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<br />001449 <br /> <br />Even ~f station location has remained unchanged, a long-term <br />record may involve progressive bias owing to continual changes in <br />the environment. For example, over the years more and more of the <br />waters in a given river basin may have been diverted for use, and <br />streamflowa may have been regulated more and more by construction <br />of successive reservoirs. Even if available, a record of such <br />diversions commonly does not show the effect on streamflow; a <br />substantial fraction of the water diverted may return to the stream <br />far below the point of diversion. Similarly, owing to evaporation <br />and other losses that commonly are not measured, records of reservoir <br />inflow, cohtent, and outflow yield a distorted measure of the <br />regulating effect. <br /> <br />Some effects of major diversions and regulation may be documented <br />in supplemental water records, so that gaging-station records can be <br />adjusted accordingly. The "remarks" section of the record in Water- <br />Supply Papers should be scanned for relevant clues; some but not all <br />environmental changes will be identified there. Both increasing <br />water-surface area in reservoirs and increasing use of water deplete <br />streamflow; usually, such depletions are not documented. Also not <br />usually documented, numerous small diversions may have been made <br />successively, small but significant quantities of water may by-pass <br />gaging stations, and numerous other works of man may have added some <br />effect. Changes in pattern of land use commonly have been large over <br />a long terin of years and are claimed widely to have caused substantial <br />changes in, the yield of water to streams. That such change has occurred <br />is likely; the magnitude of change, apart from variability owing to <br />natural causes, commonly is difficult to demonstrate. <br /> <br />In this connection, emphasis perhaps is warranted in regard to <br />the effects of dikes, levees, and other river-training works. Commonly <br />these work$ are constructed progressively in a particular stream basin. <br />As a result, stage at some key station may seem to rise progressively <br />over many years, not because the basin yields more water but because <br />training works upstream constrain a greater and greater portion of the <br />flow to the main channel, or training works downstream cause higher and <br />higher backwater, or both. Effects may reach many miles from the works <br />that cause them; they may be large and relatively obvious, or small and <br />obscure. Commonly neither their magnitude nor their timing can be <br />discriminated precisely. Failure of a levee or other training work <br />reverses the effect. <br /> <br />Such change m~ occur not only in agricultural and rural areas but <br />also in urban and industrial areas. For example, in the urban-industrial <br />environment the rate and volume of storm runoff, the volume of fluid <br />wastes, and stream temperatures are likely to increase--perhaps <br />substantially and at times sharply, but in most instances gradually and <br />obscurely. Ground-water levels and potential yields are likely to be <br />affected, and commonly depressed. <br /> <br />4 <br /> <br /> <br />l <br />