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001452 1. STREAMFLOW Gage heights 1.1 Prior to about 1930, most stream-gaging stations were equipped only with ~ non-recording staff or weight gage, commonly vertical but in some inatances inclined, from which stream stage or gage height was read once or twice daily by a local observer. Each once-daily reading of stage, or the average of twice-daily readings, was assumed to indicate the amount. of flow over a 24-hour period according to the station "rating," "Daily mean gage heights" and "daily mean discharge" were derived accordingly and published by the Geological Survey in its serial Water- Supply Papers. Exoepting errors in gage heights as recorded by the observer, a flow record so produced is reasonably consistent for periods during whi~h gage height was constant, or rose or fell slowly and steadilY for days in succession. However, for periods during which gage height fluctuated diurnally (clear-weather snowmelt or discharge from a "peaking" hydroelectric plant), or fluctuated widely over brief and irregular intervals of time (rain-generated floods), the published values of daily mean discharge may be substantially in error. The possibility of such errors may or may not be suggested by "remarks" in Water-Supply Papers. In the case of regular diurnal fluctuation, if the once- or twice- daily gage :readings were at the same hour each day, the published values of daily mean discharge tend to be systematically too large or too small. Published values of runoff, monthly or yearly, tend to be systematically in error allso, in about the same degree for the term of fluctuation. Some such ~rroneous records can be adjusted reasonably after deriving from subsequent stage-recorder charts, an average relation between daily mean gage height and gage height taken at the known hour each day. In the case of "flashy" rain-generated runoff, errors are generally random in magnitude and algebraic sign. They mayor may not compensate in the published values of monthly and yearly runoff. Some instances can be adjU;sted on the basis of "routing" comparisons with stations up- and down-st~eam. In other instances, the original gage-reader's books list supplemental readings from which the true "hydrograph" and true daily mean ~age heights can be approximated. Between about 1930 and 1945, nearly all staff-gage stations were converted tp stage-recorder stations. In most subsequent records, serious errors of the .sort just described have been eliminated.