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<br />In gaining and in losing streams, water <br />and dissolved chemicals can move repeatedly <br />over short distances between the stream and the <br />shallow subsurface below the streambed. The <br />resulting subsurface environments, which contain <br />variable proportions of water from ground water <br />and surface water, are referred to as hyporheic <br />zones (see Figure 15). Hyporheic zones can be <br />active sites for aquatic life. For example, the <br />spawning success of fish may be greater where <br />flow from the stream brings oxygen into contact <br />with eggs that were deposited within the coarse <br />bottom sediment or where stream temperatures <br />are modulated by ground-water inflow. The effects <br />of ground-water pumping on hyporheic zones and <br />the resulting effects on aquatic life are not well <br />known. <br /> <br /> <br />Interface of ground-water- <br />flow system, hyporheic zone, <br />and stream <br /> <br /> <br />----- <br /> <br />I <br />I <br />\ <br />\ <br />\ ... <br /> <br />" " <br />" Y I> <br />.............-.:: r h e i C <br />----- <br /> <br />--- <br />Direction of <br />ground-water <br />flow <br /> <br />\ <br />I <br />I <br />/ <br />. / <br />~ / <br />1.0// <br />-- <br /> <br /> <br />Figure 15. The dynamic interface between ground <br />water and streams. (Modified from Winter and others, <br />1998.) <br /> <br />Streambeds are unique environments where ground <br />water that drains mudl of the subsurface of landscapes <br />interacts with surface water tlzat drains much of the <br />surface of landscapes. Mixing of surface water m1d <br />ground water takes place in the hyporheic zone where <br />microbial activity and chemical transformations <br />commonly are enhanced. <br /> <br />35 <br />