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GENERAL PROCEDURE FOR GAGING STREAMS 3 where backwater occurs must be accepted, a uniform reach for measurement of slope should be sought, in addition to the proper placement of an auxiliary gage. Un- steady flow such as occurs in tide-affected stream channels requires similar considera- tion but, in addition, line power must be available to insure simultaneous record- ing of stage at the two gages. 4. Availability of a nearby cross section where good discharge measurements can be luude. 5. Proper placement of a stage gage with re- spect to the measuring section and to that part of the channel which controls the stage-discharge relation. 6. Suitability of existing structures for use in making high-flow discharge measure- ments, or the proper placement of a cab]e- way for this purpose. 7. Possibility of flow bypassing the site in ground water or in flood channels. 8. A vai]abi]ity of line power or telephone lines where needed, for special instrumentation or for Te]emark units. 9. Accessibility of the site by roads, particu- larly during flood periods. The gage on Kaskaskia River at Bondville, III., shown in figure 2, satisfies severa] of the above requirements. Low-flow measurements are, made by wading upstream from the artifi- cial control, and high-flow measurements are made from the bridge. The bridge site provides accessibility, convenience to power lines, and a good location for an outside gage, shown on the downstream handrail. Artificial Controls f , .\rtificial controls are structures built in a stream channel to stabilize the stage-discharge relation and thereby simplify the procedure of obtaining accurate records of discharge. They may hl' low dalll~, hro,ld-l'l'C'stec1 weirs COll- formin~' to the ~eneral shape and height of the streambed, or flumes similar in design to the Parshall flume. The adverse cffects of unstable eonditions due to shifting bed or banks. the forlllation of it'e in wintt'l', IH'Og-l'P~:-,i \'l' f!l'owth of aqmlti(' veg:etntioll during tlw :-i1l1ll1llt'l'. ilnd other phenomena whi('h at- tillle~ ati't"cr tlw 28-5-99G Q--...68--2 stage-discharge relation at low stages may gen- erally be eliminated or alleviated by the con- struction of an artificial control. The structure is seldom designed to function as a complete control throughout the entire range of stage. Generally it is impracticable to build it high enough to eliminate the effects of downstream conditions at high stages unless there is a steep fall below the gage. If the downstream slope is flat, so that with an increase in discharge the water below the control rises faster than the water above it, the control may be completely effective only for low and medium stages. Fig- ure 8 shows the artficial control on Mill Creek near Coshocton, Ohio. A differently shaped artificial control is shown in fignre 4, for the gage on the De]aware River near Red Bluff, N. Mex. Note the shallow V-notch in the broad- crested weir, to improve sensitiveness. .\Jthough the m1ificia] control is usually con- struded in the form of a dam or a weir, it is seldom if ever desirable to attempt the nse of a weir rormu]a as its rating. The rating for each staltion should be determined by a current- meter or other method of measuring discharge. The conditions or bcilities for the accurate measurement or small streams and for the measurement or the low-water flow of larger streams commonly can he improved hy the use or artifical controls. Tn the design of artificial controls the follow- ing four major points should he considered: 1. The shape or the structure should pelmit the the passage of water without creating un- desirahle disturbances in the channel a;bove or below the control. ~. The struot.ure must be or sufficient height to eliminate the effoots of variable down- stream conditions. a. The profile of the crest of the control should be designed so that a small change. in dis- ('hat.g-e llt low stages will cause a Ine-asura- hIe ehange in stage, and the relation of ('hang-es ill stage to ehunges in discharge should produce n. rating" ('urve of n shape t.hat mOlY be extended to peak stages with- out serious error. ~. TIll' eontrol should have stmelnral stahilit,y and should he permanent. TIHl nrtificial wntrol should be self-denning and