physical and temporal isolation of mountain headwater streams …€¦ · physical and temporal...

PHYSICAL AND TEMPORAL ISOLATION OF MOUNTAIN HEADWATERSTREAMS IN THE WESTERN MOJAVE DESERT, SOUTHERN CALIFORNIA1

John A. Izbicki2

ABSTRACT: Streams draining mountain headwater areas of the western Mojave Desert are commonly physic-ally isolated from downstream hydrologic systems such as springs, playa lakes, wetlands, or larger streams andrivers by stream reaches that are dry much of the time. The physical isolation of surface flow in these streamsmay be broken for brief periods after rainfall or snowmelt when runoff is sufficient to allow flow along the entirestream reach. Despite the physical isolation of surface flow in these streams, they are an integral part of thehydrologic cycle. Water infiltrated from headwater streams moves through the unsaturated zone to recharge theunderlying ground-water system and eventually discharges to support springs, streamflow, isolated wetlands, ornative vegetation. Water movement through thick unsaturated zones may require several hundred years andsubsequent 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 MojaveDesert, Southern California. Journal of the American Water Resources Association (JAWRA) 43(1):26-40. DOI:10.1111/j.1752-1688.2007.00004.x

INTRODUCTION

The Clean Water Act regulates the discharge ofpollutants from point sources and the discharge of fillmaterial into ‘‘navigable waters,’’ which the actdefines as ‘‘waters of the United States.’’ The extentto which ‘‘waters of the United States’’ include smallisolated hydrologic systems was questioned in a 2001U.S. Supreme Court decision that limited the U.S.Army Corps of Engineers jurisdiction under theClean Water Act over isolated waters (SWANCC vs.U.S. Army Corps of Engineers, 98-2277). Since theSWANCC decision, many Federal Court decisions

have discussed the extent of ‘‘waters of the UnitedStates,’’ including streams in arid areas that are iso-lated from larger hydrologic systems. Several of theserecent decisions find that waters that can convey pol-lutants to downstream navigable waters for evenbrief periods are jurisdictional because ‘‘pollutantsneed not reach interstate bodies of water immediatelyor continuously in order to inflict serious environmen-tal damage’’ (United States vs. Eidson, 94–2330).

Surface flow in streams draining mountain head-water areas in the arid western United States is com-monly physically isolated from downstream playalakes, wetlands, or larger streams and rivers bystream 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]).

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Vol. 43, No. 1 AMERICAN WATER RESOURCES ASSOCIATION February 2007

physical isolation of surface flow in mountain head-water streams (whether perennial or intermittent)from downstream systems may be broken for briefperiods after rainfall or snowmelt when runoff is suf-ficient to allow flow along the entire downstreamreach. Despite the physical isolation of surface flowin these streams, they are an integral part of hydro-logic systems in arid regions. Water infiltrated fromheadwater streams moves through the unsaturatedzone to recharge the underlying ground-water sys-tem. This ground water eventually discharges to sup-port springs, streamflow, isolated wetlands, or nativevegetation far from recharge areas. In some systems,ground-water movement from recharge areas to dis-charge areas may require many thousands of years.

In addition to their physical and temporal isola-tion, the mountain headwater streams in the westernMojave Desert are further isolated from other hydro-logic systems by their geologic setting within theBasin and Range physiographic province. Under ‘‘pre-sent-day’’ climatic conditions, many internallydrained basins (also known as ‘‘closed basins’’) withinthe Basin and Range physiographic province arephysically isolated from larger drainages that flow tointerstate waters or discharge to the ocean by inter-vening mountain ranges.

The purpose of this paper is to summarize on thebasis of existing data and published work (1) the briefphysical connection of selected mountain headwaterstreams in the western Mojave Desert to downstreamhydrologic systems, (2) the connection of water infil-trated from these streams through the unsaturatedzone to the underlying ground-water system, and (3)the longer time-scale connection through the ground-water system to discharge areas farther downgradi-ent. Only brief descriptions of methods are given inthis paper and the reader is referred to the citedwork for a more thorough explanation of the meth-ods, data, and results.

HYDROGEOLOGIC SETTING

The western Mojave Desert east of Los Angeles(Figure 1) is arid with hot, dry summers, and coldwinters. With the exception of the higher altitudes inthe San Gabriel and San Bernardino Mountains, pre-cipitation is generally about 150 mm ⁄ yr or less, butamounts vary greatly from year to year. In most ofthe area, precipitation is greater during the winterrainy season (November-March) and occurs as aresult of cyclonic storms moving inland from the Paci-fic Ocean. During winter cyclonic storms, moist airfrom the Pacific Ocean can enter the Mojave Desert

through Cajon Pass and precipitate without passingover the higher altitudes of the San Gabriel and SanBernardino Mountains. Precipitation near the passcan give rise to streamflow along the entire length ofthe Mojave River and flow in smaller streams nearthe pass, such as Oro Grande Wash. A similar gapbetween the San Bernardino and San Jacinto Moun-tains, San Gorgornio Pass, to the southeast of thestudy area (not shown in Figure 1), also allows coolmoist air to enter the desert and gives rise to winterprecipitation and intermittent streamflows in thatarea – although the effect is smaller than near CajonPass (Izbicki, 2004). Although summer thunderstormsoccur, especially in the eastern part of the study area,summer monsoonal precipitation is of lesser import-ance in the western Mojave Desert than elsewhere inthe southwestern United States.

With the exception of some small streams thatdrain the higher altitudes of the San Gabriel and SanBernardino Mountains and short reaches of theMojave River where ground-water discharges at landsurface, there are no perennial streams in the area.Physical connection between mountain headwaterstreams (whether perennial or intermittent) anddownstream hydrologic systems in the westernMojave Desert occurs only during brief periods ofstreamflow after precipitation or snowmelt along nor-mally dry downstream reaches that cross alluvialfans and basin fill deposits.

There are a number of internally drained alluvialbasins in the western Mojave Desert each having dis-tinct ground-water-flow systems often separated byfaults and bedrock outcrops. Alluvial deposits in somebasins are more than 1,000 m thick and saturateddeposits may be separated from land surface by unsat-urated alluvium as much as 300 m thick near themountain front. Ground-water movement in thesebasins is generally from recharge areas near the moun-tain front and along larger stream channels toward dis-charge areas that include springs, wetlands, or nativevegetation near dry lakes. Prior to ground-waterpumping in the Mojave River ground-water basin, thedirection of ground-water movement was from alluvialdeposits (collectively known as the regional aquifer) tothe floodplain aquifer along the Mojave River. In mostof the regional aquifer, ground-water recharge is smallin relation to the volume of water in storage and traveltimes through the aquifer system are often many thou-sands of years (Izbicki et al., 1995; Izbicki and Michel,2004). In contrast, the floodplain aquifer is more lim-ited in areal and vertical extent (typically less than2.5 km wide and 80 m thick) than the surroundingalluvial aquifers and is readily recharged by infiltra-tion of streamflow in the Mojave River.

Numerous water-level maps have been prepared ofaquifers in the area (Stamos and Predmore, 1995;

ISOLATION OF MOUNTAIN HEADWATER STREAMS

JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION 27 JAWRA

FIGURE 1. Location of Study Area.

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Mendez and Christensen, 1997; Smith and Pimentel,2000; Smith et al., 2004; Stamos et al., 2004). Severalregional-scale ground-water flow models simulatingground-water flow have been completed for theMojave River ground-water basin (Hardt, 1971; Sta-mos et al., 2001) and the Antelope Valley (Leightonand Phillips, 2003). Smaller scale flow models havebeen completed for some subbasins in the Morongoground-water basin (Londquist and Martin, 1991;Nishikawa et al., 2004).

STREAMFLOW

For the purposes of this paper, streamflow in theMojave River, the largest stream in the study area isdiscussed separately from the streamflow characteris-tics in smaller streams that drain the mountains.

The Mojave River

The Mojave River, the largest stream in the studyarea, drains about 5,500 km2, of which 540 km2 arein the San Bernardino Mountains. The Mojave Riverflows past Afton Canyon more than 160 km down-stream and splits with separated channels flowingeast toward East Cronese and Soda (dry) Lakes (notshown 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).

The physical connection of headwater reaches of theMojave River, the largest stream in the study area, todownstream reaches was assessed by Lines (1996)during water years 1992–94 (Figure 2). Perennial flowduring this period occurred only at the Upper Nar-rows, the Lower Narrows, downstream from a regionalwastewater treatment plant serving the Victorvillearea, and at Afton Canyon. Records from early trave-lers and explorers in the area suggest that perennialflow was more extensive prior to ground-water pump-ing (Lines, 1996). During each winter, runoff from theheadwaters, coupled with seasonal decreases inground-water pumping and evapotranspiration fromriparian habitat extended the seasonal surface flow.Stamos et al. (2001) showed that pumping along theriver decreased the magnitude and frequency of sea-sonal surface flow in the Mojave River along streamreaches farther downstream from the mountain front.The river flowed along its entire main stem down-stream to Afton Canyon for a few weeks during wateryear 1993 as a result of a series of large storms (Lines,1996). During 1993, the total annual flow from

headwater areas to downstream reaches of the MojaveRiver was almost 500 hm3 (5 · 108 m3). Annual flowsof this magnitude have a recurrence interval of greaterthan 50 years (Lines, 1996) and this was the first timethe river flowed continuously since 1983. Morethorough analyses of the magnitude and frequency ofsurface flows in the Mojave River from stream gagingstations are available in Lines (1996) and Stamos et al.(2001).

Smaller Streams

Smaller streams are obviously more numerous thanlarger streams, such as the Mojave River. About 140mountain headwater streams draining at least0.9 km2 were identified along the mountain frontbetween Palmdale and Twentynine Palms (Figure 3).Streamflow quantity and frequency data have beenestimated using a variety of techniques for reaches ofseveral smaller streams discussed in this article. QuailWash, Big Rock Creek, and Sheep Creek are amongthe larger streams identified in Figure 3; streamflowquantity and frequency for the more numerousstreams draining less than 20 km2 are largely unavail-able. Oro Grande Wash discussed in the article is notshown in Figure 3 because it originates near CajonPass and does not drain the mountain front.

Streamflow data from gaging stations are lessavailable for smaller intermittent streams than forlarger streams such as the Mojave River; as a

FIGURE 2. Reaches of the Mojave River That Had StreamfowDuring Water Years 1992–94 (modified from Lines, 1996).



consequence, the frequency of surface flow in smallerintermittent streams along the front of the San Gab-riel Mountains to downstream channel reaches wasestimated on the basis of streambed temperaturedata. During the winter months, when most precipi-tation occurs, streamflow is relatively cold, often onlyslightly above 0�C. Cold streamflow causes measur-able changes in streambed temperature that do notoccur in ground temperature measurements at con-trol sites adjacent to, but outside, the wash (Con-stantz et al., 2001, 2003). Streambed temperaturedata are relatively easy and inexpensive to collectand numerous measurement stations can be installedalong a wash reach to determine the downstreamextent and duration of winter storm flows. Stream-flow interpreted from temperature data was verifiedby examination of the channel during site visits afterstorms. The approach is attractive in areas where itis impractical or prohibitively expensive to install tra-ditional stream gages that may be damaged or des-troyed during large streamflows. Streambedtemperature data were collected along three selectedwashes: Oro Grande Wash, Sheep Creek Wash, andBig Rock Creek Wash. Oro Grande Wash flows to theMojave 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 washesare among the largest in the western Mojave Desertand study reaches total almost 70 km. Each washrepresents a range of hydrologic conditions (Table 1).

FIGURE 3. Rank-Order Distribution of Drainage BasinsGreater Than 0.9 km2 on the Northern Slope of the San Gabriel,San Bernardino, and Little San Bernardino Mountains Between

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An example of streambed temperature changesinterpreted as streamflow is shown for selected meas-urement sites along Sheep Creek Wash, February17–27, 2000 (Figure 4). The interpreted streamflow isof greater duration along the upstream sections ofthe wash at the mountain front. Runoff from precipi-tation is directed away from the active channel ofSheep Creek Wash by the conical shape of the allu-vial fan and streamflow decreases in duration withdistance downstream as water infiltrates into theunderlying streambed.

Streamflow is more difficult to interpret fromstreambed temperature data during the summerwhen the difference between precipitation, runoff,and streambed temperatures may be small. Theinterpretation may be further complicated becausesummer precipitation in arid areas is often highlyvariable spatially, limited in areal extent. An

example of streambed temperature changes inter-preted as streamflow is shown for a site alongOro Grande Wash, July 5–12, 1999 (Figure 5). Ana-lysis of temperature data suggests that streamflowmight not have occurred at upstream or down-stream temperature measurement sites during thisperiod.

If interpretations of streambed temperature dataare not constrained by meteorological data and fre-quent site visits, all measured streambed tempera-ture anomalies could be interpreted as streamflow –producing a higher frequency of flow than might haveoccurred (Figure 6). Despite the inherent uncertaintyassociated with this approach, estimates of stream-flow occurrence inferred from temperature data canbe assembled into statistical representations ofstreamflow frequency that reflect the regional hydrol-ogy of the study area.

FIGURE 4. Precipitation, Streambed Temperature, Control Temperature (Collected Outside of Streambed), and InferredDuration of Streamflow Along Sheep Creek Wash, Western Mojave Desert, Southern California, February 17–27, 2000.



Oro Grande Wash is the smallest of the threewashes studied and the wash does not drain the SanGabriel Mountains. The frequency of flow along mostof Oro Grande Wash is less than the frequency offlow along mountain front reaches of the otherwashes and the duration of flows is less – typicallyabout 1 hour. Given a frequency of flow of 0.05days ⁄ yr and a duration of 1 hour, Oro Grande Washmay only flow for as few as 18 hours each year (365days ⁄ yr · 0.05 stormflows ⁄ day · 1 hour ⁄ stormflow).Although during large winter storms Oro GrandeWash may flow uninterrupted from its headwatersnear Cajon Pass through the study reach to theMojave River (Izbicki et al., 2000), flows along shorterreaches of the wash are more common. This is especi-ally true along the downstream urbanized reach ofOro Grande Wash where runoff from imperviousurban areas contributes to increased streamflow.

Frequency and duration of flow in Sheep CreekWash are greater than in Oro Grande Wash becauseSheep Creek drains a larger area in the higher alti-tudes in the San Gabriel Mountains. Although not per-ennial, Sheep Creek may flow for extended periodsduring the winter and during spring runoff. For exam-ple, the duration of a single flow in Sheep Creek at themountain front between February 24 and February 26,2000 exceeded the estimated cumulative annual flowduration along Oro Grande Wash. Unlike Oro GrandeWash, where flows along only the downstream reachesare common, flow in both Sheep Creek and Big RockCreek Washes decreases in frequency and durationwith distance downstream (Figure 6).

UNSATURATED FLOW

In arid alluvial valleys of the western MojaveDesert, areal recharge from precipitation and subse-quent movement of water through the unsaturatedzone is negligible. In fact, thick unsaturated zonesoverlying alluvial aquifers in the Mojave Desertwithin California have been proposed as storagerepositories for toxic and nuclear waste (NationalResearch Council, 1995). However, along intermittentstream channels water may infiltrate to depths belowthe root zone and ultimately reach the underlyingwater table. In these areas where the volume ofwater infiltrated is small, and the unsaturated zoneis thick, or relatively impermeable, the slow move-ment of water through the unsaturated zone maycontribute to the temporal isolation of small head-water streams from underlying aquifers and down-gradient hydrologic systems.

Infiltration from streamflow commonly occurs ingreater amounts along upstream reaches near themountain front (Izbicki et al., 2002). Measurementsof water content, water potential, and low concentra-tions of soluble salts (such as chloride) in the unsat-urated zone beneath upstream reaches of SheepCreek Wash (Figure 7) are consistent with the move-ment of infiltrated water to depths below the rootzone and presumably to the underlying water tableas much as 300 m below land surface (Izbicki et al.,2002). Similarly, water infiltrated during stormflowmoves downward to the water table along upstream

FIGURE 5. Precipitation, Streambed Temperature, Control Temperature (Collected Outside of Streambed), andInferred Duration of Streamflow Along Oro Grande Wash, Western Mojave Desert, Southern California, July 5–12, 1999.

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reaches of Oro Grande Wash near Cajon Pass (Figure7). In contrast, Nishikawa et al. (2004) demonstratedthat infiltrated water did not move downwardthrough the unsaturated zone near the mountainfront along upstream reaches of Quail Wash in thesouthern part of the study area (Figure 1). However,water did move to depths below the root zone andpresumably to the water table beneath stream rea-ches farther downstream and along Yucca Wash.Flows along the downstream reaches have increasedin recent years as a result of upstream urbanization(Nishikawa et al., 2004).

The rate of downward movement of infiltratedwater beneath the channels of Oro Grande and SheepCreek Washes was calculated on the basis of tritiumconcentrations in water extracted from core materialcollected from the unsaturated zone (Figure 7).

Tritium is a radioactive isotope of hydrogen having ahalf-life of 12.3 years. Tritium is a part of the watermolecule and is an excellent tracer of the movementof water. Although tritium is naturally occurring, itspresence in the environment has increased as aresult of nuclear weapons testing beginning in 1952.For the purposes of this paper, water that does notcontain tritium was interpreted as water that infil-trated into the ground prior to 1952 and water thatcontains tritium was interpreted as infiltrated after1952. The peak tritium concentration was presumedto coincide with water that infiltrated in about 1962– the peak in the atmospheric testing of nuclearweapons (Michel, 1976).

Downward rates of movement calculated on thebasis of tritium data range from 0.3 to 0.8 m ⁄ yr, and180 to 600 years or more, depending on the thicknessof the unsaturated zone, may be required for water toreach the underlying water table (Izbicki et al.,2002). However, small amounts of water movingdownward through preferential pathways in theunsaturated zone may move more rapidly (Izbicki etal., 2000). Because water spreads laterally away fromthe wash as it moves downward, the rate of down-ward movement decreases with depth (Izbicki et al.,2000, 2002; Nimmo et al., 2002). Simulations ofunsaturated flow (Izbicki, 2002) show that lateralspreading can be increased by low permeabilitylayers within the unsaturated zone that impede thedownward movement of water (Figure 8). The simula-ted downward rate of movement of infiltrated waterclosely matches the rate of movement beneath OroGrande Wash estimated on the basis of tritium data.

Although precipitation, runoff, and subsequentstreamflow are highly variable, water potential anddownward rates of water movement damp to a con-stant value with increasing depth (Nimmo et al.,2002). For example, seasonal water potential (andtemperature data) collected beneath Quail and YuccaWashes damp to near constant values within 15 m ofland surface (Nishikawa et al., 2004). Recharge fromthese small streams at the water table hundreds ofmeters below land surface is not likely to be affectedby short-term climatic cycles, such as El Nino or thePacific Decadal Oscillation, even though infiltrationat the streambed surface may vary greatly duringthese periods.

In areas where the rate of downward movement isslow and the unsaturated zone is thick, it is possiblethat geomorphic processes that lead to channel aban-donment may effectively strand infiltrated water inthe unsaturated zone before it reaches the watertable. For example, water more than 100 m deep inthe unsaturated zone underlying Sheep Creek Washwas recharged at a time in the geologic past whenthe climate was wetter and cooler. This water is iso-

FIGURE 6. Frequency of Temperature Anomalies andFrequency of Days Interpreted to Have Flow as a Functionof 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.



lated from surface sources and effectively stranded inthe unsaturated zone (Izbicki et al., 2002). Channelabandonment processes do not occur along OroGrande Wash, which is incised into the regional allu-vial fan surface, and the position of the active chan-nel of the wash has not changed greatly for the last500,000 years (Izbicki et al., 2000, 2002).

Infiltration from successive winter streamflows coolsthe unsaturated zone beneath the streambed in com-parison with the surrounding material. Izbicki andMichel (2002) showed a good comparison between themagnitude of the annualized temperature difference inthe unsaturated zone beneath Oro Grande and SheepCreek Washes and the surrounding alluvium withother tracers of water movement through stream chan-nels (Figure 9), and used the data to estimate theinfiltration from streamflow. The average annual infil-tration along the study reaches of Oro Grande andSheep Creek Washes was then estimated as the aver-age infiltration rate times the width of the wash timesthe length of the wash reach between measurementpoints (Table 2). Comparison of the average annualinfiltration along the study reaches with estimates ofaverage annual streamflow (Table 1) suggests that

only about 20 percent of the average annual stream-flow infiltrated into the streambed along the study rea-ches, only a smaller fraction actually infiltrates todepths below the root zone, and that most waterwas transmitted through the study reaches as surfaceflow.

Water that flowed through the study reaches eitherdirectly reached the downstream hydrologic systemsas streamflow, or infiltrated into the streambed fartherdownstream. Accumulations of soluble salts beneaththe downstream reach of Sheep Creek Wash suggestthat water infiltrated along these downstream reachesof smaller streams may not infiltrate to depth belowthe root zone and move downward toward the watertable (Figure 9). Temperature data collected along thedownstream reach of Sheep Creek Wash also suggestthat streamflow and infiltration, while not occurringevery year, average about 0.7 m ⁄ yr (Izbicki and Mi-chel, 2002). This value may represent a thresholdbelow which infiltration to depths below the root zonedoes not occur. This threshold probably differs withchanges in stream channel morphology and may beless in wider channels having less vegetation or inchannels composed of highly permeable material.

FIGURE 7. Water Content, Water Potential, Chloride, and Tritium Data in the Unsaturated Zone at Selected SitesUnderlying Oro Grande and Sheep Creek Washes Western Mojave Desert, Southern California (Modified from Izbicki et al., 2002).

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GROUND-WATER AGE

For the purposes of this article, the cumulativeeffect of ground-water recharge to alluvial aquifersunderlying the western Mojave Desert was evaluatedon the basis of changes in the isotopic compositionof ground water. Deuterium, a stable isotope of

FIGURE 8. Simulated Movement of Water Through aThick Unsaturated Zone Having Areally Extensive

Clay Layers, Oro Grande Wash, Western Mojave Desert,Southern California (Modified from Izbicki, 2002).

FIGURE 9. Difference in Temperature with Depth BetweenAccess Tubes in Intermittent Streams and Their RespectiveControl Sites, and Chloride and Tritium Data Collected BeneathStreams, Oro Grande and Sheep Creek Washes, SouthernCalifornia, 1996–97 (Modified from Izbicki and Michel, 2002).



hydrogen, was used to evaluate the source of water.Tritium and carbon-14, radioactive isotopes of hydro-gen and carbon, were used to evaluate the age (timesince recharge) of ground water to assess the tem-poral connectivity of mountain streams to downgradi-ent hydrologic systems.

Deuterium is a naturally occurring stable isotopeof hydrogen and deuterium abundances areexpressed as ratios in delta notation (d) as per mil(parts per thousand) differences relative to thestandard known as Vienna Standard Mean OceanWater (Gonfiantini, 1978). Water that condensed atcooler temperatures associated with higher altitudesor cooler climatic conditions has less of the heavierisotopes and more negative values than water thatcondensed at warmer temperatures associated withlower altitudes or present-day climatic conditions. Incontrast, water that has been partly evaporated isenriched in the heavy isotopes relative to its originalcomposition.

Orographic effects near Cajon Pass between theSan Gabriel and San Bernardino Mountains allow airmasses laden with moisture from the Pacific Ocean toenter the Mojave Desert during the winter rainy sea-son and precipitate without uplift over the higheraltitudes in the mountains (Izbicki, 2004). As it con-denses at lower altitudes and warmer temperatures,precipitation near Cajon Pass is isotopically heavierthan precipitation that condenses over the moun-tains. Winter precipitation near Cajon Pass gives riseto streamflow in the Mojave River. Cumulativerecharge from infiltration of streamflow along theMojave River has resulted in a large body of isotopi-cally heavy ground water extending 160 km alongthe floodplain aquifer into the Mojave Desert (Figure10).

The isotopically heaviest water sampled in thestudy area is to the west of the Mojave River. Thiswater originated from precipitation near the passthat has not been fractionated by orographic upliftover the mountains and subsequent runoff and infil-tration of streamflow in Oro Grande Wash andother similar washes near the pass (Izbicki et al.,1995). Despite its heavy dD composition, comparisonwith oxygen-18 data shows no evidence of evapora-tive effects (Izbicki et al., 1995; Izbicki, 2004).Although the quantity of water from these sourcesis small, it is locally important. Similar processeshave resulted in isotopically heavy ground water inthe eastern part of the study area near San Gorgo-nio Pass (Figure 10), and along the western edge ofAntelope Valley (not shown in Figure 1) where thealtitudes of the San Gabriel Mountains are lower(Smith et al., 1992).

Much of the water in the floodplain aquifer alongthe Mojave River contains tritium (Figure 10). This

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water was distributed more than 160 km from CajonPass and the mountain front along the channel of theMojave River by infiltration from occasional surfaceflow in the river. In contrast, only a small amount ofwater containing tritium was present near the SanGabriel and San Bernardino Mountains where smal-ler intermittent streams flow from the mountains.Although infiltration from intermittent streamsdraining the San Gabriel and San Bernardino Moun-tains is locally important, especially in canyons nearthe mountain front, the amount of water from thesesources containing tritium is small when comparedwith the volume of water in storage and the volumeof water infiltrated from the Mojave River.

Like tritium, carbon-14 also provides informationon the age, or time since recharge, of ground water.Carbon-14 is a naturally occurring radioactive isotopeof carbon having a half-life of about 5,730 years(Mook, 1980). Carbon-14 data are expressed as per-cent modern carbon (pmc) by comparing carbon-14activities to the specific activity of National Bureau ofStandards oxalic acid: 13.56 disintegrations ⁄ min ⁄ g ofcarbon equals 100 pmc (Kalin, 2000). Carbon-14 wasproduced, as was tritium, by the atmospheric testingof nuclear weapons. As a result, carbon-14 activitiesmay exceed 100 pmc in areas where ground watercontains tritium. Because of its longer half-life, car-bon-14 preserves information on the cumulative vol-ume of water infiltrated from headwater streamsover a longer time scale than does tritium. For exam-ple, ground water having a carbon-14 activity of50 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 reac-tions between ground water and the alluvial depositsthat compose the aquifer.

Unlike tritium, carbon-14 is not a part of the watermolecule, and carbon-14 activities are affected bychemical reactions between ground water and aquifermaterial. Carbon-14 activities shown in Figure 10 donot account for these reactions. Ground-water agesestimated from uncorrected carbon-14 activities mayoverestimate ground-water age by as much as 30 per-cent compared with estimated ages that account forchemical reactions between the ground water andaquifer material (Izbicki et al., 1995). Despite thisuncertainty, uncorrected carbon-14 ages are a usefulapproximation of ground-water age.

The spatial distribution of carbon-14 activitiesgreater than 90 pmc is similar to the distribution oftritium data with high activities along the floodplainaquifer and small areas near the mountain front(Figure 10). Carbon-14 activities greater than 50 pmcshow the cumulative effect of as much as 5,730 years(one half-life) of streamflow infiltration near the frontof the San Gabriel and San Bernardino Mountains

FIGURE 10. Delta Deuterium, Tritium, and Carbon-14 Compositionof Water From the Wells in the Western Mojave Desert, Southern

California (Modified from Izbicki, 2004, and Izbicki and Michel, 2004).



and from streams, such as Oro Grande Wash, nearCajon Pass. Carbon-14 activities greater than 50 pmcalong the channel of Pipes Wash and Yucca Washesin the southern part of the study area suggest thatoccasional flow in these washes infiltrates throughthe unsaturated zone to the water table for tens of ki-lometers into the Mojave Desert. Carbon-14 and dDdata also show the cumulative recharge from infiltra-tion of streamflow in intermittent streams near CajonPass, such as Oro Grande Wash (Izbicki et al., 1995).Although small in magnitude, the cumulative effectof flow and subsequent ground-water recharge fromthese smaller streams is increasingly important overthe longer time-scales measure by carbon-14 than bytritium.

The complex distribution of recent and olderground-water ages and ground-water flow pathsunder predevelopment conditions in the alluvial aqui-fers underlying the Mojave ground-water basin weresimulated using a regional ground-water flow modellinked to a particle-tracking model (Stamos et al.,2001; Izbicki et al., 2004). The model results identi-fied the ground-water flow paths from the mountainfront through the regional aquifer to ground-waterdischarge areas near El Mirage (dry) Lake, and tothe floodplain aquifer (Figure 11). The model alsoidentified the ground-water flow paths through thefloodplain aquifer to discharge areas near Harper(dry) Lake, Coyote (dry) Lake, and Afton Canyon anddefined the complex interaction between the flood-plain aquifer, the Mojave River, and the surroundingand underlying regional aquifer. Under present-dayconditions, ground-water pumping is the largest dis-charge from many aquifers in the western MojaveDesert. Ground-water pumping has altered the prede-velopment water levels and ground-water flow paths.Water from mountain headwater streams that even-tually discharged to downgradient hydrologic systemsunder predevelopment conditions would, under pre-sent-day conditions, likely discharge as pumpagefrom wells – further contributing to the isolation ofmountain headwater streams from downgradienthydrologic systems.

DISCUSSION AND CONCLUSIONS

Mountain headwater streams in arid areas areoften physically isolated from downstream hydrologicsystems such as springs, playa lakes, wetlands, orthrough-flowing streams and rivers by reaches of drychannels across alluvial fan or basin fill deposits. Thephysical isolation of surface flow in mountain head-water streams from downstream systems may be

FIGURE 11. Particle-Tracking Model Resultsfor the Mojave Ground-Water Basin (Modified from

Stamos et al., 2001; Izbicki et al., 2004).

IZBICKI


broken for brief periods after rainfall or snowmelt inthe higher mountains when runoff is sufficient toallow flow along the entire downstream wash reach.Larger streams, such as the Mojave River and to alesser extent Pipes and Yucca Washes in the westernMojave Desert, may occasionally produce flows thatextend many kilometers from the mountain front intothe desert and briefly provide a physical connectionfrom mountain headwater streams to downstreamhydrologic systems. The recurrence interval of theselarge flows is known for many larger streams in aridareas and can be estimated for smaller streams.

Despite the physical isolation of surface flow inheadwater streams, they are an integral part of thehydrologic cycle in arid regions. Water infiltratedfrom surface flow in headwater streams moves down-ward through the unsaturated zone to the underlyingground-water system. Under predevelopment condi-tions, this infiltrated water eventually discharged tosprings, streamflow, isolated wetlands, or nativevegetation. However, infiltrated water may be tem-porally isolated from downgradient discharge areasas it flows through thick unsaturated zones and alonglong flowpaths through underlying aquifers. Forexample, travel times through the unsaturated zoneunderlying Oro Grande and Sheep Creek Washes areseveral hundred years. Travel times through theunderlying regional aquifer are longer and groundwater ages may be as great as a thousand to severaltens of thousands of years at the downgradient end oflong flowpaths through the regional aquifer. In con-trast, ground water in the floodplain aquifer underly-ing the Mojave River commonly contains tritium andground-water age is measured in decades.

The selection of a time period as the cutoff for defi-ning isolation of water infiltrated from surfacestreams through ground-water systems is arbitraryand depends on the nature of the problem being con-sidered. Winter and LaBaugh (2003) speculated thatwetlands should not be considered isolated even ifseveral decades are required for water to reach down-gradient hydrologic systems. Studies on the suitabil-ity of sites in arid areas for toxic or radioactive wastedisposal must consider the need for hydrologic isola-tion of thousands of years in duration and changinglong-term climate cycles (National Research Council,1995). Regardless of the criteria ultimately selectedfor management of mountain headwater streams inarid areas under the Clean Water Act, under present-day conditions, water infiltrated from headwaterstreams into aquifers may ultimately reach down-stream hydrologic systems through pumping forwater supply and subsequent discharge from waste-water treatment plants rather than as ground-waterdischarge through the hydrologic cycle.

Information on streamflow characteristics, traveltimes through unsaturated zones and underlyingaquifers may have transfer value from the westernMojave Desert to other arid areas in the southwest-ern United States. However, headwater streams inthe western Mojave Desert (even those tributary tothe Mojave River) flow from mountain areas to closedbasins. Under present-day geologic and climatic con-ditions, these internally drained basins are physicallyisolated by the intervening mountain ranges from thelarger drainages that flow to interstate waters or todischarge to the ocean.

ACKNOWLEDGMENTS

Funding for this paper was provided by the U.S. Geological Sur-vey’s Office of Ground Water. Previous studies on which this workwas based were funded by the Mojave Water Agency, and JoshuaBasin Water District. The author thanks James Bowers, StevenPhillips, and Peter Martin of the U.S. Geological Survey and TracieNodeau of the U.S. Environmental Protection Agency for their con-structive comments during the preparation of this manuscript.

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physical and temporal isolation of mountain headwater streams …€¦ · physical and temporal...

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