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2 TECHNIQUES OF WATER-RESOURCES INVESTIGATIONS available condition for stage and discharge measurements and for developing discharge ratings. A continuous record of stage is obtained by installing instruments that sense and record the water-surface elevation in the stream. Dis- charge measurements are initially made at vari- OUR stages to define the relation between stage aud discharge. Discharge measurements are then made at periodic intervals. usually monthly, to verify the stage-discharge relation 01' to define any chauJ?e in the relation owin~ to ChUllg'PS in channel geometry. At many sites the discharge is not. a unique function of stage: variables other than stage also must he continuously measured to obtain It rlischarg-e record. For example, streanl slope i" measured by the installation of an auxiliary stng-e gage downstream if variable hack,vater occnrs. At other sites a ('ontinuOllS measure of stream velocity at a point in the cross section i" obtainerl anrl used as an additional ,-ariable in the diRCharge rating. The rate of change of stag'e ('an h~ an important variable at sites hav- iug con"irlerable un"teadines" of flow. I.JfHY weirs and dams nre constructed at some stat.ions to stabilize tll(' stage-discharA"e rela- tions ill tl1l' low-flow range. These control struc- 1 arC's are f'ulibrnted hy stage .llul disrlUll'ge measurements in the field. The data ohtained at the gaging ~tation a1'(, 1'l',.iC'\H><l and Hnnlyzt'd hy f'1l,!!inl'Pl'in.!! 1~(,l'~nllIlPI at thf'l end of tlU' watrr year. Dis('har~p ratings :11"(11 estnhlisheil, and tIll' :.ra~:n'.Iu:'i~ht r{,col'el is l'('duced to mean values for selected time pe.ri- ods. The mean rlischarg-c for each day and ex- t.remes of discharge for the year are computed. The data arc then prepared for publication. Selection of Gaging Site The selection of gaging sites is dictated by the needs of water management or by the re- quirement" of t.he hydrologic network. In ful- Hll inp,- watt'I..lllillHlp,-ement needs t ht'l'(,. is litt If' or no freedom of choice in selecting gaging- sites, and frequently records mnst be obtained under ,'ery adverse hydraulic conditions. For example, many of the principal "treams in the TTnited St.ates have heen converted into a series of pools by the construction of dams: yet, very precise records are needed for operation. Rec- ords are also needed in tidal reaches of stream channels in connection with water supply, salin- ity contamination, or waste disposal. Hydrologic network requirements allow more choice in selecting good sites for gaging, al- t.hongh in some places gaging conditions are poor throughout an entire region. For example, all streams in a gi,'en region may have unstable beds and banks, which result in continually changing stage-discharge relations. However, before a stream-gaging station is constructed, " general reconnaissance is made in order that the most suitable site for the gage may be se- lected. This reconnaissance is facilitated by an exa.mination of geologic, topographic, and other maps of the aren. Tentati,-e sites for gaging sta- tions may be indicated on the maps, each site being "ubjed to critical examination of the phy- sical characteristic" of the stream channel. In selecting a site. con"idcration should be given to the following items: 1. Channel ('haraderi"tic" relati,-e to a fixed and pc.rmanent. relation between stage and discharge at the. gage. A rock riffle or falls, as shown in figure 1, indicates an ideal site. H a site on a stream with a movable bed must, be accepted-for example, a sand- channel stream-it is be.st to locate the ~age in as uniform a reach as possible, '''my from ob"tructions in the channel "Hch as bridges. 2. Opportunity to in"tall an artificial control. :L Possihility of hackwntel' from downstream trihutaries 01' other sources. If a sitp :-:: .'~ ,,'t'>. . ." .~.." "., -'-'"~-.: .~:~'i...~ Figure 1 ,-Gage and natural control, little Spokane River at Elkl Wash.