TEMPORAL AND SPATIAL VARIATIONS IN GROUND-WATER DISCHARGE TO STREAMS IN THE CASCADE RANGE IN OREGON, AND IMPLICATIONS FOR WATER MANAGEMENT IN THE KLAMATH BASIN

The Cascade Range volcanic arc represents what is arguably the single most important ground-water resource in Oregon. Yet, the unique hydrogeology of the Cascade Range presents challenges to resource managers. Much of the unique hydrologic character of the Cascade volcanic arc in Oregon can be attributed to two factors: (1) substantial orographic precipitation and resulting ground-water recharge, and (2) ground-water storage in extensive, highly permeable strata. These factors result in the major streams draining the eastern slopes of the southern and central Oregon Cascade Range being dominated by ground-water discharge. These streams have flows that are relatively constant compared to similar-size streams elsewhere in the region, but not without important temporal variations. The temporal variations in the Cascade streams, unlike many other streams, are not driven solely by current-year precipitation and snow pack, but strongly reflect antecedent conditions from previous years. Data from the upper Deschutes and upper Klamath Basins show that the temporal signals in ground-water discharge to springs and streams vary depending on scale of the flow system and the nature of the underlying geology. Discharge to small streams tends to be controlled by recent climate conditions, but discharge to volumetrically important larger-scale streams tends to integrate climate signals over longer periods. The hydrologic response of streams is also affected by geologic heterogeneity. Permeability and storage properties of the volcanic arc vary laterally, primarily as a function of the age of the rock. Hydraulic properties vary vertically as a function of the age of the rock and the degree of hydrothermal alteration. As a result, similar-scale flow systems may exhibit different hydrologic behavior in different areas. The unique aspects of the hydrology of the Cascade Range volcanic arc have confounded accurate streamflow forecasting and water management. Forecast uncertainty has been a particular problem in the Klamath Basin, where severe water shortages exist and year-to-year water management strategies are highly dependent on accurate forecasts. An important next step is to apply the increasing knowledge of the hydrogeology of the Cascade Range volcanic arc to water management using statistical or numerical models.