. Water Supply and Irrigation Papers of the United States Geological Survey. not applicable for known conditions of ice, log jams, or othersimilar obstructions; (c) that the increased and decreased dischargedue to change of slope on rising and falling stages is either negligibleor compensating. As already stated, the gaging stations may be divided into severalclasses, as indicated in the following paragraphs: The stations of class 1 represent the most favorable conditions foran accurate rating and are also the most economical to maintain.(See fig. 1.) The bed of the stream is usually composed
. Water Supply and Irrigation Papers of the United States Geological Survey. not applicable for known conditions of ice, log jams, or othersimilar obstructions; (c) that the increased and decreased dischargedue to change of slope on rising and falling stages is either negligibleor compensating. As already stated, the gaging stations may be divided into severalclasses, as indicated in the following paragraphs: The stations of class 1 represent the most favorable conditions foran accurate rating and are also the most economical to maintain.(See fig. 1.) The bed of the stream is usually composed of rock and isnot subject to the deposit of sediment and loose material. This classincludes also many stations located in a pool, below which is a perma-nent rocky riffle that controls the flow like a weir. Provided the con-trol is sufficiently high and close to the gage to prevent cut and fdl atthe gaging point from materially affecting the slope of the water sur-face, the gage height will for all practical purposes be a true index of INTRODUCTION. 25 GAGE HEIGHT IN FEET. 26 SURFACE WATER SUPPLY, 1907-8. the discharge. Discharge measurements made at such stations usuallyplot within 2 or 3 per cent of the mean discharge curve and the ratingdeveloped from that curve represents a very high degree of accuracy. Class 2 is confined mainly to stations on rough mountainous streamswith steep slope. The beds of such streams are as a rule compara-tively permanent during low and medium stages, and when the flowis sufficiently well defined by an adequate number of dischargemeasurements before and after each flood the stations of this classgive nearly as good results as those of class 1. As it is seldom pos-sible to make measurements covering the time of change at floodstage, the assumption is often made that the curves before and afterthe flood converged to a common point at the highest gage heightrecorded during the flood. Hence the only uncertain period occursduring the few days of highe
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