60 THE IMPROVEMENT OF RIVERS. 



able favor in this country. Their object is the same as that of spurs, but they accomplish 

 it in a- way which, while probably more certain, is sometimes more expensive. Those 

 along the Ohio River are frequently built of wooden cribs filled with broken stone. 

 Starting from the bank they describe a regular curve down stream until their direction 

 becomes parallel to that of the proposed channel, when they follow the line of the 

 latter. The space behind some of them gradually fills with sediment and even grows 

 up with willows, and a new bank is formed, while behind others considerable scour 

 takes place. They are effective in scouring out the bars and affording a greater depth 

 at low-water seasons. At ordinary stages their crests are submerged so that boats 

 may pass over them in safety. Their position is such that the dikes themselves form the 

 banks of a new, contracted channel with a more regular current than found when spurs 

 are employed; but, on the other hand, longitudinal dikes for their whole length are 

 subjected to the continual action of a current swifter than that which previously 

 existed, and under these circumstances the dikes may be undermined, and even 

 destroyed. The space included between a longitudinal dike and the bank which it is 

 designed to replace is not immediately filled up by alluvial deposits, so that when 

 a rise comes the water overflows the dike, and, pouring into the space behind it with 

 a velocity due to the height of fall, tends to undermine the dike on the inner side. 

 Their maintenance requires constant watching, and the expense of repairs is consider- 

 able. Negligence in keeping them up may be the cause of disturbances over the 

 whole area included between the two dikes, and this would happen more rapidly than 

 with spur-dikes. Lastly, the use of longitudinal dikes does not allow, except at 

 considerable expense, subsequent contraction or enlargement, if either should become 

 necessary. 



Submerged Spurs. We have seen that one of the effects of contracting a river is 

 the scouring out of its bed. This is, of course, one of the objects aimed at, but this 

 action will not always cease at the point desired, and hence it sometimes becomes a 

 menace rather than an aid to navigation. To correct this evil it was proposed many 

 years ago to adopt in connection with the dikes a system of low dams or sills placed 

 at intervals and dividing the river into a number of sections. They would rise approxi- 

 mately to the river-bed and in reality form immovable bars. This conception has 

 not, so far as we know, been exactly applied, but the same general idea has received a 

 considerable application on the Elbe, Rhine, Rhone, and other rivers. The works are 

 called submerged spurs, ground-sills, or "Grundschwellen." It will be noted that they 

 are not considered as a complete system of improvement, but are employed in con- 

 nection with regulating and contracting dikes. Their chief object is to raise the 

 bottom and induce deposits, the minor bed of the stream being at the same time held 

 to its place by supplementary works at such places as desired. 



Generally speaking the term ground -sill is applied to works constructed under 

 water, which stop at a depth somewhat greater than the normal depth of the 

 stream, and which are built in order to strengthen and consolidate the bottom of the 



