.
The resort immediately began an aggressive program to retire old or unnecessary roads by ripping, revegetating, and closing them. Seasonal-use restrictions for unsurfaced roads were tightened and, where possible, resort maintenance activities were scheduled to be undertaken during one vehicle entry rather than several. Drainage designs were upgraded for roads that would remain in use. Running distances for runoff were shortened by means of greater water-bar frequency. Stormwater discharges were removed from channels and fill slopes. Concentrated flows were level-spread and, wherever possible, stormwater drainage was routed into undisturbed areas for natural infiltration. Problem corners and soft spots on roads were armored with road-base material. The plan specified that each summer, primary maintenance roads would be regraded and treated with an enzyme to improve native soil cohesion and reduce erosion on them.
Cover Bare Slopes
Revegetation plays a major role in the resort’s fast-track water-quality protection plan. The CWE model assumed that 70% average vegetative cover would be adequate, according to Hazelhurst, whose Forest Service team monitors water quality and BMP efficacy at the resort. She notes, however, that this is more vegetation than is present in many undisturbed areas on the mountain. The low-nutrient, well-drained, sandy soils do not support abundant or luxurious ground cover, and getting plants started in this environment is a challenge. “We are starting with soils that have no organic layer and few, if any, microbes to help things along,” she explains. So the Forest Service and the resort experiment with revegetation. The Forest Service monitors seedbed preparation methods, seed mixes and application methods, tackifiers, mulches, and mulch application methods. Annually they carry out toe-point transects to monitor plant survival and vigor. They also monitor non-native plants. Contamination of some mulches with non-native plants has been a problem, observes Hazelhurst.
It is essential to increase surface roughness in some areas of the resort to decrease erosion. Crews pull woody material from undisturbed areas and place it where it can serve multiple functions: shorten the length of running surfaces, level-spread concentrated flows, provide miniature sediment traps, and create a source of organic material to build soils. In the community at the foot of the mountain, residents have collected pine needles from their yards and stockpiled them at the resort. Heavenly’s restoration team hand-broadcasts them on bare slopes. The community has also helped to restore toe-of-slope wetlands that have become centerpieces for local parks and greenways.
Today, road retrofitting is in full swing at Heavenly. Installing new utilities aboveground will avoid erosion problems that occur when trenches capture surface and groundwater. The resort’s snow-making system has been retrofitted with sprinkler heads so that it can be used to irrigate vegetation established on the runs. The focus of the plan is shifting to include improvement of stream zones. The first phase involves planning of sediment-capture facilities.
In winter, Hazelhurst, who grew up at the lake and is a former member of Heavenly’s ski patrol, now skis the resort, taking water-quality samples. Year-round, her Forest Service team monitors BMP performance. The results are used to refine practices in a constant feedback loop of adaptive management. As action items are checked off the 10-year plan, maintenance of BMPs becomes a larger element of the normal seasonal work plan.
Not long ago, regional studies pinpointed nitrogen as the limiting nutrient for Lake Tahoe’s water quality. Today nitrogen has hit saturation, and the limiting factor has shifted to phosphorus. This is puzzling, because researchers have found that background phosphorus levels in undisturbed control watersheds are very high. Studies are underway to determine the source of phosphorus and to test BMPs that trap it effectively.
By continuing to monitor BMPs and cooperate with new research, the Forest Service and Heavenly are developing erosion control models for local ski resorts and for the ski industry as a whole. Efforts such as these, undertaken by myriad watershed stakeholders, will determine the ultimate health of the fragile and world-renowned lake and, ultimately, the quality of everyday life in the magnificent Lake Tahoe Basin.
