Physical and Temporal Isolation of Mountain Headwater Streams in the Western Mojave Desert, Southern California1

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

INTRODUCTION 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 The Clean Water Act regulates the discharge of recent decisions find that waters that can convey pol- pollutants from point sources and the discharge of fill lutants to downstream navigable waters for even material into ‘‘navigable waters,’’ which the act brief periods are jurisdictional because ‘‘pollutants defines as ‘‘waters of the United States.’’ The extent need not reach interstate bodies of water immediately to which ‘‘waters of the United States’’ include small or continuously in order to inflict serious environmen- isolated hydrologic systems was questioned in a 2001 tal damage’’ (United States vs. Eidson, 94–2330). U.S. Supreme Court decision that limited the U.S. Surface flow in streams draining mountain head- Army Corps of Engineers jurisdiction under the water areas in the arid western United States is com- Clean Water Act over isolated waters (SWANCC vs. monly physically isolated from downstream playa U.S. Army Corps of Engineers, 98-2277). Since the lakes, wetlands, or larger streams and rivers by SWANCC decision, many Federal Court decisions stream reaches that are dry much of the time. The

1Paper 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. 2Research Hydrologist, U.S. Geological Survey, 4165 Spruance Road, San Diego, California (E-Mail ⁄ Izbicki: [email protected]).

JAWRA 26 JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION ISOLATION OF MOUNTAIN HEADWATER STREAMS physical isolation of surface flow in mountain head- through Cajon Pass and precipitate without passing water streams (whether perennial or intermittent) over the higher altitudes of the San Gabriel and San from downstream systems may be broken for brief Bernardino Mountains. Precipitation near the pass periods after rainfall or snowmelt when runoff is suf- can give rise to streamflow along the entire length of ficient to allow flow along the entire downstream the Mojave River and flow in smaller streams near reach. Despite the physical isolation of surface flow the pass, such as Oro Grande Wash. A similar gap in these streams, they are an integral part of hydro- between the San Bernardino and San Jacinto Moun- logic systems in arid regions. Water infiltrated from tains, San Gorgornio Pass, to the southeast of the headwater streams moves through the unsaturated study area (not shown in Figure 1), also allows cool zone to recharge the underlying ground-water sys- moist air to enter the desert and gives rise to winter tem. This ground water eventually discharges to sup- precipitation and intermittent streamflows in that port springs, streamflow, isolated wetlands, or native area – although the effect is smaller than near Cajon vegetation far from recharge areas. In some systems, Pass (Izbicki, 2004). Although summer thunderstorms ground-water movement from recharge areas to dis- occur, especially in the eastern part of the study area, charge areas may require many thousands of years. summer monsoonal precipitation is of lesser import- In addition to their physical and temporal isola- ance in the western Mojave Desert than elsewhere in tion, the mountain headwater streams in the western the southwestern United States. Mojave Desert are further isolated from other hydro- With the exception of some small streams that logic systems by their geologic setting within the drain the higher altitudes of the San Gabriel and San Basin and Range physiographic province. Under ‘‘pre- Bernardino Mountains and short reaches of the sent-day’’ climatic conditions, many internally Mojave River where ground-water discharges at land drained basins (also known as ‘‘closed basins’’) within surface, there are no perennial streams in the area. the Basin and Range physiographic province are Physical connection between mountain headwater physically isolated from larger drainages that flow to streams (whether perennial or intermittent) and interstate waters or discharge to the ocean by inter- downstream hydrologic systems in the western vening mountain ranges. Mojave Desert occurs only during brief periods of The purpose of this paper is to summarize on the streamflow after precipitation or snowmelt along nor- basis of existing data and published work (1) the brief mally dry downstream reaches that cross alluvial physical connection of selected mountain headwater fans and basin fill deposits. streams in the western Mojave Desert to downstream There are a number of internally drained alluvial hydrologic systems, (2) the connection of water infil- basins in the western Mojave Desert each having dis- trated from these streams through the unsaturated tinct ground-water-flow systems often separated by zone to the underlying ground-water system, and (3) faults and bedrock outcrops. Alluvial deposits in some the longer time-scale connection through the ground- basins are more than 1,000 m thick and saturated water system to discharge areas farther downgradi- deposits may be separated from land surface by unsat- ent. Only brief descriptions of methods are given in urated alluvium as much as 300 m thick near the this paper and the reader is referred to the cited mountain front. Ground-water movement in these work for a more thorough explanation of the meth- basins is generally from recharge areas near the moun- ods, data, and results. tain 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 HYDROGEOLOGIC SETTING 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 The western Mojave Desert east of Los Angeles of the regional aquifer, ground-water recharge is small (Figure 1) is arid with hot, dry summers, and cold in relation to the volume of water in storage and travel winters. With the exception of the higher altitudes in times through the aquifer system are often many thou- the San Gabriel and San Bernardino Mountains, pre- sands of years (Izbicki et al., 1995; Izbicki and Michel, cipitation is generally about 150 mm ⁄ yr or less, but 2004). In contrast, the floodplain aquifer is more lim- amounts vary greatly from year to year. In most of ited in areal and vertical extent (typically less than the area, precipitation is greater during the winter 2.5 km wide and 80 m thick) than the surrounding rainy season (November-March) and occurs as a alluvial aquifers and is readily recharged by infiltra- result of cyclonic storms moving inland from the Paci- tion of streamflow in the Mojave River. fic Ocean. During winter cyclonic storms, moist air Numerous water-level maps have been prepared of from the Pacific Ocean can enter the Mojave Desert aquifers in the area (Stamos and Predmore, 1995;

JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION 27 JAWRA IZBICKI

FIGURE 1. Location of Study Area.

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Mendez and Christensen, 1997; Smith and Pimentel, headwater areas to downstream reaches of the Mojave 2000; Smith et al., 2004; Stamos et al., 2004). Several River was almost 500 hm3 (5 · 108 m3). Annual flows regional-scale ground-water flow models simulating of this magnitude have a recurrence interval of greater ground-water flow have been completed for the than 50 years (Lines, 1996) and this was the first time Mojave River ground-water basin (Hardt, 1971; Sta- the river flowed continuously since 1983. More mos et al., 2001) and the Antelope Valley (Leighton thorough analyses of the magnitude and frequency of and Phillips, 2003). Smaller scale flow models have surface flows in the Mojave River from stream gaging been completed for some subbasins in the Morongo stations are available in Lines (1996) and Stamos et al. ground-water basin (Londquist and Martin, 1991; (2001). 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 repor- ted to have overflowed its banks upstream from Bar- stow and flowed northwestward into Harper (dry) Lake (Lines, 1996). FIGURE 2. Reaches of the Mojave River That Had Streamfow The physical connection of headwater reaches of the During Water Years 1992–94 (modified from Lines, 1996). Mojave River, the largest stream in the study area, to downstream reaches was assessed by Lines (1996) Smaller Streams during water years 1992–94 (Figure 2). Perennial flow during this period occurred only at the Upper Nar- Smaller streams are obviously more numerous than rows, the Lower Narrows, downstream from a regional larger streams, such as the Mojave River. About 140 wastewater treatment plant serving the Victorville mountain headwater streams draining at least area, and at Afton Canyon. Records from early trave- 0.9 km2 were identified along the mountain front lers and explorers in the area suggest that perennial between Palmdale and Twentynine Palms (Figure 3). flow was more extensive prior to ground-water pump- Streamflow quantity and frequency data have been ing (Lines, 1996). During each winter, runoff from the estimated using a variety of techniques for reaches of headwaters, coupled with seasonal decreases in several smaller streams discussed in this article. Quail ground-water pumping and evapotranspiration from Wash, Big Rock Creek, and Sheep Creek are among riparian habitat extended the seasonal surface flow. the larger streams identified in Figure 3; streamflow Stamos et al. (2001) showed that pumping along the quantity and frequency for the more numerous river decreased the magnitude and frequency of sea- streams draining less than 20 km2 are largely unavail- sonal surface flow in the Mojave River along stream able. Oro Grande Wash discussed in the article is not reaches farther downstream from the mountain front. shown in Figure 3 because it originates near Cajon The river flowed along its entire main stem down- Pass and does not drain the mountain front. stream to Afton Canyon for a few weeks during water Streamflow data from gaging stations are less year 1993 as a result of a series of large storms (Lines, available for smaller intermittent streams than for 1996). During 1993, the total annual flow from larger streams such as the Mojave River; as a

JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION 29 JAWRA IZBICKI consequence, the frequency of surface ﬂow in smaller intermittent streams along the front of the San Gab-

riel Mountains to downstream channel reaches was am gage estimated on the basis of streambed temperature data. During the winter months, when most precipi- , cubic hectome- rn California. tation occurs, streamﬂow is relatively cold, often only 3 slightly above 0C. Cold streamﬂow causes measur- ntain front was esti- Average Slope of able changes in streambed temperature that do not Study Reach (percent) occur in ground temperature measurements at control sites adjacent to, but outside, the wash (Con- stantz et al., 2001, 2003). Streambed temperature data are relatively easy and inexpensive to collect and numerous measurement stations can be installed (m) along a wash reach to determine the downstream

extent and duration of winter storm flows. Stream- Downstream Site flow interpreted from temperature data was verified by examination of the channel during site visits after storms. The approach is attractive in areas where it Channel Width is impractical or prohibitively expensive to install tra- (m) ditional stream gages that may be damaged or des- troyed during large streamflows. Streambed temperature data were collected along three selected Mountain Front 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 Reach (km)

are among the largest in the western Mojave Desert Length of Study and study reaches total almost 70 km. Each wash

represents a range of hydrologic conditions (Table 1). ) 3 Flow (hm Average Annual (m) Average Altitude (m) Maximum , square kilometers. 2 ) 2 Mountain Front (km Drainage Area at ); km, kilometers; km 3 m · 6

FIGURE 3. Rank-Order Distribution of Drainage Basins mated from a relationdata between at Valyermo channel 5 geometry km and upstream annual from ﬂow the developed mountain by front Lines – not (1996) applicable – because except Oro for Grande Big Wash Rock does Creek, not drain for from which the ﬂow mountains; is m, estimated meters; from hm stre ters (10 Greater Than 0.9 km2 on the Northern Slope of the San Gabriel,

San Bernardino, and Little San Bernardino Mountains Between TABLE 1. Physical Characteristics Along the Study Reaches of Oro Grande, Sheep Creek, Big Rock Creek, and Quail Washes, Western Mojave Desert, Southe Stream Palmdale and Twentynine Palms, California. Oro Grande WashSheep Creek WashBig Rock CreekQuail WashNotes: Drainage areas - 36.8 are at the mountain front. Altitudes are 108 for the drainage area are upstream from the 237 mountain front. Average 2,594 annual ﬂow at the mou - 2,829 1,948 1,768 1,769 - 1,351 3.1 16 0.5 28 18.8 27.3 22.7 15.2 80 10 3 10 3 40 20 3 4.9 1.5 1.7 3.1

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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.

An example of streambed temperature changes example of streambed temperature changes inter- interpreted as streamflow is shown for selected meas- preted as streamflow is shown for a site along urement sites along Sheep Creek Wash, February Oro Grande Wash, July 5–12, 1999 (Figure 5). Ana- 17–27, 2000 (Figure 4). The interpreted streamflow is lysis of temperature data suggests that streamflow of greater duration along the upstream sections of might not have occurred at upstream or down- the wash at the mountain front. Runoff from precipi- stream temperature measurement sites during this tation is directed away from the active channel of period. Sheep Creek Wash by the conical shape of the allu- If interpretations of streambed temperature data vial fan and streamflow decreases in duration with are not constrained by meteorological data and fre- distance downstream as water infiltrates into the quent site visits, all measured streambed tempera- underlying streambed. ture anomalies could be interpreted as streamflow – Streamflow is more difficult to interpret from producing a higher frequency of flow than might have streambed temperature data during the summer occurred (Figure 6). Despite the inherent uncertainty when the difference between precipitation, runoff, associated with this approach, estimates of stream- and streambed temperatures may be small. The flow occurrence inferred from temperature data can interpretation may be further complicated because be assembled into statistical representations of summer precipitation in arid areas is often highly streamflow frequency that reflect the regional hydrol- variable spatially, limited in areal extent. An ogy of the study area.

JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION 31 JAWRA IZBICKI

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.

Oro Grande Wash is the smallest of the three UNSATURATED FLOW 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 In arid alluvial valleys of the western Mojave flow along mountain front reaches of the other Desert, areal recharge from precipitation and subse- washes and the duration of flows is less – typically quent movement of water through the unsaturated about 1 hour. Given a frequency of flow of 0.05 zone is negligible. In fact, thick unsaturated zones days ⁄ yr and a duration of 1 hour, Oro Grande Wash overlying alluvial aquifers in the Mojave Desert may only flow for as few as 18 hours each year (365 within California have been proposed as storage days ⁄ yr · 0.05 stormflows ⁄ day · 1 hour ⁄ stormflow). repositories for toxic and nuclear waste (National Although during large winter storms Oro Grande Research Council, 1995). However, along intermittent Wash may flow uninterrupted from its headwaters stream channels water may infiltrate to depths below near Cajon Pass through the study reach to the the root zone and ultimately reach the underlying Mojave River (Izbicki et al., 2000), flows along shorter water table. In these areas where the volume of reaches of the wash are more common. This is especi- water infiltrated is small, and the unsaturated zone ally true along the downstream urbanized reach of is thick, or relatively impermeable, the slow move- Oro Grande Wash where runoff from impervious ment of water through the unsaturated zone may urban areas contributes to increased streamflow. contribute to the temporal isolation of small head- Frequency and duration of flow in Sheep Creek water streams from underlying aquifers and down- Wash are greater than in Oro Grande Wash because gradient hydrologic systems. Sheep Creek drains a larger area in the higher alti- Infiltration from streamflow commonly occurs in tudes in the San Gabriel Mountains. Although not per- greater amounts along upstream reaches near the ennial, Sheep Creek may flow for extended periods mountain front (Izbicki et al., 2002). Measurements during the winter and during spring runoff. For exam- of water content, water potential, and low concentra- ple, the duration of a single flow in Sheep Creek at the tions of soluble salts (such as chloride) in the unsat- mountain front between February 24 and February 26, urated zone beneath upstream reaches of Sheep 2000 exceeded the estimated cumulative annual flow Creek Wash (Figure 7) are consistent with the move- duration along Oro Grande Wash. Unlike Oro Grande ment of infiltrated water to depths below the root Wash, where flows along only the downstream reaches zone and presumably to the underlying water table are common, flow in both Sheep Creek and Big Rock as much as 300 m below land surface (Izbicki et al., Creek Washes decreases in frequency and duration 2002). Similarly, water infiltrated during stormflow with distance downstream (Figure 6). moves downward to the water table along upstream

JAWRA 32 JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION ISOLATION OF MOUNTAIN HEADWATER STREAMS reaches of Oro Grande Wash near Cajon Pass (Figure Tritium is a radioactive isotope of hydrogen having a 7). In contrast, Nishikawa et al. (2004) demonstrated half-life of 12.3 years. Tritium is a part of the water that infiltrated water did not move downward molecule and is an excellent tracer of the movement through the unsaturated zone near the mountain of water. Although tritium is naturally occurring, its front along upstream reaches of Quail Wash in the presence in the environment has increased as a southern part of the study area (Figure 1). However, result of nuclear weapons testing beginning in 1952. water did move to depths below the root zone and For the purposes of this paper, water that does not presumably to the water table beneath stream rea- contain tritium was interpreted as water that infil- ches farther downstream and along Yucca Wash. trated into the ground prior to 1952 and water that Flows along the downstream reaches have increased contains tritium was interpreted as infiltrated after in recent years as a result of upstream urbanization 1952. The peak tritium concentration was presumed (Nishikawa et al., 2004). to coincide with water that infiltrated in about 1962 The rate of downward movement of infiltrated – the peak in the atmospheric testing of nuclear water beneath the channels of Oro Grande and Sheep weapons (Michel, 1976). Creek Washes was calculated on the basis of tritium Downward rates of movement calculated on the concentrations in water extracted from core material basis of tritium data range from 0.3 to 0.8 m ⁄ yr, and collected from the unsaturated zone (Figure 7). 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 FIGURE 6. Frequency of Temperature Anomalies and table. For example, water more than 100 m deep in Frequency of Days Interpreted to Have Flow as a Function of Distance Downstream in Oro Grande, Sheep Creek, and the unsaturated zone underlying Sheep Creek Wash Big Rock Creek Washes in the Western Mojave Desert, was recharged at a time in the geologic past when Southern California, July 1, 1998-June 18, 2000. the climate was wetter and cooler. This water is iso-

JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION 33 JAWRA IZBICKI

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).

lated from surface sources and effectively stranded in only about 20 percent of the average annual stream- the unsaturated zone (Izbicki et al., 2002). Channel flow infiltrated into the streambed along the study rea- abandonment processes do not occur along Oro ches, only a smaller fraction actually infiltrates to Grande Wash, which is incised into the regional allu- depths below the root zone, and that most water vial fan surface, and the position of the active chan- was transmitted through the study reaches as surface nel of the wash has not changed greatly for the last flow. 500,000 years (Izbicki et al., 2000, 2002). Water that flowed through the study reaches either Infiltration from successive winter streamflows cools directly reached the downstream hydrologic systems the unsaturated zone beneath the streambed in com- as streamflow, or infiltrated into the streambed farther parison with the surrounding material. Izbicki and downstream. Accumulations of soluble salts beneath Michel (2002) showed a good comparison between the the downstream reach of Sheep Creek Wash suggest magnitude of the annualized temperature difference in that water infiltrated along these downstream reaches the unsaturated zone beneath Oro Grande and Sheep of smaller streams may not infiltrate to depth below Creek Washes and the surrounding alluvium with the root zone and move downward toward the water other tracers of water movement through stream chan- table (Figure 9). Temperature data collected along the nels (Figure 9), and used the data to estimate the downstream reach of Sheep Creek Wash also suggest infiltration from streamflow. The average annual infil- that streamflow and infiltration, while not occurring tration along the study reaches of Oro Grande and every year, average about 0.7 m ⁄ yr (Izbicki and Mi- Sheep Creek Washes was then estimated as the aver- chel, 2002). This value may represent a threshold age infiltration rate times the width of the wash times below which infiltration to depths below the root zone the length of the wash reach between measurement does not occur. This threshold probably differs with points (Table 2). Comparison of the average annual changes in stream channel morphology and may be infiltration along the study reaches with estimates of less in wider channels having less vegetation or in average annual streamflow (Table 1) suggests that channels composed of highly permeable material.

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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 FIGURE 8. Simulated Movement of Water Through a Thick Unsaturated Zone Having Areally Extensive effect of ground-water recharge to alluvial aquifers Clay Layers, Oro Grande Wash, Western Mojave Desert, underlying the western Mojave Desert was evaluated Southern California (Modiﬁed from Izbicki, 2002). on the basis of changes in the isotopic composition of ground water. Deuterium, a stable isotope of

JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION 35 JAWRA IZBICKI )

3 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 downgradi- ,cubic hectome- 3

hern California. ent hydrologic systems. Deuterium is a naturally occurring stable isotope of hydrogen and deuterium abundances are

Annual Deep Inﬁltration expressed as ratios in delta notation (d) as per mil Along Study Reach (hm (parts per thousand) differences relative to the ) 3 study reach estimated from tem- standard known as Vienna Standard Mean Ocean , cubic meters; hm 3 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

Annual Inﬁltration contrast, water that has been partly evaporated is yr, meters per year, m ⁄ Along Study Reach (hm 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 yr)

⁄ enter the Mojave Desert during the winter rainy sea-

(m son and precipitate without uplift over the higher

hr, meters per hour; m altitudes in the mountains (Izbicki, 2004). As it con- ⁄ denses at lower altitudes and warmer temperatures, precipitation near Cajon Pass is isotopically heavier Annual Infiltration Rate 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 hr) ⁄ Mojave River has resulted in a large body of isotopically heavy ground water extending 160 km along

0.04-0.14 0.7-1.2the ﬂoodplain 0.58 aquifer into 0.51 the Mojave Desert (Figure

Rate (m 10). The isotopically heaviest water sampled in the study area is to the west of the Mojave River. This Instantaneous Infiltration 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 evapora- tive effects (Izbicki et al., 1995; Izbicki, 2004). Description of

Stream Channel Although the quantity of water from these sources is small, it is locally important. Similar processes have resulted in isotopically heavy ground water in front to silt farther Downstream ). 3 the eastern part of the study area near San Gorgo- m

· nio Pass (Figure 10), and along the western edge of 6 Antelope Valley (not shown in Figure 1) where the altitudes of the San Gabriel Mountains are lower ters (10 perature data (J. Kulongoski, U.S. Geological Survey, written communication, 2006). m (Smith et al., 1992). Much of the water in the floodplain aquifer along TABLE 2. Streambed Characteristics and Infiltration Along the Study Reaches of Oro Grande, Sheep Creek, and Quail Washes, Western Mojave Desert, Sout Stream *Infiltration rate and annual infiltration calculated for 20-kilometer reach of Yucca Wash downstream from Quail Wash. Oro Grande WashSheep Creek Wash Medium sand Cobbles near mountain 0.28-0.72 0.7-2.0 0.1 0.04 Quail WashNotes: Instantaneous infiltration measured using a 1.2-meter-diameter double-ring coarse infiltrometer. sand Annual infiltration rate, infiltration along the the Mojave 0.46-0.79 River contains 0.25* tritium (Figure 10). 0.1* This -

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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 Moun- tains 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 ⁄ gof 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 FIGURE 10. Delta Deuterium, Tritium, and Carbon-14 Composition (one half-life) of streamflow infiltration near the front of Water From the Wells in the Western Mojave Desert, Southern California (Modified from Izbicki, 2004, and Izbicki and Michel, 2004). of the San Gabriel and San Bernardino Mountains

JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION 37 JAWRA IZBICKI 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 FIGURE 11. Particle-Tracking Model Results physical isolation of surface flow in mountain head- for the Mojave Ground-Water Basin (Modified from water streams from downstream systems may be Stamos et al., 2001; Izbicki et al., 2004).

JAWRA 38 JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION ISOLATION OF MOUNTAIN HEADWATER STREAMS broken for brief periods after rainfall or snowmelt in Information on streamflow characteristics, travel the higher mountains when runoff is sufficient to times through unsaturated zones and underlying allow flow along the entire downstream wash reach. aquifers may have transfer value from the western Larger streams, such as the Mojave River and to a Mojave Desert to other arid areas in the southwest- lesser extent Pipes and Yucca Washes in the western ern United States. However, headwater streams in Mojave Desert, may occasionally produce flows that the western Mojave Desert (even those tributary to extend many kilometers from the mountain front into the Mojave River) flow from mountain areas to closed the desert and briefly provide a physical connection basins. Under present-day geologic and climatic con- from mountain headwater streams to downstream ditions, these internally drained basins are physically hydrologic systems. The recurrence interval of these isolated by the intervening mountain ranges from the large flows is known for many larger streams in arid larger drainages that flow to interstate waters or to areas and can be estimated for smaller streams. discharge to the ocean. Despite the physical isolation of surface flow in headwater streams, they are an integral part of the hydrologic cycle in arid regions. Water infiltrated ACKNOWLEDGMENTS from surface flow in headwater streams moves downward through the unsaturated zone to the underlying Funding for this paper was provided by the U.S. Geological Sur- ground-water system. Under predevelopment condi- vey’s Office of Ground Water. Previous studies on which this work tions, this infiltrated water eventually discharged to was based were funded by the Mojave Water Agency, and Joshua Basin Water District. The author thanks James Bowers, Steven springs, streamflow, isolated wetlands, or native Phillips, and Peter Martin of the U.S. Geological Survey and Tracie vegetation. However, infiltrated water may be tem- Nodeau of the U.S. Environmental Protection Agency for their con- porally isolated from downgradient discharge areas structive comments during the preparation of this manuscript. as it flows through thick unsaturated zones and along long flowpaths through underlying aquifers. For example, travel times through the unsaturated zone LITERATURE CITED underlying Oro Grande and Sheep Creek Washes are several hundred years. Travel times through the Constantz, J., D. Stonestrom, A.E. Stewart, R. Niswonger and T.R. underlying regional aquifer are longer and ground Smith, 2001. Analysis of Streambed Temperatures in Ephem- water ages may be as great as a thousand to several eral Channels to Determine Streamflow Frequency and Dur- ation. Water Resources Research 37(2):317-328. tens of thousands of years at the downgradient end of Constantz, J., S.W. Tyler and E. Kwicklis, 2003. Temperature-Pro- long flowpaths through the regional aquifer. In con- file Methods for Estimating Percolation Rates in Arid Environ- trast, ground water in the floodplain aquifer underly- ments. Vados Zone Journal 2:12-24 http://vzj.scijournals.org/cgi/ ing the Mojave River commonly contains tritium and reprint/2/1/12, accessed November 10, 2005. ground-water age is measured in decades. Gonfiantini, R., 1978. Standards for Stable Isotope Measurements in Natural Compounds. Nature 271:534-536. The selection of a time period as the cutoff for defi- Hardt, W.F., 1971. Hydrologic Analysis of Mojave River Basin, ning isolation of water infiltrated from surface California, Using Electric Analog Model. U.S. Geological Survey streams through ground-water systems is arbitrary Open-File Report, 84 pp. and depends on the nature of the problem being con- Izbicki, J.A., P. Martin and R.L. Michel, 1995. Source, Movement sidered. Winter and LaBaugh (2003) speculated that and Age of Groundwater in the Upper Part of the Mojave River Basin, California, USA. In: Application of Tracers in Arid Zone wetlands should not be considered isolated even if Hydrology, E.M. Adair and Ch. Leibundgut, (Editors). Interna- several decades are required for water to reach down- tional Association of Hydrologic Sciences Publication No. 232, gradient hydrologic systems. Studies on the suitabil- pp. 43-56. ity of sites in arid areas for toxic or radioactive waste Izbicki, J.A., 2002. Geologic and Hydrologic Controls on the Move- disposal must consider the need for hydrologic isola- ment of Water Through a Thick, Heterogeneous Unsaturated Zone Underlying an Intermittent Stream in the Western Mojave tion of thousands of years in duration and changing Desert, Southern California. Water Resources Research 38(3), long-term climate cycles (National Research Council, doi: 10.1029/2000WR000197. 1995). Regardless of the criteria ultimately selected Izbicki, J.A., 2004. Source and Movement of Ground Water in the for management of mountain headwater streams in Western Part of the Mojave Desert, Southern California, USA. arid areas under the Clean Water Act, under present- U.S. Geological Survey Water Resources Investigations Report 03-4313, 18 pp. http://water.usgs.gov/pubs/wri/wrir034313, day conditions, water infiltrated from headwater accessed November 10, 2005. streams into aquifers may ultimately reach down- Izbicki, J.A. and R.L. Michel, 2002. 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