EROSION OVER TIME 

ON SEVERELY DISTUR - 

IC SOILS: A MODEL 

i \ 

1 USDA Forest Service 

Research Paper l NT-156, 1974 

INTERMOUNTAIN FOREST & 

RANGE EXPERIMENT STAT13 

507 - 25th S~reet 

Ogden, Utah 84401 
THE AUTHOR 

WALTER F. MEGAHAN is Principal Research Hydrologist and Leader of 

the Idaho Batholith Ecosystems Research Work Unit in Boise, Idaho. 

He holds bachelor's and master's degrees in Forestry from the State 

University of New York, College of Forestry, at Syracuse University, 

and a doctoral degree in Watershed Resources from Colorado State 

University. He served as Regional Hydrologist for the Intermountain 

Regionof the USDA Forest Service in Ogden, Utah, from 1960 to 1966 

and has been in his present position since 1967. 
USDA Forest Service 

Research Paper INT- 156 

September 1974 

EROSION OVER TIME 

ON SEVERELY DISTURBED 

GRANITIC SOILS: A MODEL 

W. F. Megahan 

INTERMOUNTAIN FOREST AND RANGE EXPERIMENT STATION 

Forest Service 

U. S. Department of Agriculture 

Ogden, Utah 84401 

Roger R. Bay, Director 
CONTENTS 

Page 

INTRODUCTION. ........................ .I 

EXPERIMENTAL RESULTS .................. .5 

Road Prism Erosion .................... .5 

Road Fill Slope Erosion. .................. .8 

Role of Rainfall Energy and Vegetation. .......... .9 

LITERATURE CITED. ..................... 14 

A negative exponential equation containing three parameters was derived 

to describe time trends in surface erosion on severely disturbed soils. Data 

from four different studies of surface erosion on roads constructed from the 

granitic materials found in the Idaho Batholith were used to develop equation 

parameters. Rainfall-intensity data were used to illustrate that variations 

in erosion forces, as indexed by rainfall kinetic energy times the maximum 

30-minute rainfall intensity (the erodibility index), were not the cause of the 

time trend in surface erosion. In addition, although vegetation growth can be 

an important factor in reducing accelerated erosion, it did not cause the rapid 

erosion decreases found in the cases studied. The evidence suggests that 

surface "armoring" was the dominant factor causing the time trends in surface erosion. The significance of time trends in surface erosion is discussed. 
INTRODUCTION 

Surface erosion can be defined as movement of individual soil particles along the 

soil surface, in contrast to mass erosion, the movement en masse of numeraus particles. 

The subject of this paper, surface erosion, can be expressed as 

where E = surface erosion rate; h = inherent soil erosion hazard and includes such 

factors as soil detachability and slope gradient; p 3 protection afforded the soil 

surface by vegetation and litter; and f = force applied by ~aindrsps, overland flaw, etc. 

Under a relatively stable farest ecosystem, erosion hazards tend to be minimized 

and protection afforded the soil surface tends to be maximized. Erosion forces vary 

but, over time, yield some average (or other appropriate measure of central tendency) 

value for surface erosion, which varies with individual site conditions, This average 

erosion rate (s,) has been termed the "normlf for the site in question (B;liley 1941). 

Severe soil disturbance disrupts the ecological balance, thereby increasing h and 

decreasing p. Consequently, surface erosion rates are accelerated, often to alarming 

 proportion^ (Anderson 1954; Reinhart and others 1963; Packer 1967; Copeland and Packer 

1972) , Over time, a new ecological balance is established on the disturbed site that 

leads to a new erosion norm, which may differ from the original because of permanent 

changes in site factors. Thus, on a severely disturbed site, the total erosion rate at 

any time (E~) is comprised of two components, the natural non for the site (E,) plus 

the accelerated erosion due to disturbance (E~). Therefore, 
Considering the accelerated erosion component (E~), a body of disturbed soil, (S) 

can be assumed, where disturbance has been severe enough to completely disrupt the 

original soil characteristics. Such soil is a polydispersed mixture with subsoil 

and bedrock fragments well graded throughout, with no remnants of the original soil 

horizons or structural development, and with no surface vegetation or litter. Such 

disturbances have numerous causes such as road construct ion, mining, logging, and 

landslides. 

Relation 1 above reduces to 

if it is assumed that f is constant and p is zero (i.e., there is no protection available from vegetation, litter, mulches, etc.) . Under such conditions, it is logical to 

assume that the opportunity for erosion is maximum immediately after disturbance and 

decreases with time. In other words, the erosion hazard decreases with time after disturbance. A number of causal factors might exist. For example, in a polydispersed 

mixture, the more easily detached and transported particles are most likely to be removed first. Over time, the average erodibility of the surface materials will tend to 

decrease. This "armoringfl or I1erosion pavement" phenomenon is commonly observed and is 

probably the result of an increase in the average size of surface particles, possibly, 

keying in of surface particles (fig. I), or both. Other factors, such as compaction of 

materials as settling occurs and reduction in slope gradients, may also help reduce 

erosion hazards over time. 

If erosion hazards do indeed decrease following disturbance and relation 3 is 

valid, the rate of accelerated erosion can be assumed to be directly proportional to 

erosion hazard (E ah). Further, erosion hazard is greatest when the disturbed soil 

S body is greatest and decreases as the body of soil decreases. Thus, erosion hazard 

appears to be directly proportional to the amount of soil present (has). Assuming 

linearity, constants of proportionality can be assigned to the above proportions, 

But, E~ is simply the rate of change of S, 

The negative constant of proportionality is used 

for S and integrating, 

based on the boundary condition that S = So when 

(5 

to indicate soil loss. By solving 

t = 0 . Substituting in 4 and 5, 

(6) 
Figure I. --Photograph A was taken 

shortly after construction. 

later. Note the increase in 

on a road fill on Interstate 90 in northern Idaho 

The same plot is shown in photograph B, 4 months 

average surface particle size resulting from erosion. 
In ahis farm, e represents a loss from the body of soil S and sa is a fiegative 

value. Harever, thig is not consistent with equation 2 where cg is p~~ltive. T~US 

equation 6 is modified to: 

dkt 

E: --" kSOe 

S ( 7) 

Finaf ly, when squatf on 7 is substituted into equation 2, the total erosiofi rate OR a 

disturbed site is 

-kt ct = E + ksOe , n E 8) 

where E:~, So and k are parameters to be determined. 

Interpretations 

~~(trns mid2day-I) = 

of these parameters and the units used in this ~eport are: 7 

The erosion rate to be expected on the disturtied area after a long 

pes3od, assuming no new major distu&arrce, This value is an -4 

essfmre sf the long-term norm for the site. 

the amount of material available to be e~oded at time zero after 

ctisturbance. 

m kadex of the rate of deckkne of erosion f0:f lowirig dtsturbmce, 

This ca;n: be thoxght of as an index of the recovery potential for 

the site in qeresetom, 

By measu~ing erosim rates on disttm%ed sites over time, values can be assigned to 

these parameters. Howewer, masuremento of instantaneous erosion rates are extremely 

difficult to obtain. Most sfudies of erosion depend upon periodic measurements of 

on-site soil los3es m of accumulations of eroded material at some downslope paint. 

Such data provide a measure of erosion integrated over ti-. Accordingly, equatio8 msr be integrated in mder to develop parameter estimates from periodkc' rneasuremrits 

of erosion. Integration of equation 8 with respect to time yields the model to be 

fitted, 

where ri is in units of tons mim2 and t is the days of elapsed time since disturbance. e 
EXPERIMENTAL RESULTS 

The Forest Service has been studying the effects of severe soil disturbance on 

surface erosion and sedimentation in the Idaho Batholith for a number of years. The 

Idaho Batholith is an extensive mass of granitic rock (16,000 mi2) covering a large 

portion of Idaho and parts of Montana (fig. 2). Various types of granitic rocks are 

found; however, quartz monzonite predominates. The bedrock exhibits wide variation 

in degree of weathering and fracturing (Clayton and Arnold 1972. The area is almost 

entirely mountainous and forested. Typically, shallow, coarse-textured soils are found 

on steep slopes that often exceed 70 percent gradient. Soils of this type are extremely 

erodible (Anderson 1954 ; And$e and Anderson 1961) . Localized, high-intensity rainstorms 

of short duration are common during the summer season. At other times of the year more 

general storms occur, often in conjunction with melting snowpacks. Following soil disturbance, the combination of steep topography, high-soil erodibility, and high-climatic 

stress often results in dramatically accelerated surface erosion rates (Megahan and 

Kidd, 1972). 

Data available from four different studies (fig. 2) conducted in the Idaho Batholith 

provide insight into the time trends in surface erosion following road construction. 

Two studies in the Deep Creek and Silver Creek study areas present data for erosion from 

the entire road prism (cut slopes + roadbed + fill slopes]. The other two studies, 

located on double-lane forest roads in the Bogus Basin and Deadwood River areas, were 

designed to measure erosion on road fill slopes only. 

Road Prism Erosion 

Deep Creek Study: --In the Deep Creek study (fig. 2) , about 0.2 mile (providing a 

total area disturbed of 1.1 acres) of low-standard logging-access roads was constructed 

in two small, ephemeral drainages totaling 5.2 acres. Slope gradients in the drainages 

average about 70 percent. Material eroded from the entire road prism was trapped in 

sediment dams located about 300 feet downslope. Data from the sediment dams were collected twice a year: following the snowmelt period (about June 1) and near the end of 
Figure &.--Location of study sites in the Idaho Batholith. 

6 
the water year (about September 30). Sediment accumulations behind the dams were surveyed by using a grid of closely spaced cross sections. The study was designed so that 

road erosion could be segregated from erosion occurring on unroaded portions of the 

drainages (Megahan and Kidd 1972). The roads were constructed in November 1961; collection of postconstruction data was continued on schedule until September 1967. Twelve 

measurement periods were sampled during the 6-year study. 

Accumulated surface erosion data are plotted in figure 3. Notice that eroded 

material accumulates rapidly immediately after disturbance, but that the rate of accumulation decreases with time. Equation 9 was fitted to the data by using a nonlinear 

least-squares procedure (Marquardt 1963) . The fitted regression line and the deri3ed 

equation are shown on figure 3. The relation has a standard error of 502 tons miand an r2 value of 0.98. Nonlinear confidence limits for model parameters indicate 

that all parameters are significantly greater than zero at the 95 percent level. 

Figure 3. --Accwmc Zated 

surface erosion over 

time--Deep Creek Study 

area. 

r, = 3.5t-54807.2 [e-0.00956t-~ ] 

standard error = 502 tons mi-* 

I STUDY DATES 

ACCUMULATED ELAPSED TIME SINCE DISTURBANCE (Days) 
&her Creek Study:--Other study data are available to check the validity of the 

long-term erosion rate (E ) determined from the Deep Creek data. Beginning in 1965, n annual sediment pr.oduction data have been collected from each of seven study watersheds in the Silver Creek area of the Payette River drainage (fig. 2). Six of the 

study watersheds are essentially undisturbed; the seventh, Ditch Creek, contains a 

low-standa~d forest road about 2 miles long that was constructed in 1933. The road is 

comparable to the Deep Greek road in that it is low standard, narrow, closely fitted 

to the landscape, and tyaverses similar topography for most of its length. 

Avevage sediment yields for Ditch Creek are greater than for any other watershed 

in the Silver Creek study area. However, based on watershed characteristics, Ditch 

Creek should rank approximately in the middle of the seven watersheds in terms of 

sediment production, No doubt, the increased sediment yields in Ditch Creek are due 

to erosion on the Ditch Creek road. This statement is borne out by inspection on the t 

ground; erosion is occuring on the road tread, on the cut slopes, and in isolated 

instanceg on the fill slopes (Megahan 1972). 1 

Lbgically, the undisturbed sediment yields from the 0. 41-mi2 Ditch Creek drainage 

would fall between the sediment yields from the two watersheds that flank it (Cabin 

Creek, 0840 mi2, and Eggers Creek, 0.51 mi2), because it is intermediate in respect to 

forest cover, sideslope gradient, channel gradient, and watershed size. Thus, the 

average of sedifient yields from the two undisturbed drainages provides a reasonable 

estimate of what the sediment yield for Ditch Creek would be if the watershed were in 

the undisturbed state. This amount can be subtracted from the measured sediment yield 

to obtain an estimate of the sediment yield caused by the old road. Finally, this 

amount can be prorated back to the area actually disturbed by road construction to 

provide a measure of annual road erosion, Based upon 7 years of data, annual road 

erosion ranges from 0.4 to 9.7 tons mi-2 day" and averaged 3.5 tons mi-2 day-'. The 

average erosion rate for this 37-year-old road is equal to the value of E derived from 

n the Deep Creek study. Such values would be expected to vary with site conditions. 

Consequently, such good agreement is likely to be more coincidence than fact, but it 

does illustrate that tBe E value predicated from the Deep Creek study is reasonable. 

n 

Road Fill Slope Erosion 

Bops Basin Study:--Other studies on granitic materials in the Idaho Batholith 

suppart the time-trend results reported above. These were designed to measure erosion 

rates bn plots ifistalled directly on steep road fills. One study was established in 

the fall 09 1962 on the newly constructed Bogus Basin highway leading from Boise, Idaho, 

ta the Bogus Basin Ski Resort (fig. a), The study was designed to evaluate the 

effecti~mess of various kinds of erosion-control measures and included a 1/100-acre 

unvegetated control plot that received no treatment. Erosion data were collected during 

the first year after construction: the first measurement was taken at 17 days and the 

last at 322 days (Bethlahmy and Kidd 1966). 

'I1 Accumulated erosion data are summarized in figure 4, These data appear to fit the 

postulated eiosi~n model; consequently, nonlinear regression was used to fit eguatiof 9 

ta the data. The fitted regression model has a standard error of 780 tons mi- day- Y 

and an r2 value of 0.98. All model parameters are significant at the 95 percent level I 

of 66fifidehde. 

Deadood ~iver Study : --Additional data (unpublished, USDA Forest Service, Intermourn aifi Fo~esC and Range experiment Station, Forestry Sciences Laboratory, Boise, Idaho) 

are available to check the value for E~ determined from the Bogus Basin data. These 

were c01Se~ted on another road erosion study conducted on the Deadwood River drainage 

(figi 21, The study was installed on the Deadwood road on a fill slope where site 
Figzae 4, --A~cwrmZated s~osion us 30,000 

tima--Bops Bagin 

Stud8 Area;, 

I standard error = 780 tons 

I I 1 

1 00 200 ~~ 

ACCUMULATED ELAPSED TIME SINCE DISTURBANCE (Days) 

conditions were similar to those on the Bogus Basin road, Numeraus attempts to revegetate this Toad fill by reseeding were unsuccessful because of harsh site conditions. 

?Wee Wvegetated, 1/200-acre control plots were available on this study to catnp8Pe to 

the Bogus Basin study. However, the Deadwood road fill was constructed in 1957; thus, 

the erosion data collected ddring the 1968-1972 sample period represent erosion rates 

expected from a 12- to 15-year-old unvegetated road fill, The average of these data 

should provide an approximate check on the E value computed from the Bq@s Bysin data. 

The 3-year annual erosion average on the ~eazwood plots is 9.1 tons mi- day- as 1 compared to the en value on the Bogus Basin plot of 17.2 tons mi-2 day- . The lower 

value for the olde~ study area could be caused by site differences, a tendency far 

erosiofl rates ta cmtinue to decrease beyond the time period used to fit the regPession 

Ihe, or both. In any case, considering the large variability usually associdted with 

erosion data, the value for cn is again reasonable. 

Role of Rainfall Energy and vegetation 

?'he de~i~a~iofi of the erosion model was based upon the asswiiptiofis that the erbsive 

&PG~$ 8ie constant over time and that nd surface protection is available fram vegeta- 

€303 and litter, Obviously, erosive forces afe not constant over time in natdre; 

cm%oquefitly, the tzme tfend in erosion may in fact be due td time trends in er6Siire 

f @g.c99, 
Wischmeier and Smith (1958) showed that the erodibility index, defined as the 

kinetic energy of rainfall (foot-tons per acre-inch of rainfall) times the maximum 

30-minute rainfall intensity (inches per hour) was a reliable index of the forces 

available for surface erosion. In addition, they developed a procedure for calculating 

the erodibility index that utilized rainfall-intensity data. An intensity rain gage 

was operated from early spring to late fall directly on the Deep Creek site for the 

entire study. During the remainder of the year, an intensity rain gage was operated at 

a similar elevation about 2 miles east of the Deep Creek study area. 

The rainfall-intensity data were used to develop erodibility-index values for all 

storms occurring during the course of the Deep Creek study. No calculations were made 

when a snowpack existed because the erodibility index is irrelevant during such times. 

A degree-day procedure was developed for predicting the daily fluctuation of the snowpack at the study site. This procedure was based upon a detailed analysis of 6 years 

of snow-cover data collected monthly throughout the winter at eight sites in the 

vicinity of the study area. Wet adiabatic lapse rates were used to adjust the mean 

daily temperatures at the climatic station in the town of McCall, about 20 miles southwest of the study area, to those at the study site. Finally, a computer program was 

developed to predict daily snow accumulation and melt at the study site based upon 

daily precipitation and temperature. 

The average erodibility index for each erosion-measurement period used in the 

study was computed by summing the erodibility indices for each rainstorm that occurred 

during snowless days and dividing by the number of days in the period. These data were 

then compared to the average rate of erosion for each measurement period (fig. 5). 

STUDY DATES 

@ NOTE - EACH CIRCLED VALUE IS THE MAXIMUM 

ERODIBILITY INDEX FOR ALL STORMS IN EACH PERIOD. 

h PERIOD OF LOGGING 

Figure 5. --Variations in erodibi Zity indices and erosion 

over time--Deep Creek Study 

Area. ELAPSED TIME SINCE DISTURBANCE (Days) 
The instantaneous rate of soil erosion is also shown as a continuous curve on the 

figure. This rate was developed by differentiating the nonlinear regression fit of 

equation 9. Notice there is no time trend in the erodibility index similar to that 

shown for erosion in figure 5. The maximum erodibility index for all storms in each 

period is also shown on the figure; again no time trend is evident. These data suggest 

that variation in erosive forces did not have a primary influence on the time trend 

in erosion found on Deep Creek. 

As one might expect, all vegetation within the area affected by road construction 

on Deep Creek was obliterated at the time of construction. Because of limited natural 

revegetation, the road remained essentially in this condition until after the study 

watersheds were logged a year later in 1962. At this time, standard postlogging 

erosion-control measures (water bars and grass seeding) were installed on the roads 

(fig. 5). Because of the time of year (November), no appreciable vegetation growth 

occurred until the following growing season: almost 2 years after construction. Notice 

in figure 3 that erosion had receded considerably by the end of the first year after 

construction, an indication that vegetation growth and installation of water bars was 

not the primary cause of the recession. 

Unfortunately, no recording rain gage was operated at the Bogus Basin study area. 

However, climatic data are available from two nearby stations (Deer Point and Boise 

Airport) on each side of the study site. Data from these stations were adjusted to the 

elevation of the study site to develop erodibility indices for snowless days for each 

erosion-measurement period in the study (fig. 6). Snowless days were determined by 

using a procedure similar to that used on the Deep Creek study area. Note there is no 

time trend in either the average or the peak erodibility index that corresponds to the 

time trend found for erosion. Vegetation growth on the Bogus Basin plot was not a factor because essentially no vegetation growth occurred on the plot surface during the 

course of the study. 

STUDY DATES 

Figure 6. --Variations in erodibility ind<ces and erosion over 

time--Bogus Basin Study Area. 

NOTE- EACH Cl RCLED VALUE IS THE 

MAXIMUM ERODlRlLlTY INDEX FOR 

ALLSTORMS IN EACH PERIOD. 

ELAPSED TIME SINCE DISTURBANCE (Days) 
Based upon experimental data on granitic materials from four different study sf tes , the derived exponential model for surface erosion over time appears to be appropriate. 

The model is useful to persons working in the area because it provides an estimate of 

the quantity and timing of surface erosion following disturbance. The need for such 

estirnptes is appreciated when surface erosion rates on disturbed lands are compared to 

those on undisturbed lands, Availability of a considerable bdy of sediment-yield 

data makes possible an estimate of the average erosion rate for undisturbed lands 

similar to the lands disturbed by road construction in these studies (Megahan 19721, 

The erosion rate for undisturbed lands averages about 0.07 ton mi-2 day-'. FOP the 

first year pfter disturbanee, ~rosian rates per unit of area involved in mad canstruetioq aTe three orders of magnitude greater than those on similar undisturbed lands: 

after almost 40 years, they are still one order of magnitude greater, The potential 

for &image by such accele~ated erosion should be apparent. 

Fortunately, accelerated erosion of this magnitude need not be accepted as an 

inevitqble consequence of severe soil disturbance because it is possible to reduce the 

erosiop with a variety of treatments, However, surface erosion follows an expanential 

relatinoship such that by far the largest percentage of soil loss occurs within 1 to 2 

years after disturbance. Thus, erosion-control measures must be applied immediately 

after disturbance to be effective. A common approach is to reseed disturbed areas. 

Howevep, seeding alone will have minimal effect because of slow initial grawtb 

respqnges; additional protection must be provided on the soil surface during the 

critical high-erosion period, Studies have shown that treatments such as mulching and 

transplanting on granitic road fills can reduce total soil losses dramatically, For 

example, Bethlahmy and Kidd (1966) yeported that soil losses on the Bogus Basin study 

plots we+e not reduced at all by grass seeding alone, but were reduced about 70 and 

98 percgnt when seeding was accompanied by mulching and mulch plus netting, yespectively? Soil losses from erosion plots on the Deadwood road were reduced 98 percent 

by combfining ponderosa pine planting with a straw mulch plus netting. 
As with most studies where results are related solely to time, the true causes of 

the time trends are not identified. The studies reported here simply were not designed 

to evaluate the causes of time trends in erosion, but they were conducted in a manner 

that helps eliminate some likely causal factors. Obviously, variations in erosive 

forces will tend to modify erosion rates over time. However, these studies suggest 

that a widely used index of erosive forces (the erodibility index) was not well 

related to the time trends in erosion, at least during the initial period of rapid 

recession. Time trends in other erosive forces, such as wind or seepage flows, are a 

possibility, although no such effects were noted. Numerous studies attest to the 

ability of vegetation and mechanical measures to reduce erosion. Such treatment measures were applied late in Deep Creek and not at all on the road fill studies, which 

eliminates them as a cause of the rapid decrease in erosion. The foregoing suggests 

that variations in erosive forces (f) and protection (p) as defined in equation 1 did 

not cause the time trend. Therefore, it is postulated that the dominant causal factor 

was "armoringtt of the erosion surface. 

Wischmeier and others (1971) developed a procedure for estimating soil losses from 

farmland and construction sites based upon the universal soil-loss equation. No time 

trends in erosion were indicated by these studies. However, the study procedure and 

the fact that soil disturbance was not severe enough to include underlying parent 

material may have affected the results. Studies by other researchers suggest that 

time trends in erosion do occur under other soil and geologic conditions. Vice and 

others (1969) reported such trends following suburban highway construction in a 4. 54-mi2 

drainage basin in Virginia. In this case, soil disturbance ultimately included about 

11 percent of the study drainage. Anderson (1972) found decreasing time trends in 

sediment from poorly logged areas in California following severe flooding. Such areas 

contained logging roads next to channels and landings in draws. Sedimentation resulting 

from surface erosion on logging roads in Oregon decreased rapidly after an initial high 

rate following construction (Fredriksen 1970) . 

As is true of the Idaho studies reported here, the actual processes causing the 

erosion decreases with time were not evaluated. However, one additional study in West 

Virginia was more definitive; Reinhart and others (1963) found that sedimentation 

increases following logging resulted from erosion on skidroads. They observed the 

erosion process over time and reported: 

The impact on water quality was greatest during and imediately after 

the logging operation. . . . Repeated disturbance during logging 

continually brought to the road surface a new supply of fine soil 

particles. Erosion decreased rapidly after logging, due first to the 

development on our soils of a partial erosion pavement (a surface cover 

of small stones) and later to vegetation growth on roads. 

Obviously, the coefficients developed for the studies in Idaho will not apply elsewhere, but apparently the form of the basic time trend relationship is appropriate. It 

is important to note that the relationship applies only to surface erosion fizzawing 

disturbance : any additional disturbance will initiate a new surface erosion cycle. 

Furthermore, it applies only to surface erosion, not mass erosion. 
LITERATURE CITED 

Anderson, H. W. 

1954. Suspended sediment discharge as related to streamflow, topography, soil and 

use. Am. Geophys. Union Trans. 35:268-281. 

Anderson, H. W. 

1972. Major floods, poor land use delay return of sedimentation to normal rates. 

USDA For. Serv. Res. Note PSW-268, 4 p. 

Andr6, J. E., and H. W. Anderson 

1961. Variation of soil erodibility with geology, geographic zone, elevation and 

vegetation type in northern California wildlands. J. Geophys . Res . 66 : 

3351-3358. 

Bailey, R. W. 

1941. Land erosion--normal and accelerated--in the semiarid West. Am. Geophys . Union Trans., Part 11, p. 240-250. 

Bethlahmy, N., and W. J. Kidd, Jr. 

1966. Controlling soil movement from steep road fills. U. S. For. Serv. Res. 

Note INT-45, 4 p. 

Clayton, J. L., and J. F. Arnold. 

1972. Practical grain size, fracturing density, and weathering classification of 

intrusive rocks of the Idaho Batholith. USDA For. Serv. Gen. Tech. Rep. 

INT-2, 17 p. 

Copeland, 0. L., and P. E. Packer 

1972. Land use aspects of the energy crisis and western mining. 

70~671-675. 

Fredriksen, R. L. 

1970. Erosion and sedimentation following road construction and 

unstable soils in three small western Oregon watersheds. 

Res. Pap. PNW-104, 15 p. 

Marquardt, D. W. 

J. For. 

timber harvest on 

USDA For. Serv. 

1963. An algorithm for least-squares estimation of nonlinear parameters. Soc . Ind. Appl. Math. 11:431-441. 

Megahan, W. F. 

In press. Sedimentation in relation to logging activities in the mountains of 

central Idaho. Paper prepared for Sediment Yield Workshop Proc., Oxford, 

Miss., Nov. 28-30, 1972. 

Megahan, W. F., and W. J. Kidd 

1972. Effect of logging roads on sediment production rates in the Idaho Batholith. 

USDA For. Serv. Res. Pap. INT-123, 14 p. 

Packer, P. E. 

1967. Forest treatment effects on water quality. Int. Symp. on Forest Hydrology 

Proc. , p. 687-699. 

Reinhart, K. G., A. R. Eschner, and G. R. Trimble, Jr. 

1963. Effect of streamflow of four forest practices in the mountains of West 

Virginia. U.S. For. Serv. Res. Pap. NE-1, 79 p. 

Vice, R. B., H. P. Guy, and G. E. Ferguson. 

1969. Sediment movement in an area of suburban highway construction, Scott Run 

Basin, Fairfax County, Virginia, 1961-64. Geol. Surv. Water-Supply Pap. 

1591-E, 41 p. 

Wischmeier, W. H., C. B. Johnson, and B. V. Cross. 

1971. A soil erodibility nomograph for farmland and construction sites. J. Soil 

Water Conserv. 26:189-193. 

Wischmeier, W. H., and D. D. Smith. 

1958. Rainfall energy and its relationship to soil loss. Trans. Am. Geophys. 

Union 39:285-291. 

14 

f? U.S. GOVERNMENT PRINTING OFFICE: 1974 - 677-088 1 13 REGION NO. 8 
MEGAHAN, W. F. 

1974. Erosion over time on severely disturbed granitic soils: a 

model. USDA For. Serv. Res. Pap. INT-156, 14 p., 

illus. (Intermountain Forest and Range Experiment Station, Ogden, Utah 84401.) 

A negative exponential equation containing three parameters was 

derived to describe time trends in surface erosion on severely disturbed soils. Data from four different studies of surface erosion on 

roads constructed from the granitic materials found in the Idaho 

Batholith were used to develop equation parameters. The evidence 

suggests that surface "armoring" was the dominant factor causing 

the time trends in surface erosion. The significance of time trends 

in surface erosion is discussed. 

OXFORD: 116: 116.3. KEYWORDS: surface erosion, vegetation, erosion energy, erodibility index, disturbed soil, soil erosion. 

MEGAHAN, W. F. 

1974. Erosion over time on severely disturbed granitic soils: a 

model. USDA For. Serv. Res. Pap. INT-156, 14 p., 

illus. (Intermountain Forest and Range Experiment Station, Ogden, Utah 84401. ) 

A negative exponential equation containing three parameters was 

derived to describe time trends in surface erosion on severely disturbed soils. Data from four different studies of surface erosion on 

roads constructed from the granitic materials found in the Idaho 

Batholith were used to develop equation parameters. The evidence 

suggests that surface "armoring" was the dominant factor causing 

the time trends in surface erosion. The significance of time trends 

in surface erosion is discussed. 

OXFORD: ll6:ll6.3. KEYWORDS: surface erosion, vegetation, 

erosion energy, erodibility index, disturbed soil, soil erosion. 

MEGAHAN, W. F. 

1974. Erosion over time on severely disturbed 

model. USDA For. Serv. Res. Pap. 

illus. (Intermountain Forest and Range 

tion, Ogden, Utah 84401.) 

A negative exponential equation containing thre

derived to describe time trends in surface erosion 

turbed soils. Data from four different studies of s

roads constructed from the granitic materials fo

Batholith were used to develop equation parameter

suggests that surface "armoring" was the domina

the time trends in surface erosion. The significa

in surface erosion is discussed. 

OXFORD: 116: 116.3. KEYWORDS: surface ero

erosion energy, erodibility index, disturbed soil, 

MEGAHAN, W. F. 

1974. Erosion over time on severely disturbed 

model. USDA For. Serv. Res. Pap. 

'illus. (Intermountain Forest and Range 

tion, Ogden, Utah 84401.) 

A negative exponential equation containing thre

derived to describe time trends in surface erosion 

turbed soils. Data from four different studies of s

roads constructed from the granitic materials fo

Batholith were used to develop equation parameter

suggests that surface "armoring" was the domina

the time trends in surface erosion. The significan

in surface erosion is discussed. 

OXFORD: 116: 116.3. KEYWORDS: surface er

erosion energy, erodibility index, disturbed soil, 
Headquarters for the Intermountain Forest and 

Range Experiment Station are in Ogden, Utah. 

Field Research Work Units are maintained in: 

Boise, Idaho 

Bozeman, Montana (in cooperation with 

Montana State University) 

Logan, Utah (in cooperation with Utah 

State University) 

Missoula, Montana (in cooperation with 

University of Montana) 

Moscow, Idaho (in cooperation ~~6th the

University of Ida: 

Provo, Utah (in coop^^ atir , righ ; 33- 

Young University) 

Reno, Nevada (in cooperat -\l~i!~ the 

University of Nevada)