## 1. Introduction

[2] The spatial variability of hydraulic conductivity (*K*) or permeability determines groundwater flow and solute transport at various spatial and timescales [*Gelhar et al.*, 1992; *Gelhar*, 1993]. Thus one of the most basic pursuits of modern hydrogeology is the determination of the heterogeneity of porous media [*Webb and Davis*, 1998].

[3] The generation of maps of heterogeneous aquifers has been categorized into three approaches [*Koltermann and Gorelick*, 1996]: structure-imitating, process-imitating, and descriptive. Structure-imitating methods include geostatistical interpolation techniques and sedimentation geometry-matching approaches. Process-imitating techniques generate images via solving governing mass and momentum balance equations of fluid and sediment flow and transport. Descriptive approaches involve delineation of domains based on direct observations and genetic conceptual models (facies).

[4] These approaches have various advantages and limitations [*Koltermann and Gorelick*, 1996]. Structure-imitating models commonly result in unrealistic images. Process-based models generate models that are more physically plausible, although it is difficult to condition them to actual data. Descriptive methods are able to include physical insight but do not generate results with sufficient accuracy. It is therefore apparent that integration of these techniques will allow effective and more realistic representation of spatial variability of hydraulic properties of aquifers [*Webb and Davis*, 1998].

[5] In this paper, we present a three-dimensional (3D) model of the modern streambed of a meander bend that is generated exclusively through a structure-imitating method- 3D kriging. Our realization is based on extensive direct small-scale hydraulic tests that cover most of the study domain. These *K* estimates are augmented with data from cores and ground-penetrating radar (GPR) profiles. This data set allowed us to examine the 3D geostatistical properties (variograms) of the tested media. Additionally, we are able to favorably compare our observations to results from detailed investigations of modern surface deposits in similar settings [*Bridge*, 1977]. Such studies directly link processes and products and allow us to interpret our results in a process-oriented framework. Through this integrative interpretive approach of data of different nature (hydraulically based, core-based, and geophysically based), we are able to separate the streambed from its adjacent hydrostratigraphic units. In addition to identification of the streambed as a distinct hydrofacies, we also present its effective hydraulic parameters derived through various upscaling methods.

[6] In a practical sense, delineation and characterization of the modern streambed has significant implications on groundwater-surface water interactions [*Sophocleous et al.*, 1995; *Zlotnik and Huang*, 1999; *Hunt et al.*, 2001; *Butler et al.*, 2001]. Our research was conducted within this context although most investigations of heterogeneity are commonly related to contaminant transport and remediation problems. A standing issue in water-resource management is how streams interact with groundwater. Unfortunately, most studies that address this problem have considered the streambed as an entity of well-defined geometric and effective hydraulic properties. Our results show that the modern streambed (sometimes referred to as the “hyporheic zone” in special cases [*Woessner*, 2000]) at our study site, which is part of a fluvial system typical of the High Plains of the Midwestern United States, is far from its common representation in both numerical and analytical models.

[7] Considering the important role of the streambed in various natural processes, the focus of this paper is the development of a realistic 3D model of the modern streambed from analyses of spatial variations of hydraulic properties used as proxies of architectural elements. The developed model should allow us to confidently separate it from its surrounding units. Although we chose to represent the 3D *K* field of the streambed as a continuum (which is inherent in the kriging process) and not through simulating discrete units, we are able to show that our realization is realistic by comparing it with deposits in similar modern environments. We present a case where a purely structure-imitating model agrees with well-documented modern surface deposits, which allows us to confidently delineate the streambed. This approach of characterizing modern streambed deposits along meander bends, whose depositional processes are readily observed, has numerous implications not only in studies related to realistic image generation of porous media but also in existing hydrogeologic models that include the streambed.