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., " .. -4- Even it etation looation haa remained unohanged, a long-term record may involve progressive bias owing to continual changes in the environ~ , ment. For example, over the years more and more of the waters in a given' river basin may have been diverted for use, and streamflows may have . been regulated more and more by construction of successive reservoirs. E\'en if available, a record of such diversions commonly does not show all the effect on streamflow} for eX1\lllPle, a substantial fraction of the water diverted may return to the stream far below the point of diversion. Similarly, owing to evaporation and ~ther losses that commorily are not measured, records of reservoir inflow, content, and outflow yield a distorted measure of the regulating effect. SOMe effects of major diversions and regulation may be documented in supplemental water records, so that gaging-station records can be adjusted accordingly. The "remarks"section of the record in Water- Supply Papers should be scanned for relevant clues} some but not all environmental changes will be identified there. Both increasing water- s'1Xface area in reservoirs and increasing use of water deplete streamflow; usually, such depletions are not documented. Also not usually documented, numerous SMall diversions may, have be.en made successiveJ,y, small but significant quantities of water may bY-pass gaging stations, and numerous other works of man may have added some effect. Changes in pattern of land use commonly have been large over a long term of years and are claimed widely to have caused substantial changes in the yield of water to streams. That such change has occurred is likely; the magnitude of change, apart from variability owing to natural causes, commonly is difficult to demonstrate. In this connection, emphasis perhaps is warranted in regard to the effects of dikes, levees, and other river-training works. Commonly these works are constructed progressively in a particular stream basin. Asa result, stage at some key station'may seem to rise progressively over many years, not because the basin yields more water but because training works upstream constrain a greater a~d greater portion of the flow to the main channel, or training works downstream cause higher and higher backwater, or both. Effects may reach many miles frOM the works that cause them} they may be large and relatively obvious, or small and obscure. Commonly neither their'magnitude nor'their timing can-be discriminated precisely. Failure of a levee or other training work reverses the, effect. Such change may occur not only in agricultural and rural areas but also in urban and industrial areas. For example, in the urban-industrial enviro!m1ent the rate and volume of storm runoff, the volume of fluid w!\stes, and stream temperatures are likely to iricrease--perhaps substlln- t:l,ally and at times sharply, but in most instances gradually and obscurely. Ground-water levels and potential yields are likely to be affected, and commonly depressed.