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<br />O()1796 <br /> <br />drainage system and associated streamflow. Natural scientists and engineers in the second half <br />of the nineteenth century had developed an understanding of the hydrologic cycle recognizing <br />precipitation, runoff, evaporation, and transpiration as the fundamental elements of the cycle <br />(Meinzer, 1942). Although some believed that forest cover might enhance precipitation by <br />altering the radiation balance, panicularly in arid regions, it was generally recognized that forest <br />cover might increase infiltration of precipitation into the ground, perhaps enhancing base flow. <br />or low flow, in streams. In contrast, it was also recognized that by evapotranspiration (water <br />returned to the atmosphere in the form of vapor from the earth surface and respired by plants). <br />forests abstracted or "consumed" water returning it to the atmosphere. <br /> <br />To the extent that forests enhance rather than deplete stream flows, preserve the <br />continuity of the drainage system, and attenuate streamflow associated with either drought or <br />flood, the National Forests contribute to "the purpose of securing favorable conditions of water <br />flows." In this instance, the contribution of the National Forests moves beyond "the forest <br />within the boundaries" to the provision of water to a variety of potential users downstream. Such <br />provision of water and flow assumes the continuity of the natural drainage system for the <br />effective and efficient delivery of water from the watershed to users. By definition a channel <br />must exist and be maintained to achieve these functions and, as more fully explained below, this <br />requires streamflow. These flows, known as channel maintenance flows, are intended to <br />maintain the physical characteristics of the stream channel such that the transport capacity of the <br />channel is preserved. Channel maintenance flows are the minimum amount of natural streamflow <br />which must be left in a stream for the channel to remain in a fully functioning condition. <br /> <br />Where channels are reduced in size, flows are impeded, flooding exacerbated, and <br />efficient delivery to the downstream channel system impaired or precluded. Assuring <br />maintenance of stream channels contributes to "securing favorable conditions of water flows" <br />and retains the ability of the channel to deliver water to downstream users to meet irrigation, <br />domestic and industrial uses. <br /> <br />B. Concept of Channel Maintenance <br /> <br />1. Principle <br /> <br />A stream channel is maintained if it retains its capacity, over time, to carry the water and <br />sediment delivered to it. The concept of channel maintenance derives from an understanding of <br />the behavior and characteristics of self-formed alluvial channels.6 Alluvial refers to material <br />moved by running water. Alluvial channels develop a size and shape over time that is <br />determined by the dynamic interaction of the flow and sediment delivered to the channel and the <br />channel boundaries. Thus, at a fundamental level, the concept of channel maintenance is derived <br />from the principles governing the behavior of self-formed alluvial channels. These principles are <br /> <br />6 Alluvial refers to material composed of alluvium: Le, deposited by a stream or running water. <br />United Slates' Expert Report Disdosing Melllodologies for Cuanlificallon of Organic Ad Claims Coosolidated Subcase No, 63-25243 <br /> <br />6 <br />