JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION
Vol. 43, No. 1
AMERICAN WATER RESOURCES ASSOCIATION
February 2007
PHYSICAL AND TEMPORAL ISOLATION OF MOUNTAIN HEADWATER
STREAMS IN THE WESTERN MOJAVE DESERT, SOUTHERN CALIFORNIA1
John A. Izbicki2
ABSTRACT: Streams draining mountain headwater areas of the western Mojave Desert are commonly physically isolated from downstream hydrologic systems such as springs, playa lakes, wetlands, or larger streams and
rivers by stream reaches that are dry much of the time. The physical isolation of surface flow in these streams
may be broken for brief periods after rainfall or snowmelt when runoff is sufficient to allow flow along the entire
stream reach. Despite the physical isolation of surface flow in these streams, they are an integral part of the
hydrologic cycle. Water infiltrated from headwater streams moves through the unsaturated zone to recharge the
underlying ground-water system and eventually discharges to support springs, streamflow, isolated wetlands, or
native vegetation. Water movement through thick unsaturated zones may require several hundred years and
subsequent movement through the underlying ground-water systems may require many thousands of years –
contributing to the temporal isolation of mountain headwater streams.
(KEY TERMS: hydrologic cycle; infiltration; recharge; vadose zone; surface water ⁄ ground-water interactions;
arid lands.)
Izbicki, J.A., 2007. Physical and Temporal Isolation of Mountain Headwater Streams in the Western Mojave
Desert, Southern California. Journal of the American Water Resources Association (JAWRA) 43(1):26-40. DOI:
10.1111/j.1752-1688.2007.00004.x
have discussed the extent of ‘‘waters of the United
States,’’ including streams in arid areas that are isolated from larger hydrologic systems. Several of these
recent decisions find that waters that can convey pollutants to downstream navigable waters for even
brief periods are jurisdictional because ‘‘pollutants
need not reach interstate bodies of water immediately
or continuously in order to inflict serious environmental damage’’ (United States vs. Eidson, 94–2330).
Surface flow in streams draining mountain headwater areas in the arid western United States is commonly physically isolated from downstream playa
lakes, wetlands, or larger streams and rivers by
stream reaches that are dry much of the time. The
INTRODUCTION
The Clean Water Act regulates the discharge of
pollutants from point sources and the discharge of fill
material into ‘‘navigable waters,’’ which the act
defines as ‘‘waters of the United States.’’ The extent
to which ‘‘waters of the United States’’ include small
isolated hydrologic systems was questioned in a 2001
U.S. Supreme Court decision that limited the U.S.
Army Corps of Engineers jurisdiction under the
Clean Water Act over isolated waters (SWANCC vs.
U.S. Army Corps of Engineers, 98-2277). Since the
SWANCC decision, many Federal Court decisions
1
Paper No. J06013 of the Journal of the American Water Resources Association (JAWRA). Received February 3, 2006; accepted July 17,
2006. ª 2007 American Water Resources Association. No claim to original U.S. government works.
2
Research Hydrologist, U.S. Geological Survey, 4165 Spruance Road, San Diego, California (E-Mail ⁄ Izbicki: jaizbick@usgs.gov).
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through Cajon Pass and precipitate without passing
over the higher altitudes of the San Gabriel and San
Bernardino Mountains. Precipitation near the pass
can give rise to streamflow along the entire length of
the Mojave River and flow in smaller streams near
the pass, such as Oro Grande Wash. A similar gap
between the San Bernardino and San Jacinto Mountains, San Gorgornio Pass, to the southeast of the
study area (not shown in Figure 1), also allows cool
moist air to enter the desert and gives rise to winter
precipitation and intermittent streamflows in that
area – although the effect is smaller than near Cajon
Pass (Izbicki, 2004). Although summer thunderstorms
occur, especially in the eastern part of the study area,
summer monsoonal precipitation is of lesser importance in the western Mojave Desert than elsewhere in
the southwestern United States.
With the exception of some small streams that
drain the higher altitudes of the San Gabriel and San
Bernardino Mountains and short reaches of the
Mojave River where ground-water discharges at land
surface, there are no perennial streams in the area.
Physical connection between mountain headwater
streams (whether perennial or intermittent) and
downstream hydrologic systems in the western
Mojave Desert occurs only during brief periods of
streamflow after precipitation or snowmelt along normally dry downstream reaches that cross alluvial
fans and basin fill deposits.
There are a number of internally drained alluvial
basins in the western Mojave Desert each having distinct ground-water-flow systems often separated by
faults and bedrock outcrops. Alluvial deposits in some
basins are more than 1,000 m thick and saturated
deposits may be separated from land surface by unsaturated alluvium as much as 300 m thick near the
mountain front. Ground-water movement in these
basins is generally from recharge areas near the mountain front and along larger stream channels toward discharge areas that include springs, wetlands, or native
vegetation near dry lakes. Prior to ground-water
pumping in the Mojave River ground-water basin, the
direction of ground-water movement was from alluvial
deposits (collectively known as the regional aquifer) to
the floodplain aquifer along the Mojave River. In most
of the regional aquifer, ground-water recharge is small
in relation to the volume of water in storage and travel
times through the aquifer system are often many thousands of years (Izbicki et al., 1995; Izbicki and Michel,
2004). In contrast, the floodplain aquifer is more limited in areal and vertical extent (typically less than
2.5 km wide and 80 m thick) than the surrounding
alluvial aquifers and is readily recharged by infiltration of streamflow in the Mojave River.
Numerous water-level maps have been prepared of
aquifers in the area (Stamos and Predmore, 1995;
physical isolation of surface flow in mountain headwater streams (whether perennial or intermittent)
from downstream systems may be broken for brief
periods after rainfall or snowmelt when runoff is sufficient to allow flow along the entire downstream
reach. Despite the physical isolation of surface flow
in these streams, they are an integral part of hydrologic systems in arid regions. Water infiltrated from
headwater streams moves through the unsaturated
zone to recharge the underlying ground-water system. This ground water eventually discharges to support springs, streamflow, isolated wetlands, or native
vegetation far from recharge areas. In some systems,
ground-water movement from recharge areas to discharge areas may require many thousands of years.
In addition to their physical and temporal isolation, the mountain headwater streams in the western
Mojave Desert are further isolated from other hydrologic systems by their geologic setting within the
Basin and Range physiographic province. Under ‘‘present-day’’ climatic conditions, many internally
drained basins (also known as ‘‘closed basins’’) within
the Basin and Range physiographic province are
physically isolated from larger drainages that flow to
interstate waters or discharge to the ocean by intervening mountain ranges.
The purpose of this paper is to summarize on the
basis of existing data and published work (1) the brief
physical connection of selected mountain headwater
streams in the western Mojave Desert to downstream
hydrologic systems, (2) the connection of water infiltrated from these streams through the unsaturated
zone to the underlying ground-water system, and (3)
the longer time-scale connection through the groundwater system to discharge areas farther downgradient. Only brief descriptions of methods are given in
this paper and the reader is referred to the cited
work for a more thorough explanation of the methods, data, and results.
HYDROGEOLOGIC SETTING
The western Mojave Desert east of Los Angeles
(Figure 1) is arid with hot, dry summers, and cold
winters. With the exception of the higher altitudes in
the San Gabriel and San Bernardino Mountains, precipitation is generally about 150 mm ⁄ yr or less, but
amounts vary greatly from year to year. In most of
the area, precipitation is greater during the winter
rainy season (November-March) and occurs as a
result of cyclonic storms moving inland from the Pacific Ocean. During winter cyclonic storms, moist air
from the Pacific Ocean can enter the Mojave Desert
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FIGURE 1. Location of Study Area.
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headwater areas to downstream reaches of the Mojave
River was almost 500 hm3 (5 · 108 m3). Annual flows
of this magnitude have a recurrence interval of greater
than 50 years (Lines, 1996) and this was the first time
the river flowed continuously since 1983. More
thorough analyses of the magnitude and frequency of
surface flows in the Mojave River from stream gaging
stations are available in Lines (1996) and Stamos et al.
(2001).
Mendez and Christensen, 1997; Smith and Pimentel,
2000; Smith et al., 2004; Stamos et al., 2004). Several
regional-scale ground-water flow models simulating
ground-water flow have been completed for the
Mojave River ground-water basin (Hardt, 1971; Stamos et al., 2001) and the Antelope Valley (Leighton
and Phillips, 2003). Smaller scale flow models have
been completed for some subbasins in the Morongo
ground-water basin (Londquist and Martin, 1991;
Nishikawa et al., 2004).
STREAMFLOW
For the purposes of this paper, streamflow in the
Mojave River, the largest stream in the study area is
discussed separately from the streamflow characteristics in smaller streams that drain the mountains.
The Mojave River
The Mojave River, the largest stream in the study
area, drains about 5,500 km2, of which 540 km2 are
in the San Bernardino Mountains. The Mojave River
flows past Afton Canyon more than 160 km downstream and splits with separated channels flowing
east toward East Cronese and Soda (dry) Lakes (not
shown in Figure 1). During 1983, the river was reported to have overflowed its banks upstream from Barstow and flowed northwestward into Harper (dry)
Lake (Lines, 1996).
The physical connection of headwater reaches of the
Mojave River, the largest stream in the study area, to
downstream reaches was assessed by Lines (1996)
during water years 1992–94 (Figure 2). Perennial flow
during this period occurred only at the Upper Narrows, the Lower Narrows, downstream from a regional
wastewater treatment plant serving the Victorville
area, and at Afton Canyon. Records from early travelers and explorers in the area suggest that perennial
flow was more extensive prior to ground-water pumping (Lines, 1996). During each winter, runoff from the
headwaters, coupled with seasonal decreases in
ground-water pumping and evapotranspiration from
riparian habitat extended the seasonal surface flow.
Stamos et al. (2001) showed that pumping along the
river decreased the magnitude and frequency of seasonal surface flow in the Mojave River along stream
reaches farther downstream from the mountain front.
The river flowed along its entire main stem downstream to Afton Canyon for a few weeks during water
year 1993 as a result of a series of large storms (Lines,
1996). During 1993, the total annual flow from
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FIGURE 2. Reaches of the Mojave River That Had Streamfow
During Water Years 1992–94 (modified from Lines, 1996).
Smaller Streams
Smaller streams are obviously more numerous than
larger streams, such as the Mojave River. About 140
mountain headwater streams draining at least
0.9 km2 were identified along the mountain front
between Palmdale and Twentynine Palms (Figure 3).
Streamflow quantity and frequency data have been
estimated using a variety of techniques for reaches of
several smaller streams discussed in this article. Quail
Wash, Big Rock Creek, and Sheep Creek are among
the larger streams identified in Figure 3; streamflow
quantity and frequency for the more numerous
streams draining less than 20 km2 are largely unavailable. Oro Grande Wash discussed in the article is not
shown in Figure 3 because it originates near Cajon
Pass and does not drain the mountain front.
Streamflow data from gaging stations are less
available for smaller intermittent streams than for
larger streams such as the Mojave River; as a
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Notes: Drainage areas are at the mountain front. Altitudes are for the drainage area are upstream from the mountain front. Average annual flow at the mountain front was estimated from a relation between channel geometry and annual flow developed by Lines (1996) – except for Big Rock Creek, for which flow is estimated from stream gage
data at Valyermo 5 km upstream from the mountain front – not applicable because Oro Grande Wash does not drain from the mountains; m, meters; hm3, cubic hectometers (106 · m3); km, kilometers; km2, square kilometers.
3.1
4.9
1.5
1.7
3
3
40
20
3
80
10
10
22.7
18.8
27.3
15.2
0.5
3.1
16
28
1,948
1,769
1,351
Oro Grande Wash
Sheep Creek Wash
Big Rock Creek
Quail Wash
36.8
108
237
2,594
2,829
1,768
Downstream Site
(m)
Mountain Front
(m)
Maximum
(m)
Average
(m)
Average Annual
Flow (hm3)
Length of Study
Reach (km)
Channel Width
Altitude
Stream
FIGURE 3. Rank-Order Distribution of Drainage Basins
Greater Than 0.9 km2 on the Northern Slope of the San Gabriel,
San Bernardino, and Little San Bernardino Mountains Between
Palmdale and Twentynine Palms, California.
Drainage Area at
Mountain
Front (km2)
TABLE 1. Physical Characteristics Along the Study Reaches of Oro Grande, Sheep Creek, Big Rock Creek, and Quail Washes, Western Mojave Desert, Southern California.
consequence, the frequency of surface flow in smaller
intermittent streams along the front of the San Gabriel Mountains to downstream channel reaches was
estimated on the basis of streambed temperature
data. During the winter months, when most precipitation occurs, streamflow is relatively cold, often only
slightly above 0C. Cold streamflow causes measurable changes in streambed temperature that do not
occur in ground temperature measurements at control sites adjacent to, but outside, the wash (Constantz et al., 2001, 2003). Streambed temperature
data are relatively easy and inexpensive to collect
and numerous measurement stations can be installed
along a wash reach to determine the downstream
extent and duration of winter storm flows. Streamflow interpreted from temperature data was verified
by examination of the channel during site visits after
storms. The approach is attractive in areas where it
is impractical or prohibitively expensive to install traditional stream gages that may be damaged or destroyed
during
large streamflows. Streambed
temperature data were collected along three selected
washes: Oro Grande Wash, Sheep Creek Wash, and
Big Rock Creek Wash. Oro Grande Wash flows to the
Mojave River, Sheep Creek Wash flows to El Mirage
(dry) Lake, and Big Rock Creek Wash flows to Rogers
(dry) Lake in the Antelope Valley. The three washes
are among the largest in the western Mojave Desert
and study reaches total almost 70 km. Each wash
represents a range of hydrologic conditions (Table 1).
Average Slope of
Study Reach (percent)
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FIGURE 4. Precipitation, Streambed Temperature, Control Temperature (Collected Outside of Streambed), and Inferred
Duration of Streamflow Along Sheep Creek Wash, Western Mojave Desert, Southern California, February 17–27, 2000.
example of streambed temperature changes interpreted as streamflow is shown for a site along
Oro Grande Wash, July 5–12, 1999 (Figure 5). Analysis of temperature data suggests that streamflow
might not have occurred at upstream or downstream temperature measurement sites during this
period.
If interpretations of streambed temperature data
are not constrained by meteorological data and frequent site visits, all measured streambed temperature anomalies could be interpreted as streamflow –
producing a higher frequency of flow than might have
occurred (Figure 6). Despite the inherent uncertainty
associated with this approach, estimates of streamflow occurrence inferred from temperature data can
be assembled into statistical representations of
streamflow frequency that reflect the regional hydrology of the study area.
An example of streambed temperature changes
interpreted as streamflow is shown for selected measurement sites along Sheep Creek Wash, February
17–27, 2000 (Figure 4). The interpreted streamflow is
of greater duration along the upstream sections of
the wash at the mountain front. Runoff from precipitation is directed away from the active channel of
Sheep Creek Wash by the conical shape of the alluvial fan and streamflow decreases in duration with
distance downstream as water infiltrates into the
underlying streambed.
Streamflow is more difficult to interpret from
streambed temperature data during the summer
when the difference between precipitation, runoff,
and streambed temperatures may be small. The
interpretation may be further complicated because
summer precipitation in arid areas is often highly
variable spatially, limited in areal extent. An
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FIGURE 5. Precipitation, Streambed Temperature, Control Temperature (Collected Outside of Streambed), and
Inferred Duration of Streamflow Along Oro Grande Wash, Western Mojave Desert, Southern California, July 5–12, 1999.
UNSATURATED FLOW
Oro Grande Wash is the smallest of the three
washes studied and the wash does not drain the San
Gabriel Mountains. The frequency of flow along most
of Oro Grande Wash is less than the frequency of
flow along mountain front reaches of the other
washes and the duration of flows is less – typically
about 1 hour. Given a frequency of flow of 0.05
days ⁄ yr and a duration of 1 hour, Oro Grande Wash
may only flow for as few as 18 hours each year (365
days ⁄ yr · 0.05 stormflows ⁄ day · 1 hour ⁄ stormflow).
Although during large winter storms Oro Grande
Wash may flow uninterrupted from its headwaters
near Cajon Pass through the study reach to the
Mojave River (Izbicki et al., 2000), flows along shorter
reaches of the wash are more common. This is especially true along the downstream urbanized reach of
Oro Grande Wash where runoff from impervious
urban areas contributes to increased streamflow.
Frequency and duration of flow in Sheep Creek
Wash are greater than in Oro Grande Wash because
Sheep Creek drains a larger area in the higher altitudes in the San Gabriel Mountains. Although not perennial, Sheep Creek may flow for extended periods
during the winter and during spring runoff. For example, the duration of a single flow in Sheep Creek at the
mountain front between February 24 and February 26,
2000 exceeded the estimated cumulative annual flow
duration along Oro Grande Wash. Unlike Oro Grande
Wash, where flows along only the downstream reaches
are common, flow in both Sheep Creek and Big Rock
Creek Washes decreases in frequency and duration
with distance downstream (Figure 6).
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In arid alluvial valleys of the western Mojave
Desert, areal recharge from precipitation and subsequent movement of water through the unsaturated
zone is negligible. In fact, thick unsaturated zones
overlying alluvial aquifers in the Mojave Desert
within California have been proposed as storage
repositories for toxic and nuclear waste (National
Research Council, 1995). However, along intermittent
stream channels water may infiltrate to depths below
the root zone and ultimately reach the underlying
water table. In these areas where the volume of
water infiltrated is small, and the unsaturated zone
is thick, or relatively impermeable, the slow movement of water through the unsaturated zone may
contribute to the temporal isolation of small headwater streams from underlying aquifers and downgradient hydrologic systems.
Infiltration from streamflow commonly occurs in
greater amounts along upstream reaches near the
mountain front (Izbicki et al., 2002). Measurements
of water content, water potential, and low concentrations of soluble salts (such as chloride) in the unsaturated zone beneath upstream reaches of Sheep
Creek Wash (Figure 7) are consistent with the movement of infiltrated water to depths below the root
zone and presumably to the underlying water table
as much as 300 m below land surface (Izbicki et al.,
2002). Similarly, water infiltrated during stormflow
moves downward to the water table along upstream
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Tritium is a radioactive isotope of hydrogen having a
half-life of 12.3 years. Tritium is a part of the water
molecule and is an excellent tracer of the movement
of water. Although tritium is naturally occurring, its
presence in the environment has increased as a
result of nuclear weapons testing beginning in 1952.
For the purposes of this paper, water that does not
contain tritium was interpreted as water that infiltrated into the ground prior to 1952 and water that
contains tritium was interpreted as infiltrated after
1952. The peak tritium concentration was presumed
to coincide with water that infiltrated in about 1962
– the peak in the atmospheric testing of nuclear
weapons (Michel, 1976).
Downward rates of movement calculated on the
basis of tritium data range from 0.3 to 0.8 m ⁄ yr, and
180 to 600 years or more, depending on the thickness
of the unsaturated zone, may be required for water to
reach the underlying water table (Izbicki et al.,
2002). However, small amounts of water moving
downward through preferential pathways in the
unsaturated zone may move more rapidly (Izbicki et
al., 2000). Because water spreads laterally away from
the wash as it moves downward, the rate of downward movement decreases with depth (Izbicki et al.,
2000, 2002; Nimmo et al., 2002). Simulations of
unsaturated flow (Izbicki, 2002) show that lateral
spreading can be increased by low permeability
layers within the unsaturated zone that impede the
downward movement of water (Figure 8). The simulated downward rate of movement of infiltrated water
closely matches the rate of movement beneath Oro
Grande Wash estimated on the basis of tritium data.
Although precipitation, runoff, and subsequent
streamflow are highly variable, water potential and
downward rates of water movement damp to a constant value with increasing depth (Nimmo et al.,
2002). For example, seasonal water potential (and
temperature data) collected beneath Quail and Yucca
Washes damp to near constant values within 15 m of
land surface (Nishikawa et al., 2004). Recharge from
these small streams at the water table hundreds of
meters below land surface is not likely to be affected
by short-term climatic cycles, such as El Nino or the
Pacific Decadal Oscillation, even though infiltration
at the streambed surface may vary greatly during
these periods.
In areas where the rate of downward movement is
slow and the unsaturated zone is thick, it is possible
that geomorphic processes that lead to channel abandonment may effectively strand infiltrated water in
the unsaturated zone before it reaches the water
table. For example, water more than 100 m deep in
the unsaturated zone underlying Sheep Creek Wash
was recharged at a time in the geologic past when
the climate was wetter and cooler. This water is iso-
reaches of Oro Grande Wash near Cajon Pass (Figure
7). In contrast, Nishikawa et al. (2004) demonstrated
that infiltrated water did not move downward
through the unsaturated zone near the mountain
front along upstream reaches of Quail Wash in the
southern part of the study area (Figure 1). However,
water did move to depths below the root zone and
presumably to the water table beneath stream reaches farther downstream and along Yucca Wash.
Flows along the downstream reaches have increased
in recent years as a result of upstream urbanization
(Nishikawa et al., 2004).
The rate of downward movement of infiltrated
water beneath the channels of Oro Grande and Sheep
Creek Washes was calculated on the basis of tritium
concentrations in water extracted from core material
collected from the unsaturated zone (Figure 7).
FIGURE 6. Frequency of Temperature Anomalies and
Frequency of Days Interpreted to Have Flow as a Function
of Distance Downstream in Oro Grande, Sheep Creek, and
Big Rock Creek Washes in the Western Mojave Desert,
Southern California, July 1, 1998-June 18, 2000.
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FIGURE 7. Water Content, Water Potential, Chloride, and Tritium Data in the Unsaturated Zone at Selected Sites
Underlying Oro Grande and Sheep Creek Washes Western Mojave Desert, Southern California (Modified from Izbicki et al., 2002).
only about 20 percent of the average annual streamflow infiltrated into the streambed along the study reaches, only a smaller fraction actually infiltrates to
depths below the root zone, and that most water
was transmitted through the study reaches as surface
flow.
Water that flowed through the study reaches either
directly reached the downstream hydrologic systems
as streamflow, or infiltrated into the streambed farther
downstream. Accumulations of soluble salts beneath
the downstream reach of Sheep Creek Wash suggest
that water infiltrated along these downstream reaches
of smaller streams may not infiltrate to depth below
the root zone and move downward toward the water
table (Figure 9). Temperature data collected along the
downstream reach of Sheep Creek Wash also suggest
that streamflow and infiltration, while not occurring
every year, average about 0.7 m ⁄ yr (Izbicki and Michel, 2002). This value may represent a threshold
below which infiltration to depths below the root zone
does not occur. This threshold probably differs with
changes in stream channel morphology and may be
less in wider channels having less vegetation or in
channels composed of highly permeable material.
lated from surface sources and effectively stranded in
the unsaturated zone (Izbicki et al., 2002). Channel
abandonment processes do not occur along Oro
Grande Wash, which is incised into the regional alluvial fan surface, and the position of the active channel of the wash has not changed greatly for the last
500,000 years (Izbicki et al., 2000, 2002).
Infiltration from successive winter streamflows cools
the unsaturated zone beneath the streambed in comparison with the surrounding material. Izbicki and
Michel (2002) showed a good comparison between the
magnitude of the annualized temperature difference in
the unsaturated zone beneath Oro Grande and Sheep
Creek Washes and the surrounding alluvium with
other tracers of water movement through stream channels (Figure 9), and used the data to estimate the
infiltration from streamflow. The average annual infiltration along the study reaches of Oro Grande and
Sheep Creek Washes was then estimated as the average infiltration rate times the width of the wash times
the length of the wash reach between measurement
points (Table 2). Comparison of the average annual
infiltration along the study reaches with estimates of
average annual streamflow (Table 1) suggests that
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FIGURE 9. Difference in Temperature with Depth Between
Access Tubes in Intermittent Streams and Their Respective
Control Sites, and Chloride and Tritium Data Collected Beneath
Streams, Oro Grande and Sheep Creek Washes, Southern
California, 1996–97 (Modified from Izbicki and Michel, 2002).
GROUND-WATER AGE
For the purposes of this article, the cumulative
effect of ground-water recharge to alluvial aquifers
underlying the western Mojave Desert was evaluated
on the basis of changes in the isotopic composition
of ground water. Deuterium, a stable isotope of
FIGURE 8. Simulated Movement of Water Through a
Thick Unsaturated Zone Having Areally Extensive
Clay Layers, Oro Grande Wash, Western Mojave Desert,
Southern California (Modified from Izbicki, 2002).
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Quail Wash
hydrogen, was used to evaluate the source of water.
Tritium and carbon-14, radioactive isotopes of hydrogen and carbon, were used to evaluate the age (time
since recharge) of ground water to assess the temporal connectivity of mountain streams to downgradient hydrologic systems.
Deuterium is a naturally occurring stable isotope
of hydrogen and deuterium abundances are
expressed as ratios in delta notation (d) as per mil
(parts per thousand) differences relative to the
standard known as Vienna Standard Mean Ocean
Water (Gonfiantini, 1978). Water that condensed at
cooler temperatures associated with higher altitudes
or cooler climatic conditions has less of the heavier
isotopes and more negative values than water that
condensed at warmer temperatures associated with
lower altitudes or present-day climatic conditions. In
contrast, water that has been partly evaporated is
enriched in the heavy isotopes relative to its original
composition.
Orographic effects near Cajon Pass between the
San Gabriel and San Bernardino Mountains allow air
masses laden with moisture from the Pacific Ocean to
enter the Mojave Desert during the winter rainy season and precipitate without uplift over the higher
altitudes in the mountains (Izbicki, 2004). As it condenses at lower altitudes and warmer temperatures,
precipitation near Cajon Pass is isotopically heavier
than precipitation that condenses over the mountains. Winter precipitation near Cajon Pass gives rise
to streamflow in the Mojave River. Cumulative
recharge from infiltration of streamflow along the
Mojave River has resulted in a large body of isotopically heavy ground water extending 160 km along
the floodplain aquifer into the Mojave Desert (Figure
10).
The isotopically heaviest water sampled in the
study area is to the west of the Mojave River. This
water originated from precipitation near the pass
that has not been fractionated by orographic uplift
over the mountains and subsequent runoff and infiltration of streamflow in Oro Grande Wash and
other similar washes near the pass (Izbicki et al.,
1995). Despite its heavy dD composition, comparison
with oxygen-18 data shows no evidence of evaporative effects (Izbicki et al., 1995; Izbicki, 2004).
Although the quantity of water from these sources
is small, it is locally important. Similar processes
have resulted in isotopically heavy ground water in
the eastern part of the study area near San Gorgonio Pass (Figure 10), and along the western edge of
Antelope Valley (not shown in Figure 1) where the
altitudes of the San Gabriel Mountains are lower
(Smith et al., 1992).
Much of the water in the floodplain aquifer along
the Mojave River contains tritium (Figure 10). This
Notes: Instantaneous infiltration measured using a 1.2-meter-diameter double-ring infiltrometer. Annual infiltration rate, infiltration along the study reach estimated from temperature data (J. Kulongoski, U.S. Geological Survey, written communication, 2006). m ⁄ hr, meters per hour; m ⁄ yr, meters per year, m3, cubic meters; hm3,cubic hectometers (106 · m3).
*Infiltration rate and annual infiltration calculated for 20-kilometer reach of Yucca Wash downstream from Quail Wash.
0.1*
0.25*
0.46-0.79
0.04
0.51
0.1
0.58
0.7-2.0
0.7-1.2
0.28-0.72
0.04-0.14
Medium sand
Cobbles near mountain
front to silt farther Downstream
coarse sand
Oro Grande Wash
Sheep Creek Wash
Annual Deep Infiltration
Along Study Reach (hm3)
Annual Infiltration
Along Study Reach (hm3)
Annual Infiltration Rate
(m ⁄ yr)
Instantaneous Infiltration
Rate (m ⁄ hr)
Description of
Stream Channel
Stream
TABLE 2. Streambed Characteristics and Infiltration Along the Study Reaches of Oro Grande, Sheep Creek, and Quail Washes, Western Mojave Desert, Southern California.
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water was distributed more than 160 km from Cajon
Pass and the mountain front along the channel of the
Mojave River by infiltration from occasional surface
flow in the river. In contrast, only a small amount of
water containing tritium was present near the San
Gabriel and San Bernardino Mountains where smaller intermittent streams flow from the mountains.
Although infiltration from intermittent streams
draining the San Gabriel and San Bernardino Mountains is locally important, especially in canyons near
the mountain front, the amount of water from these
sources containing tritium is small when compared
with the volume of water in storage and the volume
of water infiltrated from the Mojave River.
Like tritium, carbon-14 also provides information
on the age, or time since recharge, of ground water.
Carbon-14 is a naturally occurring radioactive isotope
of carbon having a half-life of about 5,730 years
(Mook, 1980). Carbon-14 data are expressed as percent modern carbon (pmc) by comparing carbon-14
activities to the specific activity of National Bureau of
Standards oxalic acid: 13.56 disintegrations ⁄ min ⁄ g of
carbon equals 100 pmc (Kalin, 2000). Carbon-14 was
produced, as was tritium, by the atmospheric testing
of nuclear weapons. As a result, carbon-14 activities
may exceed 100 pmc in areas where ground water
contains tritium. Because of its longer half-life, carbon-14 preserves information on the cumulative volume of water infiltrated from headwater streams
over a longer time scale than does tritium. For example, ground water having a carbon-14 activity of
50 pmc was recharged 5,730 years before present,
and 30 pmc was recharged 9,950 years before present
– assuming that there have been no chemical reactions between ground water and the alluvial deposits
that compose the aquifer.
Unlike tritium, carbon-14 is not a part of the water
molecule, and carbon-14 activities are affected by
chemical reactions between ground water and aquifer
material. Carbon-14 activities shown in Figure 10 do
not account for these reactions. Ground-water ages
estimated from uncorrected carbon-14 activities may
overestimate ground-water age by as much as 30 percent compared with estimated ages that account for
chemical reactions between the ground water and
aquifer material (Izbicki et al., 1995). Despite this
uncertainty, uncorrected carbon-14 ages are a useful
approximation of ground-water age.
The spatial distribution of carbon-14 activities
greater than 90 pmc is similar to the distribution of
tritium data with high activities along the floodplain
aquifer and small areas near the mountain front
(Figure 10). Carbon-14 activities greater than 50 pmc
show the cumulative effect of as much as 5,730 years
(one half-life) of streamflow infiltration near the front
of the San Gabriel and San Bernardino Mountains
FIGURE 10. Delta Deuterium, Tritium, and Carbon-14 Composition
of Water From the Wells in the Western Mojave Desert, Southern
California (Modified from Izbicki, 2004, and Izbicki and Michel, 2004).
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and from streams, such as Oro Grande Wash, near
Cajon Pass. Carbon-14 activities greater than 50 pmc
along the channel of Pipes Wash and Yucca Washes
in the southern part of the study area suggest that
occasional flow in these washes infiltrates through
the unsaturated zone to the water table for tens of kilometers into the Mojave Desert. Carbon-14 and dD
data also show the cumulative recharge from infiltration of streamflow in intermittent streams near Cajon
Pass, such as Oro Grande Wash (Izbicki et al., 1995).
Although small in magnitude, the cumulative effect
of flow and subsequent ground-water recharge from
these smaller streams is increasingly important over
the longer time-scales measure by carbon-14 than by
tritium.
The complex distribution of recent and older
ground-water ages and ground-water flow paths
under predevelopment conditions in the alluvial aquifers underlying the Mojave ground-water basin were
simulated using a regional ground-water flow model
linked to a particle-tracking model (Stamos et al.,
2001; Izbicki et al., 2004). The model results identified the ground-water flow paths from the mountain
front through the regional aquifer to ground-water
discharge areas near El Mirage (dry) Lake, and to
the floodplain aquifer (Figure 11). The model also
identified the ground-water flow paths through the
floodplain aquifer to discharge areas near Harper
(dry) Lake, Coyote (dry) Lake, and Afton Canyon and
defined the complex interaction between the floodplain aquifer, the Mojave River, and the surrounding
and underlying regional aquifer. Under present-day
conditions, ground-water pumping is the largest discharge from many aquifers in the western Mojave
Desert. Ground-water pumping has altered the predevelopment water levels and ground-water flow paths.
Water from mountain headwater streams that eventually discharged to downgradient hydrologic systems
under predevelopment conditions would, under present-day conditions, likely discharge as pumpage
from wells – further contributing to the isolation of
mountain headwater streams from downgradient
hydrologic systems.
DISCUSSION AND CONCLUSIONS
Mountain headwater streams in arid areas are
often physically isolated from downstream hydrologic
systems such as springs, playa lakes, wetlands, or
through-flowing streams and rivers by reaches of dry
channels across alluvial fan or basin fill deposits. The
physical isolation of surface flow in mountain headwater streams from downstream systems may be
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FIGURE 11. Particle-Tracking Model Results
for the Mojave Ground-Water Basin (Modified from
Stamos et al., 2001; Izbicki et al., 2004).
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Information on streamflow characteristics, travel
times through unsaturated zones and underlying
aquifers may have transfer value from the western
Mojave Desert to other arid areas in the southwestern United States. However, headwater streams in
the western Mojave Desert (even those tributary to
the Mojave River) flow from mountain areas to closed
basins. Under present-day geologic and climatic conditions, these internally drained basins are physically
isolated by the intervening mountain ranges from the
larger drainages that flow to interstate waters or to
discharge to the ocean.
broken for brief periods after rainfall or snowmelt in
the higher mountains when runoff is sufficient to
allow flow along the entire downstream wash reach.
Larger streams, such as the Mojave River and to a
lesser extent Pipes and Yucca Washes in the western
Mojave Desert, may occasionally produce flows that
extend many kilometers from the mountain front into
the desert and briefly provide a physical connection
from mountain headwater streams to downstream
hydrologic systems. The recurrence interval of these
large flows is known for many larger streams in arid
areas and can be estimated for smaller streams.
Despite the physical isolation of surface flow in
headwater streams, they are an integral part of the
hydrologic cycle in arid regions. Water infiltrated
from surface flow in headwater streams moves downward through the unsaturated zone to the underlying
ground-water system. Under predevelopment conditions, this infiltrated water eventually discharged to
springs, streamflow, isolated wetlands, or native
vegetation. However, infiltrated water may be temporally isolated from downgradient discharge areas
as it flows through thick unsaturated zones and along
long flowpaths through underlying aquifers. For
example, travel times through the unsaturated zone
underlying Oro Grande and Sheep Creek Washes are
several hundred years. Travel times through the
underlying regional aquifer are longer and ground
water ages may be as great as a thousand to several
tens of thousands of years at the downgradient end of
long flowpaths through the regional aquifer. In contrast, ground water in the floodplain aquifer underlying the Mojave River commonly contains tritium and
ground-water age is measured in decades.
The selection of a time period as the cutoff for defining isolation of water infiltrated from surface
streams through ground-water systems is arbitrary
and depends on the nature of the problem being considered. Winter and LaBaugh (2003) speculated that
wetlands should not be considered isolated even if
several decades are required for water to reach downgradient hydrologic systems. Studies on the suitability of sites in arid areas for toxic or radioactive waste
disposal must consider the need for hydrologic isolation of thousands of years in duration and changing
long-term climate cycles (National Research Council,
1995). Regardless of the criteria ultimately selected
for management of mountain headwater streams in
arid areas under the Clean Water Act, under presentday conditions, water infiltrated from headwater
streams into aquifers may ultimately reach downstream hydrologic systems through pumping for
water supply and subsequent discharge from wastewater treatment plants rather than as ground-water
discharge through the hydrologic cycle.
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ACKNOWLEDGMENTS
Funding for this paper was provided by the U.S. Geological Survey’s Office of Ground Water. Previous studies on which this work
was based were funded by the Mojave Water Agency, and Joshua
Basin Water District. The author thanks James Bowers, Steven
Phillips, and Peter Martin of the U.S. Geological Survey and Tracie
Nodeau of the U.S. Environmental Protection Agency for their constructive comments during the preparation of this manuscript.
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