(KEY TERMS: hydrologic cycle; infiltration; recharge; vadose zone; surface water ground-water interactions; arid lands.)

Transcription

1 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 CALIFORNIA 1 John A. Izbicki 2 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., 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): DOI: /j x 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, ). Since the SWANCC decision, many Federal Court decisions 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, ). 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 1 Paper No. J06013 of the Journal of the American Water Resources Association (JAWRA). Received February 3, 2006; accepted July 17, 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 ( Izbicki: jaizbick@usgs.gov). JAWRA 26 JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION

2 ISOLATION OF MOUNTAIN HEADWATER STREAMS 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 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; JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION 27 JAWRA

3 IZBICKI FIGURE 1. Location of Study Area. JAWRA 28 JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION

4 ISOLATION OF MOUNTAIN HEADWATER STREAMS 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). headwater areas to downstream reaches of the Mojave River was almost 500 hm 3 ( m 3 ). 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 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). 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 km 2, of which 540 km 2 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 (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 FIGURE 2. Reaches of the Mojave River That Had Streamfow During Water Years (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 km 2 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 km 2 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 JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION 29 JAWRA

5 IZBICKI 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 0 C. 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). FIGURE 3. Rank-Order Distribution of Drainage Basins Greater Than 0.9 km 2 on the Northern Slope of the San Gabriel, San Bernardino, and Little San Bernardino Mountains Between Palmdale and Twentynine Palms, California. TABLE 1. Physical Characteristics Along the Study Reaches of Oro Grande, Sheep Creek, Big Rock Creek, and Quail Washes, Western Mojave Desert, Southern California. Channel Width Altitude Average Slope of Study Reach (percent) Downstream Site (m) Mountain Front (m) Length of Study Reach (km) Average Annual Flow (hm 3 ) Average (m) Maximum (m) Drainage Area at Mountain Front (km 2 ) Stream Oro Grande Wash Sheep Creek Wash ,594 1, Big Rock Creek 108 2,829 1, Quail Wash 237 1,768 1, 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; hm 3, cubic hectometers (10 6 m 3 ); km, kilometers; km 2, square kilometers. JAWRA 30 JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION

6 ISOLATION OF MOUNTAIN HEADWATER STREAMS 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, 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 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. JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION 31 JAWRA

7 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, 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). UNSATURATED FLOW 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 JAWRA 32 JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION

8 ISOLATION OF MOUNTAIN HEADWATER STREAMS 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, 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 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 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- JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION 33 JAWRA

9 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 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 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. JAWRA 34 JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION

10 ISOLATION OF MOUNTAIN HEADWATER STREAMS 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, (Modified from Izbicki and Michel, 2002). GROUND-WATER AGE 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). 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 JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION 35 JAWRA

11 IZBICKI TABLE 2. Streambed Characteristics and Infiltration Along the Study Reaches of Oro Grande, Sheep Creek, and Quail Washes, Western Mojave Desert, Southern California. Annual Deep Infiltration Along Study Reach (hm 3 ) Annual Infiltration Along Study Reach (hm 3 ) Annual Infiltration Rate (m yr) Instantaneous Infiltration Rate (m hr) Description of Stream Channel Stream Oro Grande Wash Medium sand Sheep Creek Wash Cobbles near mountain front to silt farther Downstream Quail Wash coarse sand * 0.1* - 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, m 3, cubic meters; hm 3,cubic hectometers (10 6 m 3 ). *Infiltration rate and annual infiltration calculated for 20-kilometer reach of Yucca Wash downstream from 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 JAWRA 36 JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION

12 ISOLATION OF MOUNTAIN HEADWATER STREAMS 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). 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: 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 JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION 37 JAWRA

13 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 physical isolation of surface flow in mountain headwater streams from downstream systems may be FIGURE 11. Particle-Tracking Model Results for the Mojave Ground-Water Basin (Modified from Stamos et al., 2001; Izbicki et al., 2004). JAWRA 38 JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION

14 ISOLATION OF MOUNTAIN HEADWATER STREAMS 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. 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. 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. LITERATURE CITED Constantz, J., D. Stonestrom, A.E. Stewart, R. Niswonger and T.R. Smith, Analysis of Streambed Temperatures in Ephemeral Channels to Determine Streamflow Frequency and Duration. Water Resources Research 37(2): Constantz, J., S.W. Tyler and E. 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