The cold-desert climate, with its cold, wet winters and springs and dry, hot summers, predisposed many sagebrush steppe communities to an evolutionary history with recurring fire.  Estimates of fire return intervals for sagebrush steppe range from ca. 20 to 100 or more years (Houston 1973, Wright et al. 1979, Wright and Bailey 1982).  Wright et al. (1979) surmised that the interval between fires must have been sufficiently long for big sagebrush, which does not resprout and must recolonize burned sites from seeds, to regain dominance;  otherwise, the extensive areas dominated by sagebrush would have been dominated by root-sprouting shrubs such as horsebrush or rabbitbrush.  Nevertheless, it is clear that fire played an important role in the evolution of many plant species that comprise cold desert communities.  The vast majority of shrubs and perennial grasses and forbs can survive wildfires, especially fires that occur in late summer or fall when many plants are dormant.  Some species respond vigorously to postfire conditions (Wright et al. 1979, Cole 1987, Ratzlaff and Anderson 1995).

Numerous fire scars are apparent in satellite images of the INEEL and vicinity.  These abrupt linear features generally trend along the prevailing wind directions from southwest to northeast.  They mark abrupt changes in albedo that result from differences in soil surfaces and vegetative cover.  The scars of 10 fires that are known to have burned during this century have been mapped from satellite imagery or aerial surveys.  These range in size from a few to over 6,900 ha (17,000 acres).  Wildfires have been aggressively controlled at the INEEL since 1950, which may have decreased the area that otherwise would have burned.  Nevertheless, the two largest known fires at the INEEL occurred in the last two years.  The largest, ignited by burning rubber from a flat tire on a horse trailer, occurred in early July of 1994;  it started near the junction of Highways 20 and 22 on the western boundary of the INEEL and burned to the northeast across “Deadman Flats,” traversing some 25 km just inside the INEEL’s western border.  The conditions for a large fire likely were established by a very wet growing season in 1993, which resulted in the accumulation of abundant fine dry fuels that persisted through the summer of 1994.  The second of these recent fires burned some 2,500 ha (6,000 acres) near the Argonne National Laboratory West facility in August of 1995.  It was ignited by a cigarette dropped from a vehicle on Highway 20. 

The next largest fire burned approximately 1,400 ha in the Tractor Flat area on the east side of the INEEL.  This fire is thought to have occurred in 1910, which is likely because the summer of 1910 was one of the most severe fire seasons in the recorded history of the region.  It has been estimated that 2,500 fires were burning simultaneously in central and northern Idaho and western Montana during that year and that about 1.2 million ha (3 million acres) of forests were burned.  Vestiges of the Tractor Flat fire scar can be distinguished on the satellite image (Figure 2).  The scar of another large fire, which burned about 1,200 ha in 1949, is readily identified on the satellite image.  It lies just north of Highway 20 near the junction of Highways 20 and 26.  The bright triangular feature at the junction is an area that was plowed and seeded to crested wheatgrass in the late 1950’s. 

The Principal Lineament.  A distinct linear feature, which became known as the principal lineament, was conspicuous on the earliest aerial photographs of the INEEL.  This feature is  discernible on the satellite image as a dark line extending north and south from the corner of the INEEL boundary just northeast of East Butte;  it lies along the eastern edge of a large area of relatively high albedo that extends to the northeast from Middle Butte.  Most early investigators assumed that this feature was of geologic origin (see Morin-Jansen 1987 for a review of hypotheses).  One hypothesis was that a lateral blast from Middle Butte deposited a layer of ash that radiated to the northeast.  Walker (1964) described the principal lineament as “a prominent and continuous rift which can be traced for 12 miles.”  Bonilla and Chase (1967 as cited by Malde 1971) noted that the lineament extended for about 17 miles northward from East Butte and argued that it was not caused by a fault but was a surface feature that probably formed during volcanic episodes.  Malde (1971) determined that the lineament was not a result of basalt emplacement or of a fault.  He concluded that the feature consisted of an anomalous strip of sand on the surface that increased infiltration and storage of water.  The improved water availability resulted in a corresponding strip of lush vegetation.  Connelly et al. (1984 as cited by Morin-Jansen 1987) postulated that the lineament was the remnant of a cattle trail.  They found little difference in the plant communities on either side of the lineament, but reported that sagebrush cover was about 25% lower to the west than in the areas to the east.  Downs (1984 as cited by Morin-Jansen 1987) suggested that the principal lineament controlled the eastern edge of a fire.  Downs thought that higher water content of the vegetation growing over a perched water table along the lineament may have stopped the fire. 

The principal lineament is indeed a surface feature that can be readily distinguished from surrounding vegetation.  Its main features were summarized by Morin-Jansen (1987) as follows: It is characterized by accumulations of sand on the surface, by stands of Great Basin wildrye in topographic depressions and by vigorous stands of big sagebrush on higher ground.  Its location is independent of topography;  it crosses depressions, volcanic vents, lava lobes, lava channels, and pressure ridges along its 27.8 km course.  The lineament is somewhat sinuous, varying in width from 20 to 125 m.  Its western boundary is distinct, but the eastern edge is diffuse and scalloped, consisting of northeast-trending sand lobes that have been deposited by wind. 

The most reasonable hypothesis to account for the principal lineament is that it represents the eastern edge of a fire scar where wind-blown sediments accumulated.  The higher albedo to the west of the lineament is likely a consequence of lower sagebrush cover and, perhaps, postfire erosion of sandy sediments from the soil surface.  The extent of this area is clearly visible on aerial photographs and satellite images (see back cover).  Assuming that this hypothesis is correct, this would have been a fire of similar magnitude to that of the 1994 Deadman Flats fire.  It appears that the fire burned in a more northerly direction than the prevailing southwest to northeast wind direction;  thus, the stage was set for substantial accumulations of wind-blown sediments, which, in turn, resulted in higher infiltration of moisture and higher availability of water for plant growth.  Similar accumulations of sand to depths of 30 to 50 cm were observed in August of 1994 along the eastern side of the Deadman Flats burn.

When did the fire that formed the principal lineament occur?  The fact that no charcoal has been found on the putative fire scar, the observation that sagebrush plants on both sides of the lineament are the same age (Morin-Jansen 1987), and the fact that no written record of a large fire in that area has been found suggest that it occurred prior to settlement of the region.  It seems likely that it burned sometime during the 1800’s, assuming that differences in vegetation and soil reflectance would gradually disappear.  One thing is clear, assuming that the fire hypothesis is correct:  disturbances due to wildfire can have effects that persist for a very long time in cold-desert communities.