A person looking down on the mountains from an airplane flying south over the Honey Lake Basin toward Reno, NV, will see a crumpled brown landscape give way to green timber as the Sierra Nevada mountain range rises steeply out of the high desert. Ranches hug the eastern footslopes of this mountain front, watered by streams whose headwaters rise on rainmaker peaks that loom thousands of feet above. As the tawny land slides by below, lakes and wetlands appear intermittently in the desert along the abrupt Sierra Front. Green pastures spread, towns bloom, and tangles of roads etch the narrow zone at the mountains’ foot where the shrub-scrub desert leaves off and the open stands of Jeffrey pine begin.
People are flocking to the eastside, as locals of the Sierra-Cascade mountains’ eastern footslope call their home. Yet, as they settle in intentionally sparse communities on the alluvial fans and open forests of the mountain front, many have unwittingly put themselves and their houses in danger of the very land-sculpting processes that created the dynamic landscape that drew them there.
Water Is Key
It is a beckoning landscape; one that offers space, blue sky, great weather, and a lingering flavor of the Old West. It possesses scenery and recreation opportunities that result from the juncture of the Great Basin with the continent’s westernmost mountain wall. And it has water. This fact impresses itself on those who first meet the Sierra Front traveling westward through places such as Panamint Valley or the Smoke Creek Desert. Here is a world of sage-dotted valleys divided by dry, mostly treeless ridges. Occasionally, an island ridge juts up, tall enough to comb water from air masses already wrung out by the Cascade-Sierra, and wet enough to support an island forest of juniper or pine in the midst of the brown sea of the high desert. But for the most part, the Great Basin is dry right down to its very bones.
So when the car prowls westward over the pass of the westernmost fault block ridge of the Great Basin, the scene below can be breathtaking. Dun-colored cheatgrass and gray sage give way to irrigated hay fields; streams meander into marshes where clouds of birds settle into the tall rushes; juniper and pine crowd the alluvial fans at the base of a mountain front whose highest peaks glisten with snow. It seems like an enchanted world in which tall, woody vegetables stand miraculously upright on impossibly steep slopes, where water conjures the color green and lends life-giving complexity to the gaunt desert.
Diversity Draws People
This biological and geologic complexity spills over into the local economies of towns and cities that dot the eastside from Mazama, WA, to Bishop, CA, supporting a diverse blend of economic activities that includes ranching, logging, mining, irrigated agriculture, transportation, resource management, municipal government, local business, light manufacturing, and tourism. People are choosing to relocate to this eastside to live affordably in aesthetic surroundings where communities retain a human scale and the mild climate allows a physical lifestyle year-round. Industry has begun to follow this wave of settlement, and for many eastside communities, business has never been better.
A glance at 1990’s census data shows populations swelling in regional centers such as Bend and Klamath Falls, OR; Yakima, WA; and Reno and Carson City, NV. In fact, Nevada has outpaced the growth of all states in the nation for the past 14 years. Paralleling this is the growth of tiny, remote towns such as Twisp, WA; Christmas Valley, OR; Hallelujah Junction, CA; and Genoa, NV. As people hurry to claim million-dollar views in these out-of-the-way places, the landscape of the Old West is undergoing a rapid transformation. Spacious subdivisions and spare mobile-home communities are springing up near exotic shade trees that once graced the grounds of ranch homes. Golf courses and estate houses are sprouting on alluvial fans. Grids of roads are being laid out on the edges of dry lakebeds. Newly constructed dirt roads thread into the sparse forests above the valley floors where, in communities from Janesville, CA, to Galena, NV, people are staking out a little piece of heaven.
Alluvial-Fan-Forming Processes Can Be Sudden
Shortly after noon on May 30, 1983, following a winter that brought 200% of normal snow pack, a supercharged joint of the snow-covered granitic Sierra batholith let loose from Slide Mountain on the flank of Mount Rose, just south of Steamboat, NV. On hearing the noise, a man out hiking with his girlfriend looked down at his watch, as he later told Patrick Glancy of the United States Geological Survey in Carson City, NV. Overhead, according to Glancy, a hang glider mistook the sound for a jet plane and watched as the landslide tore away from the mountain, careened into a lake high in the narrow canyon of Ophir Creek, and tumbled downward.Below, a couple and three friends were hanging Sheetrock in a house located on the gently sloping alluvial-fan surface of Ophir Creek, not far upslope from old Highway 395. It was Memorial Day weekend, and temperatures had climbed into the 90ÂºF range. According to the husband, whom Glancy later interviewed, one of the friends suddenly asked whether the small creek nearby always made so much noise. The husband looked up to see a wall of water and debris roaring toward them. The five ran out of the house and downhill toward their cars. The husband looked back over his shoulder to see his house catching up with him. All five were overtaken by the viscous flow. Overhead, the hang glider watched as the spectacular debris flow rushed downslope, pushing aside cars and houses in its path, and sprawled over old Highway 395, burying it 9 ft. deep in sand, gravel, and boulders.
Glancy spent 16 years studying the six-minute event on Ophir Creek, tracking down witnesses and survivors and piecing together a topology of the flow and a chronology of events. One thing he learned early on is that the maximum flow of the event was 50,000 ft.3/sec., about 25 times greater than had previously been estimated by flood-hazard mapping for the 100-year flood peak.
Intense Convective Rainstorms Are Responsible
This seeming anomaly is characteristic of alluvial-fan flood events, Glancy notes, because of atmospheric pulses that create these dynamic land-surface responses. For the most part, alluvial-fan floods derive from differential heating of the earth’s surface, a phenomenon that results in intense, localized summer thunderstorms of brief duration. During such events, soil macropores are clogged by dashing rain, and most of the precipitation results in runoff. Rapid runoff on the steep slopes detaches soils that may be loose, friable, and deeply weathered. Rills and gullies are excavated by converging waters, which swill into the main channel, mobilizing sediments in storage there and sweeping downstream in a flow of increasing viscosity and speed. Measurements of Ophir Creek after the Slide Mountain event, and a comparison of the times noted by the hang glider and the hiker during the event, revealed a flow velocity of about 20 mph.
Glancy is quick to point out that the Slide Mountain event, which involved a super-saturated chunk of the mountain letting loose “like a deck of cards,” is a different sort of event than the high-intensity, short-duration summer storms more commonly responsible for alluvial-fan flooding. But in the long run, the depositional evidence of both types of events may be very similar.
Landscapes Hold Evidence of Past Events
This evidence-lobes of sage-covered debris lapping down into desert basins from the mouths of steep canyons, huge boulders standing far out on the gently sloping alluvial-fan surfaces that could only have been rafted there on viscous debris flows-gets Glancy’s attention where development is concerned. Deposition in alluvial-fan flooding events occurs in areas of low gradient where development is increasingly taking place. Fast-moving, viscous debris flows are capable of inflicting tremendous damage on structures andinfrastructures. The hazards are not widely understood for a number of reasons, he says. Populations along the front have always been low; therefore, firsthand experience of alluvial-fan Sflooding has been limited. Planners, engineers, and others involved in land-use decisions might not have received adequate training to recognize alluvial-fan flooding hazards. Methods for accurately assessing and mapping the hazards have been lacking. Methods that work well in one environment are not always applicable to others.
Events Are Sure to Happen, Impossible to Forecast
There have been very few studies of alluvial-fan flooding, according to Glancy. There is, however, a flood chronology published in 1977 by Victor Goodman, a forester for the US Forest Service in Carson City. He devoted an entire section to dry-mantle flood events along the Sierra Front in the vicinity of the Carson River. Through archival research at the region’s newspapers, libraries, and historical organizations, Goodwin was able to piece together a picture of alluvial-fan flooding for the 115-year period between 1861 and 1976. What he found was repeated, unpredictable, summertime convective thunderstorm events along the entire Sierra Front from Susanville to the Walker River country. These events destroyed water distribution systems, covered ranches with rocks and flood debris, and repeatedly ripped out the same roads. In Kings Canyon alone, which contains the route over Spooner Summit from Carson City to Lake Tahoe, alluvial-fan flooding occurred in 1913, 1926, 1927, 1960, and 1973.
A person driving south along the Sierra Front from Galena, NV, will see a boulder-harvest operation on the fresh lobe of the Ophir Creek fan that was created by the 1983 Slide Mountain event. It is a major supplier of riprap for construction projects throughout the region, and it does not look as if it will run out of rocks at any time in the near future. Continuing south, the car climbs easily over the lobes of older fans, then drops back down to the valley. It is easy to see where maintenance crews have scraped rocks from the blacktop in places where dry washes have only recently flowed across the road. Large boulders lie far out on fan surfaces where the ringing of hammers ushers in new neighborhoods of grand houses. In the older settlements, the stories of past flood surges are told in the many rock walls neatly constructed of fieldstones along property lines and around yards.
Across the Carson Valley, the Buckbrush Wash Flood Safety Coalition is working to protect a large community from alluvial-fan flooding. The watershed of Buckbrush Wash rises in the west-facing Pine Nut Range just east of the Sierra Front. This range gets hit repeatedly by turbulent air masses that tumble over the Sierra after evaporating water from Lake Tahoe at 6,400 ft. in elevation. Differential summertime heating of the high desert tangles with this turbulent, moisture-laden air to create dynamic dry-mantle flooding conditions in the Pine Nut Range. In some of the events of the 1990s, as few as l.2 mi.2 of drainage area were involved.
Community Turns to Alluvial-Fan Mapping
The gently inclined surface of Buckbrush fan is a favored residential area, owing to its striking views of the Sierra and its proximity to Carson City and Reno. According to Steve Lewis of the University of Nevada, Reno (UNR) Cooperative Extension Service, there are about 600 properties on the fan. Damage inflected by intense summer convectional storms for several consecutive years in the 1990s led to the first phase of a flood protection plan: to identify and map flood hazard zones on the fan.
This was undertaken by Kyle House, research geologist for the Nevada Bureau of Mines and Geology at UNR. According to his 1999 report, House used a series of aerial photographs spanning the 60-year period between 1938 and 1997 to identify the presence, degree, and nature of drainage development; the degree of connectivity of distributary channels with the active feeder channel; and patterns of faulting influencing topographic separation on the fan.
House’s report noted that the areas of the fan undisturbed the longest were veneered with a layer of aeolian sand of varying thickness. He posited that the wind-blown sands derived either from a mid-Holocene drying period on the nearby Carson River floodplain or from wind deflation of vast amounts of sediment delivered to the floodplain as a result of glacial melting at the end of the Pleistocene. Given these possibilities, he assigned the fan surfaces a probable age range of 4,500-10,000 years. House also identified a fan surface of intermediate age, characterized by a thinner and less continuous cover of aeolian sands and located topographically between the areas of oldest and most recent fan deposits.
Deposits associated with recent fan activity were identified as active flood hazard zones on the fan. House mapped three facies of these deposits, corresponding to their location relative to the active fan apex and the flow energies required to transport materials of the sediment sizes present in each zone.
Channels Can Shift Rapidly
Interestingly, the active fan apex was not found at the head of the fan, as might be expected, but farther down from an entrenched channel that, House observed, had been capable of conveying the majority of discharges associated with the flooding events of the 1990s. The active fan area includes channels with the greatest potential for avulsion, or the rapid shifting of the channel from one location to another as a result of deposition or debris blockages. If a person were to observe time-lapse photography of fan development over millions of years, he or she would see the main stem wandering from one side of the fan to the other. During this wandering, short-lived distributaries would be established and abandoned as each depositional event built a bit of the fan surface.
House mapped old, intermediate, and young fan surfaces and the locations of fault scarps and lineaments that cut across the grain of the fan deposits and hence influence the behavior of flows on the fan. The map serves as a tool to help direct new development away from the active fan apex and onto surfaces on which there is no geological evidence of flood action for thousands of years.
Because there are both existing developments and serious flood hazards below the active fan apex, the coalition looked at the potential of diverting dry-mantle flood events from the main channel above the active apex and routing the debris flows to an adjacent watershed. The price tag for this option proved prohibitive. Instead, a flood warning system is being put in place that is connected with real-time rain-gauge data collected in the upper watershed. Door-knob hangers are being distributed to give citizens information about how to maintain their properties to minimize the impact of an alluvial-fan flooding event. The main activity here is keeping driveway culverts clear, and community culvert-cleaning weekends are planned. Finally, adjacent washes and fans will be studied and mapped.
Groundwater Pumping Can Create Subsidence Hazards
Arid and semiarid regions present another dynamic erosion hazard that is receiving closer attention as population booms in the Great Basin and Intermountain West. Here, where internally draining desert basins are filled with the erosional detritus of the weathering and eroding ridges that encircle them, remarkable aquifers may develop. And no wonder: Surface-water flows from the encircling mountains can quickly infiltrate the coarse alluvium at the valleys’ edge during overland flow events, percolating into extensive aquifers. These are increasingly being tapped to support flourishing communities in lands of little rainfall. John Bell of the Nevada Bureau of Mines and Geology has studied the subsidence effects of groundwater pumping in these settings. He has found that fine-grained sediments are subject to compaction and deformation as water in the interstitial spaces is removed, which results in tension cracking. As the cracks enlarge by means of mechanical piping, underground utilities can be exposed and damaged. Furthermore, the cracks provide a depressed linear route capable of capturing surface flows and concentrating their erosive energy.
Bell has observed subsidence fissuring effects in the Las Vegas basin where groundwater pumping helps to support such basic needs as drinking water and sewage systems, as well as lifestyle needs, such as swimming pools, golf courses and green lawns-demands not felt in wetter areas of the country. Bell noted that subsidence has been occurring in the basin since about 1935. Its effects include protrusion of wellheads and differential settlement of roads and structures. Although groundwater pumping has been reduced and water districts are reinjecting water into the subsurface, Annual groundwater withdrawals exceed natural recharge levels by factors of two to three, according to Bell. Recent urban development has intensified the occurrence of fissuring and structural damage in some areas of the Las Vegas basin, he observes.Bell also found subsidence on the Sierra Front north of Reno in the rapidly growing suburbs near Stead, where heavy groundwater pumping occurred during a drought in the mid-1990s, and at Honey Lake and the Amadee geothermal area south of Susanville. These findings hold implications for long-range planning where burgeoning populations will be dependent on groundwater resources at the desert’s edge.
Semiarid Zone Erosion Hazards Can Be Increased by Wildfire
Because little rain falls in deserts, most reasonable people would guess that erosion isn’t a great concern where vegetation is scant and skies are predictably blue. But places that receive between 12 and 16 in. of annual rainfall actually have among the highest sediment production rates in the world. This is because vegetation is generally sparse in areas with this level of rainfall, but precipitation is sufficiently frequent and intense to produce significant interfluvial erosion. This fact is easy to see by looking at any planimetric map that spans the eastside ecotone between well-watered mountain slopes and the adjacent semiarid zone. Drainage density will be higher in the semiarid zone.Fire danger is enormous in this ecotone, which includes much of eastern California and western Nevada, according to Ed Smith of UNR Cooperative Extension Service. The mature big sagebrush and bitterbrush shrub communities common to this area are tall, dense, and full of dead wood. If they are ignited on a hot, windy August day, they are capable of supporting intense, fast-moving wildfires that can generate flame lengths in excess of 50 ft. Areas where sagebrush and bitterbrush interfinger with open stands of juniper and Jeffrey pine are increasingly prized for residential development, he explains, because of their position above the valley floors and potential for great views. But the fire hazard in this zone is extreme. “This vegetation type has supported some of the most horrific wildfires along the eastern Sierra Front,” says Smith, “and the scariest thing is knowing that those unburned patches are the next ones to go.” When the big burn areas are plotted along the Sierra Front, “you can pretty much connect the dots,” he observes. When wildfire does sweep through the suburban/wildland interface along the Sierra Front, “erosion is a train coming down the track,” Smith remarks. After fire has destroyed the vegetation cover, the coarse, decomposed granite soils are exposed on steep slopes where sheetwash and rill erosion in the wake of a typical summer convective storm will carry them away. Soil productivity is reduced by this erosion, and regrowth of persistent woody vegetation is slow. Cheatgrass or other non-native plants may move in, displacing other plants and degrading wildlife habitat. Chronic wind and water erosion may continue for decades.
People continue to choose to live with the threat of fire in these areas. And ironically, fire-safe practices to maintain defensible living spaces might be the long-term ticket to holding soil on these highly erodible, semiarid slopes. By managing amount, type, and arrangement of vegetation on different slope classes, communities can minimize the risk of uncontrollable wildfires and, in doing so, literally hold back the sands of time.