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<br />30.5--2 <br /> <br />. <br /> <br />WATER INFORMATION MANAGEMENT SYSTEM HANDBOOK <br /> <br />channel network so as to retain its capability for passing flood <br />discharges, minimize channel erosion and/or sediment deposition <br />associated with instability or disequilibrium conditions. The <br />consequences of not maintaining channel stability include <br />channel aggradation, channel erosion, floodplain encroachment, <br />vegetation encroachment, changes in hydrauliC geometry, and <br />reduced channel capacity with resultant increased risk of <br />flooding and associated resource damage. <br /> <br />Dav 1. An index point describing the beginning of the rise in <br />the required maintenance flows hydrograph. It generally occurs <br />on or after the day during snowmelt runoff when the mean 24-hour <br />flow equals or exceeds mean annual discharge for some user <br />defined period of time. <br /> <br />DeRradation. <br />floodplains, <br />in elevation <br />boundary. <br /> <br />The geologic process by which stream beds, <br />and the bottoms of other water bodies are lowered <br />by the removal by water of material from the <br /> <br />Deoosition. The mechanical or chemical process through which <br />sediments accumulate in a resting place. <br /> <br />~ <br /> <br />Drawdown. The difference between the flood elevation recorded <br />at a stream gage and the actual flood elevation which is caused <br />by the effects of high velocity on the gage. As used in the <br />Chapter - the lowering of a water level, as for example, depth <br />to groundwater, lake level, or stream stage. <br /> <br />Eauilibrium. (Or dynamiC equilibrium). The condition of a <br />stream where a reach is neither aggrading nor degrading and can <br />be represented by the qualitative relationship Qsd - QwS <br />'where Q = sediment discharge, Q = water discharge, d = <br />sedimen~ particle size, and S re~resents slope. <br /> <br />Erodible Bed/Bank Channels. Those natural, perennial, alluvial, <br />and fluvial stream channels or channel reaches which are free to <br />adjust their dimensions, shape, pattern, or gradient in response <br />to changes in streamflow. <br /> <br />Flood El~y~_t1Qn. The height above the thalweg at which a <br />discharge of a given recurrence interval flows. Most commonly <br />applied to heights above the level of the banks of the active <br />channel. <br /> <br />Flow-Duration Curve. A graphicaL plotting of magnitudes of <br />stream discharge (ordinate axis) against the percent of time <br />each magnitude is equaled or exceeded (abscissa axis). <br /> <br />*-FSH 4/89 AMEND 3-* <br />