Sustainability of Arizona’s few remaining perennial streams.
Mother Nature made water simple. In general, water comprises a continuum in whatever phase or phases it may occur. If you take some here, you have less somewhere else. Arizona law does not readily account for this simple concept; rather it arbitrarily divides water into different classes to be regulated as if the classes were somehow distinct and separable. This creation of different classes of water too often creates a false premise that there is more water. Estimates of the ratio of “claimed” water to real water in Arizona may run as high as 10:1. The discussion frames example situations in the context of technical aspects of current Arizona law and discusses the practical aspects of integration of a modern groundwater code with antiquated laws regarding surface water. These concepts are then evaluated in light of the interminable surface water adjudication process and the subsequent potential impacts on perennial surface waters.Examples are derived from the author’s experience in early studies of groundwater-surface water interaction in Tonto Basin, the Upper San Pedro, the Upper Santa Cruz and the Verde Valley. Insight into agency policy initiatives in the early 1970’s and 80’s with regard to groundwater-surface water interaction will be provided. With specific reference to the Upper San Pedro and the Upper Verde Valley, the discussion examines the as yet unresolved Arizona issue of contributory groundwater and the potential impacts of leaving this critical issue unaddressed. Contributory groundwater is defined as that water that sustains the base flow of perennial streams.Tonto Creek is one of two main tributaries to Roosevelt Lake. The perennial portion of the creek enters Tonto Basin from a bedrock narrows above Gun Creek. Below this point Tonto Creek is generally ephemeral as a function of the season and precipitation events. The hydrogeologic setting provides a perfect example of the subflow issue. A broad band of relatively shallow, recent coarse alluvium is embedded in thick sequences of lakebed deposits consisting of silts, clays and evaporites. Contributory groundwater flow from the valley sides is judged minimal to non-existent. Therefore the only viable source of water is direct withdrawal from the creek or pumping of wells located in the recent alluvium. Studies in which the author took part in the late 1960’s documented the fact that the surface water regime was in direct connection with the groundwater system in the recent alluvium. Thus, on a one for one basis, any water withdrawn from the recent alluvium directly depletes the surface water and/or the subflow contribution to Roosevelt Lake. Thus, on a purely technical basis, each and every well in the recent alluvium should be part of the adjudication along with diversions from the creek. Based on the surface water concept of senior rights to the water, no further withdrawals should be permitted.The Upper San Pedro River is a classic example of a perennial stream in the Basin and Range. Like many river courses in the Basin and Range, the river is set in a broad, relatively shallow ribbon of recent and relatively coarse alluvium. The surrounding valley is comprised of productive older alluvium typical of basin fill deposits in the Basin and Range. In the recent alluvium along the river the concept of subflow is well documented and has been the subject of several specific technical and legal evaluations. In the perennial reaches, the river’s base flow is also sustained in part by contributory groundwater from the sides of the valley. Studies by the author and others associated with proposed large scale developments in the early 1970’s established the concept that groundwater pumpage from the valley alluvium (contributory groundwater) would eventually have an adverse impact on the perennial reaches of the river. The only issue was one of timing, i.e., when would the contributory groundwater pumpage impact the river. Therefore, on a purely technical basis, if the stream is to be sustained, pumpage from the recent alluvium and some accommodation for the pumpage of contributory groundwater must be made in the adjudication process. Because there is no specific linkage between the surface water and groundwater code, and no significant case law regarding contributory groundwater in Arizona, the Upper San Pedro represents one of the most controversial issues in the Gila River adjudication.The Upper Santa Cruz River enters Arizona through a bedrock narrows near the Mexican border. A short reach of the river was perennial in the vicinity of the border and was utilized by the City of Nogales for much of its water supplies in the 1970’s and 80’s. The City was a recalcitrant participant when the issue of rights to use the water was raised with the advent of the adjudication. The river has a hydrogeologic setting similar to the San Pedro in that there is a broad shallow band of recent alluvium within which subflow as currently defined is an obvious concept. The concept of contributory groundwater from the valley sides is more complex because of the question of contributory flow to ephemeral reaches of the river downstream from the border. Studies by the author and others for proposed developments, and the differing conclusions reached, led to a formal hearing on the subflow issue with regard to a proposed development in the immediate vicinity of the perennial reach. The issue was generally decided in favor of Nogales, although their specific rights to use the water were unclear. The issues downstream of the perennial reaches are more technically and legally complex, and are now rendered more so by the addition of effluent from wastewater treatment facilities creating a longer reach of perennial flow and providing a technical setting in which subflow and contributory groundwater may become a more important issue.The Verde Valley, and particularly the upper Verde, whose base flow is sustained by groundwater discharge from Big Chino Valley, has the technical potential to most directly raise the contributory groundwater flow issue. The gaging station records at Verde River near Paulden offer a classic example of the base flow concept of groundwater discharge sustaining a perennial stream. The rights to the surface flow of the Verde River are some of the oldest and most legally viable in the state. Subflow issues in the upper reaches are considered minor. However, the historical and current interest in the development of large scale groundwater withdrawals from Big Chino may put the issue of contributory groundwater in a forum appropriate to developing a solution to the last remaining siginificant problem in sustaining Arizona’s few remaining perennial streams. Modern technical approaches such a digital modeling of the the Big Chino groundwater system, including the basin's contribution to the base flow of the Verde, would allow predictive capability of the magnitude and timing of the potential depletion of the river's base flow component. Such an approach may also be able to demonstrate that it may be possible to develop groundwater from parts of the Big Chino Valley without impacting the base flow component of the Verde River.In summary, the last signifcant remaining technical issue which impacts the sustainability of Arizona's few remaining perennial streams is that of contributory groundwater. Addressing this issue would allow nearly complete integration of the surface water adjudication process with Arizona groundwater law. If the issue is not addressed on a proactive basis by the legislature and/or the adjudication process, the existence of perennial streams may become a thing of the past.