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Rio Grande The wetted perimeter of the Rio Grande, thickness and hydraulic conductivity of the riverbed, and difference between river stage and hydraulic head beneath the riverbed control the rate of water movement between the Rio Grande and the aquifer system. The physical extent of the Rio Grande channel is well known and is described with digital data at various time periods (Kernodle and others, 1995, p, 10). Digital data at a source scale of 1:24,000 are available from the National Biological Service for 1935 and 1989 and at a source scale of 1:12,000 are available from the Bureau of Reclamation for 1955, 1975, and 1992. Elevation of the riverbed, important for determining the difference between river stage and the hydraulic head beneath the riverbed, is available from USGS topographic maps. These data provide sufficient detail in describing channel extent and elevation; however the wetted perimeter of the river and river stage vary depending on the volume of its flow. Riverbed hydraulic conductivity may also be dependent on volume of riverflow if the texture of channel sediments or amount of silt sealing varies across the river. To determine if changes in wetted perimeter and river stage make a significant difference in estimated seepage between the river system and the aquifer, the 1980-94 simulation of the ground-water-flow model described by Kernodle and others (1995) was modified. The wetted perimeter of the river and the thickness and hydraulic conductivity of the riverbed are reflected in the hydraulic conductance of the stream/aquifer interface (riverbed) (McDonald and Harbaugh, 1988, chap. 6). Kernodle and others (1995) assumed that the Rio Grande covered the entire channel and maintained a constant stage of 3 feet above the riverbed throughout the year, For the modified scenario, the simulated hydraulic conductance of the riverbed was reduced by half during the winter months to represent 50 percent less wetted perimeter, and the river stage was reduced by 0.75 foot to represent the generally lower flow conditions in the Rio Grande during that time period (Cruz and others, 1994, p. 201). The simulated hydraulic conductance of the riverbed was reduced by 10 percent during the summer months to represent an average condition of less than channel-capacity discharge. The resulting seepage from the river and canals was reduced about 5 percent from the original simulation in the Albuquerque area (river and canal seepage could not be separated in the model output). Because this is a combined reduction of river and canal seepage, seepage from the river alone was reduced by a larger percentage. Wetted perimeter and river stage are, therefore, important for quantifying seepage from the Rio Grande. This information can be interpolated using discharge and stage records from the existing Rio Grande streamflow-gaging stations in the Albuquerque area (fig. 4); therefore, additional information on these characteristics is not needed, As noted in the previous paragraph, riverbed hydraulic conductance includes the thickness and hydraulic conductivity of the riverbed as well as the wetted perimeter. A reduction in the ratio of riverbed hydraulic conductivity divided by riverbed thickness also reduces the simulated seepage. Kernodle and others (1995, p. 20, 110) assumed riverbed hydraulic conductivity to be 0.5 foot per day and bed thickness to be 1 foot in their ground-water-flow model of the Albuquerque Basin. Sophocleous and others (1995, p. 587-588) reported that the riverbed hydraulic conductivity relative to aquiferhydraulii: conductivity is the most significant factor contributing to overestimation of stream depletion based on the assumption that the river and aquifer are in full hydraulic connection. Therefore, it is essential to better define riverbed hydraulic conductivity. Because the hydraulic conductivity must be applied over the thickness of the riverbed to determine hydraulic conductance, the effective riverbed thickness must be determined in conjunction with hydraulic conductivity. The Bureau of Reclamation (1994c, p. 9) installed a drive point into the Rio Grande channel and encountered sand (they indicted it had a similarity to "quick sand") to a depth of 3 feet below the channel, clay from 3 to 8 feet, and a sand layer below the clay. In this example, the effective riverbed that would restrict the vertical movement of water between the river and the aquifer is the 5-feet thick clay. For calculating hydraulic conductivity across the riverbed, measuring the hydraulic head below the riverbed (about 8 feet below the river channel in the above example) and the stage of the river is essentiaL 10 0: 1616