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<br />Oln4~j <br /> <br />At st~ge-recorder stations on certain relatively swift streams, <br />it has been found that water level in the stilling well may be drawn <br />down owing to distortion of the :flow pattern by intake pipes of the <br />ordinary tyPe. Intakes of special design have been installed at <br />numerous stations to eliminate this effect in whole or in substantial <br />part. Published "daily mean gage heights" may be in error from this <br />cause, in j;hat they are in part "daily mean stilling-well heights." <br />Published values of daily mean discharge, however, will be essentially <br />correct if' the station rating is wholly in terms of stilling-well <br />heights. Adjustments generally are not warranted, except when the <br />principal interest is in the water-surface profile corresponding to <br />a given discharge, as in predicting flood stages. <br /> <br />Unstable controls <br /> <br />Owing to instability of the "contro]", the rating of a stream- <br />gaging staj;ion--that is its relation between stage and flow--may <br />change gradually over a long interval of time, or abruptly and inter- <br />mittently. Such a change, or "shift", may be either natural or man- <br />caused; tyPical causes include ice, beaver dams, drift that lodges, <br />and channel modifications by man. <br /> <br />In cOmmon practice, the first one or two abnormal discharge <br />measurements have been treated as though influenced by random errors. <br />A new rating is adopted, or "shifting-control" methods are applied in <br />computing the flow records, only after the transient or progressive <br />shift has 1;>een confirmed by several measurements in succession. In <br />net effect, published values of daily mean discharge probably are somewhat <br />in error over relatively short intervals of time but ultimately "average <br />out" or coJilpensate. Magnitude, algebraic sign, and duration ot' such error <br />can be appraised by correlating the questioned reCord with records for <br />adjacent stations on the same stream or other streams in the same <br />hydrologic environment. Instability of control during floods is a special <br />problem and is considered below. <br /> <br />Rapidly changing discharge <br /> <br />During periods of large streamflow the control for a particular <br />gaging station generally is the reach of channel downstream from that <br />station. In this situation, discharge is a function not only of stage <br />but also of the slope of the water-surface profile. If discharge is <br />steady or Changes slowly, each rate of discharge is associated uniquely <br />with a particular stage and a particular slope. However, if discharge <br />is increasing rapidly the water-surface slope is greater than the steady- <br />flow slopeifor any given discharge, and vice versa. Percentage-wise <br />these departures from steady-flow slope may be large for streams with <br />flat gradients, although they may be of no consequence for the steeper <br />streams. <br /> <br /> <br />1.2 <br />