Advancing Water Resources Research and Management
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Advancing Water Resources Research and Management |
| 1999 Annual Summer Specialty Conference Proceedings |
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| Science Into Policy: Water in the Public Realm / Wildland Hydrology |
| Bozeman, Montana, June 30 - July 2, 1999 |
James L. Fogg and Daniel P. Muller 1.
An instream flow assessment of an oasis stream in the northeastern Mojave Desert was conducted using the U.S. Bureau of Land Management's value-based process (Jackson et al., 1989). Produced in support of a water right application, the study included an institutional analysis of water availability for the site, characterization of the hydrology of the aquifer-stream system, and description and quantification of flow-dependent resource values. Specific objectives for the assessment were to (1) determine the physical and legal availability of water for management purposes; (2) understand the hydrology of Beaver Dam Wash, particularly the interaction of surface- and ground-water systems; (3) develop relationships between streamflow and resource values and evaluate flow requirements to maintain resource values; and (4) identify and evaluate flow protection strategies and related flow issues for management. The institutional analysis addressed the need for protection from extensive ground-water withdrawals that threaten surface flows in the area. The hydrologic characterization described ground water contributions to streamflow at the oasis and the influence of large floodflows on channel morphology and ecological sites. The resource assessment focused on critical habitat for several special status fish species, aesthetics and wadeability for recreation users, and consumptive use and ecological needs for riparian habitat.
KEY TERMS: Instream flows; resource values; surface-water/ground-water relationships
Beaver Dam Wash is a small stream originating in the mountains of southwest Utah and flowing first west and then south to its confluence with the Virgin River in northwest Arizona. Near the confluence, ground-water discharge supports perennial flow and a lush riparian environment that is in sharp contrast to its Mojave Desert surroundings. In 1991, the U.S. Bureau of Land Management (BLM) initiated an instream flow assessment for the lower end of Beaver Dam Wash using the value-based process described by Jackson et al. (1989). The instream flow assessment included an institutional analysis of water availability for the site, a characterization of the hydrology of the aquifer-stream system, and a description and quantification of flow-dependent resource values. Results from the assessment provide a scientific basis for protection of these flow-dependent resources.
In 1989, BLM submitted an application for an instream flow water right, pursuant to Arizona law, for the reach of Beaver Dam Wash owned by the United States and managed by BLM. The original BLM application of 1,176 acre-feet per year (1-2 cfs) was based primarily on estimated needs of threatened and endangered fish. Based on the findings of this assessment, the amount of instream flow originally applied for will need to be increased to about 4,300 acre-feet per year (i.e., 6 cfs as described later). The current total of all Arizona Beaver Dam Wash uses, existing and applied for, is approximately 18,000 acre-feet per year.
Holmes et al. (1997) estimated total discharge of the Beaver Dam Wash system at about 22,000 acre-feet per year, including subsurface outflow to the Virgin River, and subsurface losses to surrounding formations. Flow appears to be sufficient to support present uses and applications, including the BLM instream flow application. While flows may also be sufficient to support additional future uses, protection of instream flows for resource values along the Wash will be important in light of the rapid development occurring in the region.
Arizona has a bifurcated water management system in which surface water is managed separately from ground water. In Arizona, surface water may only be appropriated through a water right issued by the Arizona Department of Water Resources (ADWR), while ground water (outside of an Active Management Area) may be legally pumped by merely notifying ADWR of intent to do so. Thus, the holder of an instream flow water right is at risk from nearby ground-water development and associated pumping.
An exception to the bifurcated water management system occurs where ground water is determined to be subflow to a stream. In such cases, ground-water pumping may be managed under surface water appropriation rules. A 1994 decision by the Gila River adjudication trial court provided guidance for defining subflow and affirmed that there must be a hydraulic connection between the stream and the subflow zone. The court established five additional criteria to distinguish subflow associated with the stream from underground tributary flow toward or away from the main channel:
The hydrologic analysis focused primarily on the interaction between ground water and surface water in the area and the resultant conditions of streamflow. Effects of large flood events on channel morphology and ecological conditions also were investigated.
Ground water occurs in four hydrostratigraphic units within the study area: 1) alluvial fans along the mountain fronts, 2) thin sand and gravel layers in the upper Muddy Creek Formation, 3) sand and gravel in the alluvium overlying the Muddy Creek Formation (i.e., post-Muddy Creek Tertiary gravels), and 4) Quaternary channel alluvium of Beaver Dam Wash (described as channel fill in this report). Of these deposits, the channel fill is the primary aquifer, accounting for almost 100 percent of the water withdrawn from wells in the study area (Holmes et al., 1997). Minor quantities of ground water occur in thin zones within the Muddy Creek Formation and in the post-Muddy Creek Tertiary gravels. Ground water in alluvial fans is limited to the area near the mountain front, and is primarily a source of recharge to the post-Muddy Creek Tertiary gravels and the Muddy Creek Formation. The unit is not considered a good source of water supply, and no water supply wells have been drilled into the unit.
The channel of Beaver Dam Wash is filled with sand and gravel deposits of alluvial origin derived from erosion of the Bull Valley Mountains to the north and the Beaver Dam Mountains to the east. These sand and gravel deposits comprise the Beaver Dam Wash channel-fill aquifer. Ground water occurs in the channel fill along the entire length of the Wash from near Motoqua, Utah, to the confluence with the Virgin River in Arizona. Ground water in the channel-fill alluvium flows to the southeast, in the same direction as surface flow in the channel. The channel-fill alluvium forms a separate aquifer that is generally less consolidated, and is estimated to have a higher hydraulic conductivity than gravel layers in either the Muddy Creek Formation or the post-Muddy Creek Tertiary gravels deposited on either side of Beaver Dam Wash (Figure 1).
Figure 1. Geology of study area and location of observation holes (adapted from Moore, 1972 and Holmes et al., 1997).
Recharge to the channel-fill aquifer occurs from at least four sources: 1) infiltration of surface flow in the main channel of Beaver Dam Wash, 2) intermittent surface flow in drainages tributary to Beaver Dam Wash, 3) lateral inflow from post-Muddy Creek gravels, and 4) inflow from sand and gravel deposits in the upper part of the Muddy Creek Formation. Average annual recharge from stream infiltration (sources 1 and 2) has been estimated at about 15,600 acre-feet per year (about 21.5 cfs), and average annual inflow from the upper Muddy Creek and its overlying units (sources 3 and 4) has been placed at about 1,900 acre-feet per year (about 2.6 cfs) (Holmes et al., 1997). In the case of inflow from other formations (sources 3 and 4), almost all of the recharge to the channel alluvium is believed to occur in the last mile of Beaver Dam Wash (near the confluence) where the channel is incised to at or near the level of the Muddy Creek Formation.
Discharge from the channel-fill aquifer also occurs through several pathways. Approximately 9,000 acre-feet per year (about 12.4 cfs) is believed to be subsurface discharge, with roughly half going to the Muddy Creek Formation and its overlying sediments, and the other half going to the alluvium of the Virgin River. Another 4,000 acre-feet (about 5.5 cfs) of channel-fill ground water is consumed each year through evapotranspiration and well withdrawals. And approximately 4,300 acre-feet per year (nearly 6 cfs) discharges to streamflow in the last few miles of Beaver Dam Wash above the confluence with the Virgin River.
Because of the free interaction between ground water in the alluvial channel fill and surface flow in Beaver Dam Wash, water level changes in the channel fill are directly related to streamflow in the Wash (Holmes et al., 1997). Whereas the confined aquifers in the upper Muddy Creek Formation and overlying Tertiary gravels east of Beaver Dam Wash exhibit little, if any, variation in static water level (as measured at BLM test wells in the first year subsequent to drilling and in October 1997), water levels in the Beaver Dam Wash channel alluvium have shown pronounced changes during the same period, with rising water levels corresponding to times of increased streamflow (Holmes et al., 1997).
There are at least five wells in the channel alluvium that show a rise in water levels during 1993 1994, all reflecting an increase in water level due to increased infiltration from high streamflow (Enright, 1996). These wells are distributed from near the Arizona-Utah border to about 1 mile above the Highway 91 bridge. Thus, the channel alluvium responds as a definable unconfined aquifer, with a changing level of saturation depending on the amount of available infiltration from streamflow.
The direction of subsurface flow in the alluvial fill is linked to surface water in the Wash. Water in the channel-fill alluvium generally flows southward, following the surface drainage toward the Virgin River. Subsurface flow and surface flow move under almost identical gradients toward the confluence with the Virgin River. The gradient of the two components of flow is very similar, whether measured over a long segment (e.g., from several miles above the State line to the confluence) or over a relatively short distance (e.g., the Arizona segment only) (Figure 2). The measured hydraulic gradient of subsurface flow in the channel fill from the Iverson well (about 9 miles above the Arizona-Utah line) to the Arizona Department of Transportation well (T. 41 N., R. 15 W., sec. 33, SWNESW) near the confluence is .0096. The gradient in the surface flow is .0091. Similarly, the surface-flow gradient near the State line flattens out slightly to .007, and the subflow gradient also flattens out slightly to .008.
Figure 2. Longitudinal profile of Beaver Dam Wash showing gradient of ground water in channel-fill aquifer (horizontal distance not to scale).
Chemical quality of ground water in the channel alluvium and surface flow is consistently similar over the entire distance of Beaver Dam Wash, from high on the Utah side near Motoqua to near the confluence with the Virgin River. Water in the channel alluvium and the stream is of a calcium-bicarbonate type low in total dissolved solids (TDS) (less than 500 mg/L).
Ground water beneath the entire length of Beaver Dam Wash seems to meet the court's criteria for subflow, with the possible exception of parts of the ephemeral stream definition, and the preponderance of evidence strongly supports a subflow determination. If ADWR determines that subflow exists, then a BLM instream flow water right, as well as other existing water rights on the Wash, will have a new measure of protection against pumping in and near the channel.
Streamflow that sustains resource values near the mouth of Beaver Dam Wash derives from two very different processes. The majority of time, streamflow is sustained by discharge of ground water from the channel alluvium of the Wash. This streamflow is remarkably consistent in quantity and quality, varying little throughout the year. Superimposed on this ground-water discharge are infrequent runoff events that increase streamflow and sediment transport, occasionally in dramatic fashion. Duration of surface runoff is primarily influenced by type of precipitation event associated with the runoff. Summer thunderstorms provide high-intensity, short-duration rainfall that generates significant amounts of surface runoff; however, the short duration of these storms generally results in runoff lasting only a few hours. In contrast, winter frontal storms may produce moderate-intensity, long-duration rainfall that generates significant runoff lasting for several days.
The long-term (1970-95) average streamflow of Beaver Dam Wash at the mouth has been estimated at 12.5 cfs. Of this, about 6 cfs (about 4,300 acre-feet per year) is attributed to spring discharge to the stream near the mouth (Holmes et al., 1997). The remaining 6.5 cfs is attributed to surface runoff in response to snowmelt in the upper watershed and precipitation throughout the basin.
The public land surrounding the confluence of Beaver Dam Wash and the Virgin River is a very dynamic landscape influenced by the extremely active channels of these two desert streams. Flood events in either of these two drainages have a pronounced effect on channel location and morphology for both streams. It is the dynamic nature of the processes active here, along with the dependable supply of relatively high-quality fresh water, that gives this area its high resource values.
The large flood events that occur in the Beaver Dam Wash drainage are superimposed on the geomorphic processes of the Virgin River. Floods in Beaver Dam Wash are commonly produced by summer thunderstorms and winter frontal rains; however, the longer duration of runoff associated with frontal systems generally causes much greater changes in channel morphology. Channel evolution during and after these flood events produces a wide variety of aquatic and riparian habitats, even over a relatively short period of time.
Flood sequences are the driving force for geomorphic change along Beaver Dam Wash and are partially responsible for the rich diversity of riparian communities that occur in the area. Geomorphic response to flood events produces a variety of microhabitats for riparian reproduction and establishment. The magnitude of floods required to produce these habitats depends on the antecedent conditions before each event. When prolonged periods of base flow have allowed establishment of riparian vegetation and development of a narrow primary channel, moderately large floods are sufficient to initiate channel widening, with sediment deposition during the recession producing sites for riparian regeneration. Following a moderate flood, a much larger flow is required to bring about similar processes on a wider channel. In general, larger flood events cause greater channel adjustments and create larger areas for regeneration and ecological succession after the event. Thus, there is no single flood magnitude that is associated with these processes.
However, it is possible to estimate a minimum flood magnitude required to initiate these processes. The minimum flood required to initiate channel adjustments is the flow that slightly exceeds bankfull discharge of the stream when it is in an advanced ecological condition. Although the channel is narrow and the banks are well-vegetated, the noncohesive nature of the substrate results in channel widening and other adjustments when bankfull flow is exceeded for durations of a few hours or more. For this assessment, bankfull flow was estimated for cross sections surveyed when the channel was in such an advanced ecological condition.
It appears that channel adjustments will begin to occur with floods as small as a few hundred cfs, but that flows in the range of 1,000 to 3,500 cfs are required to access most of the streambank and initiate widespread channel adjustments. Peak flows of 5,940 and 13,000 cfs were recorded during the course of this study; thus, it appears the present magnitude and frequency of flooding in Beaver Dam Wash is adequate to maintain the geomorphic processes that are occurring.
The Virgin River and its tributaries support a largely native and unique fish population and aquatic ecosystem. Five of eleven fish species found in Beaver Dam Wash during this assessment are native to the Colorado River system, including the Virgin River roundtail chub, which is Federally listed as endangered. Habitat was modeled for five species and two or three life stages per species. It is estimated that flows required to sustain Virgin spinedace also will sustain other species in lower Beaver Dam Wash. Habitat of the adult Virgin River roundtail chub, however, increases with flows beyond those modeled. Recommended yearlong flows for Virgin spinedace should be in the range of 6 to 20 cfs, with a minimum flow requirement of 6 cfs.
The perennial stream and outstanding riparian values associated with Beaver Dam Wash provide recreational opportunities that are largely aesthetic in character. These opportunities are rare relative to other nearby Virgin River tributaries that appear more arid and are not perennial. Quantification of flows required for recreation are based on evaluations of water needed to support both aesthetics and wading/swimming opportunities. Flows less than 4 cfs cause significant portions of the channel bottom to be exposed, and thus decrease scenic beauty in the stream. Based on analysis of certain hydraulic parameters, flows ranging from 1.5 to 8 cfs are required for wading, and flows from 9 to 20 cfs are required for swimming. The recommended minimum flow for recreational opportunities, with an emphasis on wading and some lesser opportunity for swimming, is 4 cfs throughout the year.
Streamflows in Beaver Dam Wash provide drinking water and support crucial riparian habitat for a variety of wildlife, including two bird species listed as Federally endangered. Habitat in this area is comprised of very diverse plant communities, which make this relatively small riparian area important compared to nearby areas along the Virgin River. Factors contributing to the high riparian values include consistently available ground water and good water quality to sustain plant communities (i.e., lower salinity levels than in the Virgin River floodplain). Lowering the water table within the floodplain of Beaver Dam Wash at the Virgin River confluence would favor tamarisk replacement of cottonwood and willow, and thus make the riparian area less diverse. Recommended minimum flows for maintaining water levels sufficient to support the riparian area are 4 to 4.5 cfs.
The Beaver Dam Wash assessment outlined a course of action for BLM. Under Arizona law, protection from ground-water pumping for an instream flow water right is uncertain. Consequently, the following actions are recommended to assist BLM in protecting Beaver Dam Wash riparian resources and other flow-dependent values:
Enright, M. 1996. Selected Hydrological Data for Beaver Dam Wash and Adjacent Areas, Washington County, Utah, Lincoln County, Nevada and Mohave County, Arizona. U.S. Geological Survey Open-File Report 96-493.
Holmes, W.F., G.E. Pyper, J.S. Gates, M. Enright, D. Schefer, and K. Waddell. 1997. Geohydrology and Water Quality of the Beaver Dam Wash Area, Washington County, Nevada, and Mohave County, Arizona. U.S. Geological Survey Water Resources Investigations Report 97 4193.
Jackson, W.L., B. Shelby, A. Martinez, and B.P. Van Haveren. 1989. An Interdisciplinary Process for Protecting Instream Flows. Journal of Soil and Water Conservation 44(2):120-126.
Moore, R.T. 1972. Geology of the Virgin and Beaverdam Mountains, Arizona. Arizona Bureau of
Mines Bulletin 186.
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