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12 TECHNIQUES OF WATER.RESOURCES INVESTIGATIONS structures on or near a stream, The usual ap. proach is to estimate the discharge from a regional relation and to compute the stage from this discharge and a channel survey, A simpler though less accurate method relates stream depth to discharge or to basin charac- teristics (see Thomas, 1964, and Gann, 1968), A more comprehensive study was made by Stall and Yang (1970) in which stream depth (and other measures of channel geometry) were related to flow frequency and drainage area, All three of the above. referenced studies are based on the pioneering work on channel geometry by Leopold and Maddock (1953), Procedures for Other Flow Characteristics Multiple regression has been used to reo gionalize mean annual flows, mean monthly flows, annual minimum !lows, annual !lood volumes, and some other characteristics. Thomas and Benson (1970) described a study of relations for estimating stream!low charac- teristics from drainage-basin characteristics in four hydrologically differing regions of the United States. An even more comprehensive use of the multiple-regression method for regionalization of !low characteristics was performed in each State of the conterminous United States during 1970, Results of this study are given in a series of reports, gen. erally one for each State; a typical one is by Collings (1971). In most humid regions mean !low is closely related to drainage area and mean annual precipitation. Thomas and Benson (1970) found a standard error of regression of 14.4 percent using those two variables in Potomac River; they reduced it further by including channel length and mean annual snowfall. Standard errors of 10 to 15 percent have been attained in other humid regions, In semiarid regions of large relief the rela. tion of mean flow to drainage area and pre. cipitation may not be usable because of (1) the great range in precipitation with eleva- tion, (2) the lack of good information on precipitation, and (3) the strong influence of geology on mean flow. For example, the standard error of the regression equation for mean !low in New Mexico is 53 percent (Bor. land, 1970), In certain humid regions a satisfactory regionalization of mean flows is not attain- able because of the movement of ground water across topographic divides, Some reo gions exhibiting this condition are the Ump. qua River basin in Oregon, the Red Rock River basin in Montana, and the Ba1cones Fault region in Texas, Although the principles of regional analy. sis apply to all !low characteristics, the appli. cation to low flows is least successful because of the greater dependence of low !lows on basin characteristics that are imperfectly known and that cannot be described by sim- ple indexes. Geology is the chief basin charac. teristic, other than drainage area, controlling the size of low flows in a region of homoge. neous climate, Evapotranspiration, especially from the channels and flood plains, also has a substantial effect on low !lows in many basins, Most reported attempts at regionalization of low flows on a statewide basis have been unsuccessful. Forty.seven Geological Survey districts participated in and reported on their compt'ehensive regionalization studies in 1970. Most districts reported either standard errol's of low flows in excess of 100 percent (average of plus and minus percentages) or that no meaningful relation was derived. A notable exception was Connecticut; there the 7.day 10.year low flow was related to drain. age area, channel slope, mean basin elevation, and percentage of basin covered by stratified drift, with a standard error of 68 percent (Thomas and Cervione, 1970), This small standard error (relative to those found in most regions) resulted from the inclusion of the fourth parameter, In a previous paper, Thomas (1966) reported large unit base flows from stratified drift and very small ones from till, the predominant surficial glacial deposit, Regionalization of low flows in a few geo. logically homogeneous regions of limited ex. tent has produced useful results, A "Tech. niques of Water-Resources Investigations" on low flow investigations now (1972) being prepared, will consider regionalization of low flows in more detail.