Potential Overflow of Mojave 

Creek near Disposal Site, 

Edwards Air Force Base, California

By RANDAL L DINEHART and JERRY G. HARMON

U.S. GEOLOGICAL SURVEY 

Open-File Report 98-97

Prepared in cooperation with the

U.S. DEPARTMENT OF THE AIR FORCE

Sacramento, California 

1998
U.S. DEPARTMENT OF THE INTERIOR 

BRUCE BABBITT, Secretary

U.S. GEOLOGICAL SURVEY 

Thomas J. Casadevall, Acting Director

The use of firm, trade, and brand names in this report is for identification purposes only and does 

not constitute endorsement by the U.S. Geological Survey

For additional information write to:

District Chief

U.S. Geological Survey

Placer Hall, Suite 2012

6000 J Street

Sacramento, CA 95819-6129

Copies of this report can be purchased 

from:

U.S. Geological Survey 

Information Services 

Box 25286 

Federal Center 

Denver, CO 80225
CONTENTS

Abstract................................................................................................_^ 1

Introduction........................................................................................................................................... 1

Purpose and Scope........................................................................................................................ 3

Description of Study Area............................................................................................................. 3

Evidence of Flood Magnitude in Mojave Creek ................................................................................... 3

Sedimentological Evidence........................................................................................................... 5

Hydraulic and Hydrologic Information......................................................................................... 7

Flood Discharges from Regional Characteristics.................................................................................. 10

Water-Surface Profiles in Floodplain near Main Base Landfill............................................................. 11

Summary................................................................................................................................................ 14

References Cited.................................................................................................................................... 14

FIGURES

1. Map showing location of Edwards Air Force Base, California, and Main Base Landfill.............. 2

2. Map showing soils and vegetation along the Mojave Creek near the Main Base Landfill,

Edwards Air Force Base................................................................................................................ 4

3-7. Photographs showing:

3. Small channel of Mojave Creek near the Main Base Landfill, Edwards Air Force Base .......... 5

4. Thin layer of cracked mud on the floodplain surface of Mojave Creek near the Main

Base Landfill, Edwards Air Force Base..................................................................................... 6

5. Distal end of an alluvial fan at Mojave Creek near the Main Base Landfill intersected

by the railroad grade, Edwards Air Force Base......................................................................... 7

6. Crest-stage gage in a confined channel of Mojave Creek, Edwards Air Force Base................. 8

7. Wooden culvert through which water passes from south to north under the railroad grade 

immediately upstream from the flow path across the unpaved Mojave Road, Edwards 

Air Force Base........................................................................................................................... 9

8. Map showing floodplain of Mojave Creek near the Main Base Landfill boundary, Edwards 

Air Force Base................................................................................................................................ 12

9. Graph showing elevation plots of cross sections of the floodplain for the water-surface 

profile, Edwards Air Force Base .................................................................................................... 13

TABLE

1. Water-surface elevations computed using standard-step method for Mojave Creek

near the Main Base Landfill, Edwards Air Force Base............................................................. 11

Contents III
CONVERSION FACTORS AND VERTICAL DATUM 

Conversion Factors

Multiply

cubic foot per second (ft3/s) 

foot per second (ft/s)

acre 

square foot (ft2) 

square mile (mi2)

foot (ft) 

inch (in.) 

mile (mi)

By

Flow

0.0283 

0.3048

Area

0.4047 

0.0929 

2.590

Length

0.3048 

25.40 

1.609

To obtain

cubic meter per second 

meter per second

hectare 

square meter 

square kilometer

meter 

millimeter 

kilometer

Vertical Datum

Sea level: In this report, "sea level" refers to the National Geodetic Vertical Datum of 1929 a geodetic datum derived 

from a general adjustment of the first-order level nets of both the United States and Canada, formerly called Sea Level Datum of 

1929.

IV Contents
Potential Overflow Of Mojave Creek Near Disposal 

Site, Edwards Air Force Base, California

By Randal L. Dinehart and Jerry G. Harmon

ABSTRACT

Sedimentological evidence in Mojave Creek 

near Edwards, California, indicates that the largest 

discharge in the last hundred years near the disposal site of the Main Base Landfill at Edwards Air 

Force Base was a few hundred cubic feet per second. The distal ends of two alluvial fans on the 

Mojave Creek floodplain near the Main Base 

Landfill have not been eroded substantially since 

sediment supply was cut off by a railroad grade 

completed in 1884. Previous estimates of flood 

discharges were 4,000 cubic feet per second and 

larger in this reach; the estimates were calculated 

by regression equations derived from regional 

characteristics. However, a 100-year rainfall in 

1983 failed to produce erosion in Mojave Creek 

commensurate with discharges of greater than 

about 100 cubic feet per second. To test the potential for the creek to overflow and reach the disposal 

site, a hypothetical discharge was used to determine the depth of flooding at local cross sections. 

Although the access road from Mojave Boulevard 

to the Main Base Landfill may be inundated during 

a flood, the artificial grade at the disposal site 

would not be reached at a discharge of 2,000 cubic 

feet per second, which is an order of magnitude 

greater than the apparent flood discharges that 

occurred during the past hundred years in Mojave 

Creek near the present Main Base Landfill.

INTRODUCTION

The best documentation of floods in a stream valley lies in the discharge records of past floods as measured by hydrographers. Where no discharge records 

exist, the valley can be examined for sedimentological 

evidence of flooding. In addition, regression statistics 

for streams in similar watersheds can be applied to the 

stream in question. Finally, the hydraulic capacity of 

the stream can be calculated for a flood discharge 

related to the local channel conditions.

Mojave Creek is an ephemeral stream near 

Edwards Air Force Base (AFB), California (fig. 1), 

with a drainage area of 182 mi and a poorly defined 

channel for which no discharge measurements have 

been made. Managers at Edwards AFB expressed concern to the U.S. Geological Survey (USGS) about the 

possibility of flooding near the disposal site of the Main 

Base Landfill above Mojave Creek. Therefore, the 

potential for overflow of Mojave Creek near the Main 

Base Landfill was studied, in cooperation with the U.S. 

Department of the Air Force, primarily by examining 

sedimentological features and local hydraulic and 

hydrologic information. The study also included calculation of a water-surface profile for a hypothetical flood 

discharge.

Waste-discharge requirements set forth by the 

California Regional Water Quality Board in 1992 

(Earth Technology, 1993) for the Main Base Landfill 

stipulated that

The disposal site shall be adequately protected 

against inundation and/or washout from a 

storm or flood having a recurrence interval of 

once in 100 years...

Introduction 1
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-- ANTELOPE VALLEY BOUNDARY 

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Figure 1. Location of Edwards Air Force Base, California, and Main Base Landfill.
Because the stipulation involved a flood defined 

by recurrence interval, the potential for overflow was 

investigated with reference to published flood boundaries drawn for a 100-year flood (GRW Engineers, 

1993). The published flood boundaries had been 

mapped for studies of Mojave Creek near the Main 

Base using 100-year flood discharges derived from a 

combination of regional characteristics, statistics, and 

watershed simulation. A consulting firm (GRW Engineers, 1993) and the U.S. Army Corps of Engineers 

(GRW Engineers, 1993) both derived flood discharges 

for Mojave Creek greater than 4,000 ft3/s at Forbes 

Avenue near the Main Base Landfill.

Purpose and Scope

The purpose of this report is to describe the 

potential for flooding from the Mojave Creek channel 

to reach the Main Base Landfill northwest of Edwards, 

California. This study examined sedimentological features to determine flood magnitude at this site. The 

scope of this report is limited to the presentation of 

local hydrologic information and the calculation of a 

water-surface profile for a hypothetical flood discharge 

in the reach of Mojave Creek near the Main Base Landfill.

Description of Study Area

Edwards AFB is located in the Antelope Valley 

of south central California and occupies about 470 mi2, 

with the main military installation at the west side of 

Rogers Lake (fig. 1). Antelope Valley is bordered on the 

southwest by the San Gabriel Mountains and on the 

northwest by the Tehachapi Mountains. The V-shaped 

mountain border inhibits the passage of moist Pacific 

Ocean air to the Antelope Valley. Rainfall is sparse and 

widely scattered in the Antelope Valley. Average annual 

rainfall at Edwards AFB near Rogers Lake for the years 

1942-95 was 5.04 in. (Phillip Harvey, U.S. Air Force, 

written commun., 1996).

The Main Base Landfill serves Edwards AFB by 

providing trash compaction for local burial at the disposal site on a low hill above Mojave Creek (fig. 2). 

The Main Base Landfill has an area of 125 acres surrounded by a fence on a hill to the northeast of Mojave 

Creek. Borrow pits for the disposal site are located in a 

separate area (600 x 3,000 ft) between lines of power 

poles to the southwest of Mojave Creek. The name

Mojave Creek (known locally as Cache Creek) refers to 

the ephemeral stream that flows to the town of 

Edwards, California, from the stream valley north of 

Bissell Hills (fig. 1). Downstream from the Main Base 

Landfill, storm runoff flows within a depression along 

the south side of Mojave Boulevard toward Forbes 

Avenue. Storm runoff passes through a culvert under 

Forbes Avenue and continues along a storm-drainage 

channel on the north side of Mojave Boulevard.

Topographic maps indicate that the drainage 

basin for Mojave Creek extends from the town of 

Mojave to Rogers Lake. The drainage area is about 182

n

mi , most of which is upstream from Forbes Avenue. At 

the time of this study (1996), the main development in 

the watershed upstream from Forbes Avenue was the 

Main Base Landfill. About 10 mi2 of urbanized, sloped 

land produces storm runoff to Mojave Creek downstream from Forbes Avenue.

Daily rainfall records for Edwards AFB show 

that the largest rainfall for 24-hour periods between 

1942 and 1995 ranged from 1 to 1.5 in. (Phillip Harvey, 

U.S. Air Force, written commun., 1996). Localized 

cloudbursts with high rainfall intensity, but smaller 

totals can occur in summer. Widespread rain occurs 

most often in the winter months, and is usually associated with the largest rainfall amounts. Even with 

intense rainfall, hydrologic response of Mojave Creek 

is slow. The high infiltration capacity of the drainage 

basin delays the onset of stormflow. Soils beneath the 

floodplain adjacent to the Main Base Landfill are gravelly sands of decomposed granite. Borehole logs show 

that alluvial sand of Holocene age near the base of the 

disposal site extends to a depth of 40 ft (Earth Technology, 1993).

EVIDENCE OF FLOOD MAGNITUDE IN MOJAVE 

CREEK

To locate physical evidence of flood magnitude, 

USGS hydrologists inspected several miles along 

Mojave Creek during December 1995 and January 

1996. This section presents sedimentological evidence 

observed on the ground and in aerial photographs, followed by hydraulic and hydrologic information gathered in the area.

Evidence of Flood Magnitude in Mojave Creek 3
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4 Potential Overflow of Mojave Creek near Disposal Site, Edwards Air Force Base, California
Sedimentological Evidence

About 0.25 mi upstream from the Main Base 

Landfill, flow in Mojave Creek has formed several narrow channels that are less than 2 ft in depth (fig. 3). 

Between the channels, intervening areas of the floodplain have a surface of silty sand vegetated with grass 

and small desert shrubs. The floodplain was inundated 

during the rainy period of March 1995, as is indicated 

by the puffy, unmarked texture that is typical of 

recently dried ponds. A thin layer of cracked mud (fig. 

4) extends from the distal end of the alluvial fan to the 

hill bordering the northeast side of the floodplain.

The present floodplain does not appear to have 

formed in response to the current hydrologic regime of 

Mojave Creek. Dutcher and Worts (1963) concluded 

that the arid watersheds of the Antelope Valley gained 

their present form more than 10,000 years ago (during 

the Pleistocene geologic epoch), when rainfall was 

plentiful; that is, the present stream valley represents 

flows on surfaces formed in a wetter climate.

Mojave Boulevard is unpaved west of the landfill. The road diverges at the landfill, splitting to the 

abandoned railroad grade for the Atchison, Topeka, and 

Santa Fe Railway and to an adjacent, parallel road 

(unpaved Mojave Road). This study found narrow 

bands of sedimentation on the unpaved Mojave Road, 

indicating that local runoff occupies the road before 

reaching the ephemeral stream channel. At several 

places, flows have crossed the road into a low area 

between the road and the railroad grade. A larger view 

of flow along Mojave Creek was subsequently obtained 

by examining aerial photographs of the local area.

Patterns of vegetation along Mojave Creek and 

alluvial-fan deposition at the base of small tributaries to 

Mojave Creek are visible in aerial photographs taken in 

September 1984 and August 1985 as part of the Infrared High-Altitude Photograph program of the USGS. 

The photographs were examined for features relevant 

to floodflows along Mojave Creek. Infrared photographic film is sensitive to thermal differences caused 

by living vegetation and denser vegetation registers as 

darker shades. This property of the film was used to

Figure 3. Small channel of Mojave Creek near the Main Base Landfill, Edwards Air Force Base, California. Mojave Creek 

is an ephemeral stream that has formed several narrow channels with depths less than 2 feet. Flow is toward the 

foreground.

Evidence of Flood Magnitude in Mojave Creek 5
digitally enhance the vegetation patterns in a scanned 

aerial photograph of the Mojave Creek floodplain. The 

vegetation patterns were transferred to a topographic 

map (fig. 2). The darkened areas are considered to be 

the outline of soils wetted by local runoff from tributaries and streamflow along Mojave Creek. Southwest of 

the Main Base Landfill, the darkened areas form the 

outline of a meandering stream. The darkened areas on 

both sides of the unpaved Mojave Road and the railroad 

grade probably represent vegetation (grasses and/or 

desert shrubs) that survives on water from recent 

storms that is trapped in the silty sand of the relict 

stream outline.

The diagnostic value of one of the infrared photographs is demonstrated at the northwest corner of figure 2, where the main channel is north of the unpaved 

Mojave Road. Darkened patches appear alternately 

along both sides of the unpaved Mojave Road farther 

downstream, indicating probable areas where stormflow crossed the road. The fact that the darkened areas 

are not continuous on both sides of the road indicates 

that stormflows have not been large enough to inundate 

the road entirely.

Two separate drainages trend northeast toward 

the railroad grade (fig. 2). Both drainages originate at a 

hill about 440 ft above the floodplain. Although each 

drainage unites and is intersected by the railroad grade 

(fig. 5), an alluvial-fan shape is visible in the deflection 

of the darkened area on the northern side of the railroad 

grade (fig. 2).

The alluvial-fan deposition of loose sand at the 

distal ends of these tributaries leads to a useful interpretation of the vegetation pattern. The pattern is visible 

on both sides of the railroad grade, which was completed in 1884 (Mark Hagan, U.S. Air Force, written 

commun., 1996). Because the railroad grade has no 

stream crossings near the Main Base Landfill, no additional sand from the alluvial-fan source has been transported to Mojave Creek since the grade was completed. 

The crushed rock used in construction of the railroad 

grade lies on the surface of the alluvial fan and has not 

been superposed by additional sediment from the alluvial-fan source. The contour of the vegetation pattern in 

the floodplain shows that the distal end of the alluvial 

fan (now cut off) has not been eroded by subsequent 

floodflows from Mojave Creek.

Figure 4. Thin layer of cracked mud on the floodplain surface of Mojave Creek near the Main Base Landfill, Edwards Air 

Force Base, California. The layer of mud extends north from the distal end of the alluvial fan to the hill bordering the 

northeast side of the floodplain.

6 Potential Overflow of Mojave Creek near Disposal Site, Edwards Air Force Base, California
Any floodflows more than a few inches deep at 

the distal end of the alluvial fan would have had sufficient force to dislodge and transport the loose sand. To 

verify this, an experimental value of the critical shear 

velocity of sand was compared to the force of water on 

the floodplain expressed as shear velocity, w*:

(1)

where g is gravitational acceleration, d is flow depth, 

and S is energy slope. For a typical depth of 1 ft and a 

slope of 0.003, the shear velocity is 0.31 ft/s, which is 

five times the critical shear velocity of 0.06 ft/s for sand 

with mean diameter of 1 millimeter. Therefore, if the 

loose sand has remained at the distal end of the alluvial 

fan long after the railroad grade had cut off replenishment, then discharges on the floodplain have not been 

large enough to flow over the distal end of the alluvial 

fan with significant shear velocity.

The lack of erosive features from floods that 

inundate the floodplain up to the railroad grade is consistent with our observations that no features typical of 

a wide alluvial channel are visible at a distance from 

the small channels near the Main Base Landfill. For 

example, the north side of the railroad grade shows no 

signs of stream erosion, which would be expected from 

a large flow constrained in the area between the railroad 

grade and the northern margin of the floodplain.

Hydraulic and Hydrologic Information

Northwest of the Main Base Landfill, Mojave 

Creek is a small alluvial channel confined in a constriction between Bissell Hills and the base of a small butte 

(see "Flow channel at crest-stage gage," fig. 2). This 

channel follows the north side of the unpaved Mojave 

Road for a short distance before diminishing and entering the road. Erosive marks near the crest-stage gage 

indicate a bankfull channel less than 10 ft wide and 

about 2 ft deep (fig. 6). A reasonable estimate of bankfull discharge in this channel would be 20 ft2 x 5 ft/s, or

o

about 100 ft/s. Somewhat larger discharges could

Figure 5. Distal end of an alluvial fan at Mojave Creek near the Main Base Landfill intersected by the railroad grade, 

Edwards Air Force Base, California. (View to the west)

Evidence of Flood Magnitude in Mojave Creek 7
overflow this channel without detectable erosion or 

deposition. However, no flood marks or depositional 

levees from overbank flows were seen at this location. 

A wooden culvert conveys water from south to 

north under the railroad grade immediately upstream 

from the flow channel (figs. 2 and 7). The crossing 

point can be seen in figure 2 where the darkened area 

ceases its downstream extension south of the railroad 

grade. The total cross-sectional area of the culvert is

o

about 12 ft. High-water marks indicated ponding 

between the culvert at the railroad grade and the hill to 

the southwest. With a water depth of 2 to 3 ft above the 

culvert opening, the culvert capacity is about 80 to 100

ft /s. No erosion by overflow from the pond was found 

at the top of the railroad grade; all flow in Mojave 

Creek passes through this culvert.

A smaller wooden culvert (nearly filled with 

alluvium) crosses under the railroad grade about 1,500 

ft downstream from the larger culvert. No other culverts convey flow from south to north between the 

smaller culvert and the eastern end of the railroad grade 

near the Main Base Landfill. Therefore, additional 

inflow from Bissell Hills to Mojave Creek is negligible 

along the railroad grade. Although discharges near the 

Main Base Landfill would be about the same as discharges at the larger culvert and at the crest-stage gage,

Figure 6. Crest-stage gage in a confined channel of Mojave Creek, Edwards Air 

Force Base, California. About 1 mile northwest of the Main Base Landfill, 

Mojave Creek occupies a small alluvial channel in a constriction between 

Bissell Hills and the base of a small butte. The crest-stage gage is in the 

foreground.

8 Potential Overflow of Mojave Creek near Disposal Site, Edwards Air Force Base, California
the area near the Main Base Landfill was examined for 

evidence of a larger channel. Erosive features are longlived in a desert environment, commonly remaining 

visible for decades, and therefore, stream-channel erosion by any large floods in Mojave Creek would likely 

be preserved. The lack of evidence of a larger channel 

for Mojave Creek indicates that the visible alluvial 

channel probably represents the effect of the largest 

flows during the past few decades or longer.

A similar opinion was expressed by Robert Lichvar (U.S. Army Corps of Engineers, Waterways Experimental Station, oral commun., 1995), who 

investigated the geomorphological features of Bissell 

Hills. He stated that the soils in the Mojave Creek 

watershed are highly permeable (see also Earth Technology, 1993) and that storm runoff immediately infiltrates at the base of the local hills. He also stated that 

the drainage network was formed during the temperate 

climate of the late Pleistocene (more than 10,000 years 

before present) and does not represent the erosive activity of the current dry climate. These factors indicate 

that the capacity of the visible drainage channels represents the magnitude of discharges to be expected along 

Mojave Creek. That magnitude may be a few hundred 

cubic feet per second, as determined by dimensions of 

the larger culvert beneath the railroad grade, the channel dimensions at the nearby crest-stage gage, and the 

shallow flow conditions of the floodplain near the Main 

Base Landfill.

Figure 7. Wooden culvert through which water passes from south to north under 

the railroad grade immediately upstream from the flow path across the unpaved 

Mojave Road, Edwards Air Force Base, California. The opening on the right is 

partly visible; the width of each opening is 3 feet, and the height of each opening 

is 2 feet. Flow is toward the foreground.

Evidence of Flood Magnitude in Mojave Creek
Local hydrologic information tends to support 

the conclusion that flood discharges do not inundate the 

width of the low-lying area near the Main Base Landfill. At Edwards AFB, a total of 3.38 in. of rain fell on 

March 1,1983, with a calculated recurrence interval 

greater than 100 years (Blodgett, 1996). Sedimentary 

deposits of the flood discharge resulting from this 

storm were preserved in the arid environment. The farthest extent of the wide, thin layer of cracked mud (fig. 

4) probably was left by water that ponded on the floodplain following the 1983 storm, and the high-water 

marks from ponding above the railroad culvert were 

probably associated with that same storm.

During March 1995, a few storms over a 14-day 

period resulted in a rainfall total of 0.88 in. at the Main 

Base. The manager at the landfill stated that storm runoff from the paved area of the disposal site was contained by a culvert outside the weigh station at the 

landfill and was passed to the floodplain below, where 

the water ponded. He did not recall any flow in the 

Mojave Creek floodplain. If average rains in March 

1995 did not produce a noticeable flow, then the capacity of the drainage basin for infiltration must be high, as 

noted by Robert Lichvar (U.S. Army Corps of Engineers, Waterways Experimental Station, oral commun., 

1995).

A storm occurred at Edwards AFB on February 

20-21,1996, with total rainfall of 0.90 in. for a period 

of 24 hours. The rainfall total for the 24-hour period 

corresponds to an approximate 10-year recurrence 

interval (Blodgett, 1996). Three crest-stage gage sites 

on Mojave Creek (maintained by USGS for Edwards 

AFB) were checked on February 29, 1996. The creststage gage at the upstream corner of Lancaster Boulevard and Rosamond Boulevard showed a flood mark 

indicating recent flow in the channel. The channel in 

the culvert on Mojave Creek that passes under Forbes 

Avenue also showed marks that indicated recent flow. 

However, the crest-stage gage on the main channel of 

Mojave Creek, about 0.5 mi upstream from the Main 

Base Landfill, showed no flood marks from recent flow. 

During the February 1996 storm, flow in the Mojave 

Creek was not measurable in the main channel 

upstream from the Main Base Landfill but was measurable at Forbes Avenue, which is downstream from 

developed areas of the town of Edwards. This indicates 

that flow probably infiltrated completely before reaching the landfill, and the flow in the channel at Forbes 

Avenue resulted from local runoff.

FLOOD DISCHARGES FROM REGIONAL CHARACTERISTICS

Statistical data and analytical procedures were 

combined in previous studies to derive flood discharges 

for Mojave Creek without the benefit of local field data 

or discharge records (GRW Engineers, 1993). The 

techniques that apply regional characteristics (including drainage area, elevation, and soil properties) in 

regression equations are well-established and often are 

necessary to estimate discharge for ungaged streams 

(Thomas and others, 1994). However, the techniques 

yielded flood discharges for Mojave Creek above 

Forbes Avenue that are difficult to reconcile with the 

direct evidence of flood magnitude in Mojave Creek 

near the Main Base Landfill.

GRW Engineers (1993), using data curves suggested by the U.S. Corps of Engineers and the County 

of San Bernadino, developed a semi-analytical method 

to obtain an estimated 200-year flood discharge for 

Mojave Creek of 4,000 ft3/s. An independent calculation by the U.S. Corps of Engineers yielded a 100-year 

flood discharge at Mojave Creek at Forbes Avenue of 

5,350 ft3/s (GRW Engineers, 1993).

To assess the accuracy and the relative magnitude of these previous estimates, other flood-frequency 

equations were applied to Mojave Creek for the present 

study. Regional flood-frequency equations, compiled 

by Thomas and others (1994) for the Southwestern 

United States, were applied to Mojave Creek. The 

equations relate discharges of various recurrence intervals to the drainage area through the form Q = cAb, 

where A is drainage area and the values of b and c were 

determined by linear regression of values of Q and A 

for similar watersheds in the southwest (Thomas and 

others, 1994).

Use of the regional flood-frequency equation for 

the Southern Great Basin Region 10 (Thomas and others, 1994) was recommended by Robert Meyer (U.S. 

Geological Survey, written commun., 1996). The recommended least-squares regression equation from 

Thomas and others (1994), Q = 850 A6'69, gives a 100- 

year flood discharge Q of 30,800 ft3/s for the drainage 

area A of 182 mi2. This equation was developed from 

data for watersheds with intermittent streams and visible floodplains. Using the regression equation for more 

arid conditions of south-central Utah, Q = 264 A0<344, 

the 100-year flood discharge Q becomes 1,580 ft3/s for 

the same drainage area A.

Each of these regression equations results in a 

discharge estimate of thousands of cubic feet per sec10 Potential Overflow of Mojave Creek near Disposal Site, Edwards Air Force Base, California
ond. Peak discharges determined from direct evidence 

near the Main Base Landfill are in the hundreds of 

cubic feet per second. Although regression values for 

discharge and drainage area probably include a few 

watersheds similar to Mojave Creek, the generalized 

regression equation may not provide a 100-year flood 

discharge representative of Mojave Creek.

WATER-SURFACE PROFILES IN FLOODPLAIN 

NEAR MAIN BASE LANDFILL

Flood boundaries for Mojave Creek above 

Forbes Avenue were determined by GRW Engineers 

(1993). They routed an estimated flood discharge 

through cross sections of Mojave Creek using a program to compute water-surface profiles by the standard 

step method (Henderson, 1966), also known as the 

step-backwater method. The boundaries were established using hydraulic calculations from cross sections 

in the reach upstream and downstream from the Main 

Base Landfill. The cross sections shown by GRW Engineers (1993) were several thousand feet apart and did 

not include the topography at the landfill. The published flood maps showed the disposal site outside the 

flood boundaries.

For the USGS study, four cross sections (figs. 8 

and 9) were surveyed on the floodplain for Mojave 

Creek southwest of the Main Base Landfill on January 

12,1996. The cross sections between Mojave Boulevard, the access road, and the borrow area were surveyed on January 30,1996. Surveyed features included 

the railroad grade, which rises above the floodplain by 

2 ft; the center line of the unpaved Mojave Road; and 

the chain-link fence that intersects the floodplain below 

the culvert draining the asphalt pavement at the Main 

Base Landfill.

Cross-section locations were selected by judging 

the likely flow path between the Main Base Landfill

and the unpaved Mojave Road. The floodplain adjacent 

to the Main Base Landfill had been cleared of vegetation by heavy equipment sometime prior to the survey. 

At the northwest side of the cleared area, the small 

channels that cross the floodplain are intersected by the 

chain-link fence that surrounds the Main Base Landfill. 

The channels then turn south toward the access road 

that connects to Mojave Boulevard (fig. 2).

If the water surface in Mojave Creek were to 

approach the artificial grade on the southwestern side 

of the disposal site, the railroad grade and the artificial 

grade below the disposal site would control the flow 

width as depth increased. The flow width between the 

railroad grade and the Main Base Landfill boundary 

decreases downvalley toward the access road. Near the 

borrow area, the railroad grade merges with the surrounding land surface and ceases to control the flow 

width. Flow would continue through the open area 

south of Mojave Boulevard toward Forbes Avenue.

Although physical evidence was found for flood 

discharges of a few hundred cubic feet per second in 

the floodplain, a discharge that is an order of magnitude 

greater was used to test whether larger flows would 

reach the disposal site. A hypothetical discharge of 

2,000 ft3/s was applied through the standard-step 

method for flow profiles using the surveyed cross sections of the floodplain adjacent to the Main Base Landfill. A hypothetical discharge of 2,000 ft3/s was chosen 

because its magnitude was identified by the regression 

equation for arid watersheds (Thomas and others, 

1994) as the 100-year flood discharge for the given 

drainage area. Cross-sectional geometry and location 

were determined from the cross-sectional surveys. 

Water-surface elevations for the hypothetical discharge 

were calculated by the standard-step program WSPRO 

(Water Surface PROfile). The calculated water-surface 

elevations are lower than the asphalt pavement inside 

the Main Base Landfill. Some results from the WSPRO 

computation for 2,000 ft3/s are given in table 1.

Table 1. Water-surface elevations computed using standard-step method for Mojave Creek near the Main Base Landfill, 

Edwards Air Force Base, California

Cross-section 

number

1

2

3

4

Water-surface 

elevation, 

in feet

2,381.5

2,380.7

2,379.8

2,379.4

Right bank elevation 

at end of section, 

in feet

2,382.2

2,380.6

2,380.0

2,380.0

Left bank elevation 

at end of section, 

in feet

2,385.6

2,388.4

2,384.1

2,380.4

Disposal site 

ground elevation, 

in feet

2,390.0

2,390.0

2,390.0

2,390.0

Water-Surface Profiles in Floodplan near Main Base Landfill 11
118057'30"

35057'30"

VEGETATION FROM 

INFRARED PHOTOGRAPH

2

EXPLANATION

 CROSS SECTION AND 

NUMBER

Base from U.S. Geological Survey 

Edwards.California: 1:24,000,1992 0 .5 1 MILE

.5 1 KILOMETER

Figure 8. Floodplain of Mojave Creek near the Main Base Landfill boundary, Edwards Air Force Base, California. Cross-section locations for 

the water-surface profile are shown.The gray areas along Mojave Creek, indicating vegetation on wetted soils, are derived from an infrared 

aerial photograph taken in 1984.

12 Potential Overflow of Mojave Creek near Disposal Site, Edwards Air Force Base, California
_J

LLJ

>

LLJ

_J

<

LLJ

CO

LLJ

>

O 

CD

<

fLLJ

LLJ

U_

Z

z~

0

f- 

<

>

LLJ

_J 

LLJ

2,385

2,380

2,375

NEAR LANDFILL SECTION 1

I I I I I I

NEAR RAILROAD GRADE

1 1 1

9

 - ^ m ^ /

V

I I I I I I

-100 0 100 200 300 400 500

SECTION 2

2,385

2,380

2,375

I I I I I I

*-- ^_^^ ^'~""""- -^^^

^~*""~- ^^

* - 

I I I I I I

-100 0 100 200 300 400 500

SECTION 3

2,385

2

QQfi ,OoU

2,375

1 I I I I I

- *^^

~~~~~" ^^*^_ ^_ «j> ^ J

1 1 1 1 1 1

-100 0 100 200 300 400 500

SECTION 4

2,385

2,380

2,375

1 1 1 1 1 1

v

\ * ~~~^

1 1 1 1 1 1

-100 0 100 200 300 400 500

- ~ ^*

1 1 1

600 700 800 90

1 1 1

^ _ ^- /

 *~~^*fJ^J

\ \ \

600 700 800 90

1 1 1

1 1 1

600 700 800 90

1 1 1

_

1 1 1

600 700 800 90

DISTANCE FROM INITIAL POINT, IN FEET

Figure 9. Elevation plots of cross sections of the floodplain forthe water-surface profile (numbered in downstream order), Edwards Air Force 

Base, California. The initial points at left are near the artificial grade around the Main Base Landfill. Cross sections extend to the railroad 

grade, shown by points at right. Horizontal line on cross sections are water-surface elevations from table 1. (Manning's n=0.035; watersurface slope=0.0024)

Water-Surface Profiles in Floodplan near Main Base Landfill 13
The water-surface elevations for the hypothetical 

discharge would be very near or at the top of the railroad grade. However, at the railroad grade across from 

the Main Base Landfill, the railroad-grade edge shows 

no signs of stream erosion that would be expected from 

a flood of this magnitude inundating the floodplain. 

Therefore, the hypothetical discharge of 2,000 ft3/s 

must be greater than flood discharges that have 

occurred in Mojave Creek near the Main Base Landfill 

since the completion of the railroad grade.

A flood discharge identified as the largest in 100 

years does not equate with a flood discharge having a 

100-year recurrence interval. The 100-year designation 

means that the discharge has a probability of 0.01 of 

being exceeded in 100 years. The probability p is the 

inverse of the recurrence interval Tr:

(2)

from

n + 1 

m

(3)

where n is the number of years of record and m is the 

rank of the annual peak discharge. Although Mojave 

Creek has no discharge record, a hypothetical distribution of annual peak discharges is associated with any 

watershed. As the number of years in the population of 

annual peak discharges increases, the likelihood also 

increases that a flood discharge has occurred with an 

exceedance probability p equal to or less than 0.01. 

The probability J that a peak annual discharge with a 

return period of Tr years will occur in a period of n 

years is

n

(4)

For a period of 112 years between the completion of the railroad grade and this investigation of 

Mojave Creek near the landfill, the probability J of the 

occurrence of a 100-year flood is 0.68. Assuming that 

the effects of flood erosion are well preserved, there is 

a 0.32 probability that sedimentological evidence for

flood magnitude in the floodplain near the landfill does 

not include the effects of a 100-year flood discharge.

SUMMARY

Because the desert environment preserves erosive features for decades, the lack of erosive features 

along the floodplain margins near the Main Base Landfill indicates that flood discharges greater than a few 

hundred cubic feet per second have not occurred in the 

last hundred years. For this reason, reported flood discharges derived from regional characteristics in the 

range of 1,580 to 5,350 ft3/s cannot be reconciled with 

the physical evidence for the magnitude of flooding. 

The sedimentological evidence at this site also could 

not be reconciled with published estimates of 100-year 

flood discharges. The Mojave Creek floodplain does 

not show signs of flood discharges in that range since 

the completion of the railroad grade in 1884.

In the area adjacent to alluvial-fan deposition 

that is over a hundred years old, flow depths have not 

been sufficient to dislodge the sediment deposited there 

before the railroad grade was built. Discharges of a few 

hundred cubic feet per second may have occurred, as 

estimated from the dimensions of the railroad culvert 

and alluvial channel west of the Main Base Landfill. 

Given the uncertainty in a 100-year flood discharge for 

this ephemeral stream, a flood discharge as great as any 

in the last hundred years in Mojave Creek could inundate the access road from Mojave Boulevard but not the 

disposal site within the Main Base Landfill boundary.

REFERENCES CITED

Blodgett, J.C., 1996, Precipitation depth-duration and 

frequency characteristics for Antelope Valley, Mojave 

Desert, California: U.S. Geological Survey WaterResources Investigations Report 95-4056, 33 p.

Dutcher, L.C., and Worts, G.F., Jr., 1963, Geology, 

hydrology, and water supply of Edwards Air Force 

Base, Kern County, California: U.S. Geological Survey 

Open-File Report (unnumbered), 223 p.

Earth Technology, 1993, Report of disposal site information, 

main base sanitary landfill, Edwards Air Force Base, 

California: vols. I-n, 81 p. and appendices.

GRW Engineers, 1993, Edwards Air Force Base IRP Flood 

Study: 23 p. and 4 appendices.

Henderson, P.M., 1966, Open Channel Flow: New York, 

MacMillan, 522 p.

Thomas, B.E., Hjalmarson, H.W, and Waltemeyer, S.D., 

1994, Methods for estimating magnitude and frequency 

of floods in the southwestern United States: U.S. 

Geological Survey Open-File Report 93-419, 211 p.

14 Potential Overflow of Mojave Creek near Disposal Site, Edwards Air Force Base, California