The delineation of ground-water flow in alluvial valley stream-aquifer systems is important in studies of water availability, environmental impact, and aquifer remediation and is required by some states' water laws. Published potentiometric surfaces and head data, however, are not always available. When data are absent, it is commonly assumed that ground water and dissolved solutes flow toward the river. Analysis of published geomorphologic and hydraulic data in 24 alluvial systems and the results of digital simulations indicate that in some cases this assumption is only valid immediately adjacent to an effluent river. Ground-water flow in the remaining portions of an alluvial valley aquifer may be dominated by downstream or underflow components. This paper investigates the nature of ground-water flow in alluvial valleys and presents a new classification scheme for alluvial aquifers. The geomorphologic conditions that may allow underflow to be the predominant regional ground-water flow component are described.

In order to classify alluvial aquifers, two Darcy flux end-member components are defined. The baseflow component is that portion of the ground-water flux that moves perpendicular to the river. The baseflow component may flow toward the river or away from the river depending on whether the river is effluent or influent, respectively. The underflow component of the Darcy flux moves parallel to the river and in the same direction as the stream flow. The alluvial stream-aquifer systems are classified based on the predominant regional ground-water flow component as underflow-component dominated, baseflow-component dominated, or mixed. On the basis of the results of this study, we conclude that the dominant regional ground-water flow component, base-flow or underflow, can be inferred in an alluvial valley aquifer from geomorphologic data. These data include the channel slope, the river sinuosity, the degree of penetration (incision through the alluvium) of the river, the width-to-depth ratio, and the fluvial depositional system. The underflow component is demonstrably predominant when the following conditions exist: (1) the channel gradient exceeds .0008, (2) the sinuosity is less than 1.3, (3) the river penetration is less than 20%, (4) the width-to-depth ratio is greater than 60, and (5) the fluvial depositional system is either valley fill or mixed load to bed-load. The underflow component can also be dominant on flood plains where the lateral valley slope is negligible.

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