In gaining streams, groundwater seeps out into streams. In losing streams, stream water moves into groundwater systems. The flow moving through the streambed sediments under these two types of flow conditions are generally in opposite directions (upward vs. downward). The two opposite flow mechanism will affect the pore size 5 and fine particle content of streambeds. It is thus very likely that the opposite flow conditions affect the streambed hydraulic conductivity. However, comparisons of the

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... hydraulic conductivity (K ) of streambeds for losing and gaining streams are not well documented. In this study, we examined the K distribution patterns of sediments below the channel surface or stream banks for the Platte River and its tributaries in Nebraska, 10 USA. Two contrast vertical distribution patterns were observed from the test sites. In gaining reaches, hydraulic conductivity of streambed decreases with the depth of the sediment cores. In losing reaches, hydraulic conductivity increases with the depth of the sediment cores. This contrast patterns in the two types of streams were mostly attributed to flow directions during stream water and groundwater exchanges. In losing 15 reaches, downward movement of water brought fine particle into the otherwise coarse sediment matrix, partially silting the pores. For gaining reaches, upward flow winnows fine particles, increasing the pore spacing in the top parts of streambed, leading to higher hydraulic conductivity in shallower parts of streambeds. These flux directions can impact K values to depth of greater than 5 m. At each test sites, in-situ perme-20 ameter tests were conducted to measure the K values of the top streambed layer. Statistical analyses indicated that K values from the sites under losing stream condition are significantly different from the K values from the sites under gaining stream condition. 1694