CHAPTER IV. 

 FIXED DAMS. 



General. While the construction of fixed dams has been practically abandoned 

 abroad for navigation purposes, it still continues in America, and it is in fact only 

 within recent years that movable dams have been applied here, and this application 

 has been limited to two or three streams. 



Fixed dams are frequently justified by existing conditions, but they are an obstruction 

 to navigation at periods when there would be sufficient water in the natural state of 

 the stream. They have been built, and are still being built, where the conditions are 

 also favorable for the construction and operation of movable dams, because they can 

 be constructed of cheap materials, usually abundant in the locality, and because they 

 require but little attention for some years after completion, except occasional repairs. 



In streams having small commerce the use of fixed dams is not usually objection- 

 able, while for purposes of storing water and furnishing power they are well adapted. 

 This fact led largely to their use in this country, because nearly all the earlier river 

 improvements were made by corporations or by State governments, and one of the 

 chief objects was to secure power for industries as well as water upon which to trans- 

 port their products. With rare exceptions these streams have come under the control 

 of the United States, and the use of power from their pools has been greatly curtailed 

 or entirely abandoned. The improvements have been frequently extended farther 

 up the rivers for the sole purpose of navigation, and it has been done in all cases by 

 a continuance of the fixed systems already in use; generally the locks have b 

 enlarged and improved, but the dams, whatever their construction, have remained of 

 the fixed type. It has been found necessary to rebuild many of the older ones as 

 they were made of wooden cribs filled with stone, a class of construction which the 

 attacks of floods and drift and the alternate exposure to air and water injure sooner 

 or later, ultimately necessitating extensive repairs. Fixed dams also cause floods to 

 reach a greater height than would otherwise be the case. 



Alignment and Length. Fixed dams are generally placed near the head of the 

 lock, this location being adopted in order to avoid strong currents in the lower approach. 

 They are usually built straight and at right angles to the axis of the river, but other 

 forms may be seen in old structures, especially in Europe, sometimes with a curve of 

 large radius, sometimes with straight or broken arms inclined more or less to the axis 



of the river, in order to obtain a longer spillway. It is important that a dam of the 



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