362 



NATURE 



[August 20, 190^ 



for stirring up the materials of bars and shoals so 

 as to effect their removal by the current, the different 

 types of dredgers and their capabilities, and the clear- 

 ing away of trunks of trees, termed snags, and wrecks 

 from the navigable channel. The most interesting 

 work in this respect is the formation each year, during 

 the low stage of the Mississippi, of a channel for 

 navigation, about 250 feet wide and 9 feet deep, across 

 sandy shoals in certain places by suction dredgers, 

 the efficiency of which is increased by stirring up the 

 sand with water-jets ; and in 1899 five of these dredgers 

 cut about 62 miles of channel at the average rate of 

 105 lineal feet per hour. The second chapter of this 

 part lays down the general principles on which the 

 regulation of river, channels is based, with the object 

 of obtaining greater uniformity of depth ; whilst the 

 following chapter describes the construction of spur 

 and longitudinal dykes, which are sometimes sub- 

 merged, by which the regulation is effected, a system 

 which has been successfully applied to several of the 

 larger rivers of Europe, as well as in America. 



The protection of banks aims mainly at the preven- 

 tion of prejudicial changes in the course of a river by 

 the erosion of the concave banks in flood-time ; and it 

 is accomplished by pitching, rubble stone, fascines, 

 brush mattresses, or occasionally submerged spurs. 

 Levees, consisting of eartheri. embankments, formed 

 along the banks of a river to prevent the river from 

 inundating the riparian lands in flood-time, are rather 

 works for the protection of property than for river 

 improvement; but to effect their purpose they must 

 be watertight, continuous, and have their tops above 

 the highest floods, which necessarily have their water- 

 level raised by being confined within the banks. 

 Several rivers in Europfe have been controlled by em- 

 bankments, notably the Po, the Loire, and the Theiss ; 

 and levees have been extensively carried out on the 

 Mississippi below Cairo and some of its tributaries, 

 the total expenditure on these works in the United 

 States being estimated at about 10,000,000^, up to the 

 present time, for a length of 1436 miles ; whilst con- 

 siderable additions to the Mississippi levees are pro- 

 jected. These embankments, however, are liable to 

 be occasionally overtopped and breached by an ex- 

 ceptional flood; and in alluvial plains, as in the case 

 of the Mississippi, they are exposed to undermining 

 by changes . in the course of the river, in spite of 

 regulating works ; and the rush of vvater through the 

 gap formed in the bank produces considerable devasta- 

 tion over the adjacent low-lying lands. Rivers bring- 

 ing along large quantities of detritus in their torrential 

 flow down steep mountain slopes, and abruptly 

 emerging into flat plains, are liable to raise their beds 

 by the deposit of sediment, owing to loss of velocity, 

 when confined within embankments, a result which 

 occurs in the Yellow River of China and some Japanese 

 rivers; and under such conditions, when the embank- 

 ments are successively raised to compensate for the 

 rising of the river-bed, a terrible catastrophe is a mere 

 question of time, due to the precipitation of the raised 

 and imprisoned river through a weak place in the 

 embankments, with irresistible force and rapidity, into 

 the plains below. 



The chapter on " Storage Reservoirs " consists 



NO. 1764, VOL. 68] 



almost entirely of extracts from a report by Captain 

 Chittenden on " Reservoir Sites in Wyoming and 

 Colorado," a method of compilation employed in 

 several of the earlier chapters, though to a minor 

 extent, and also in the following chapter on river out- 

 lets, already referred to. Reservoirs would be valuable 

 in river valleys in serving, like lakes, for regulating 

 the flow of rivers by reducing the flood discharge and 

 augmenting the low-water flow. It is, however, only 

 under exceptional conditions that reservoirs can be 

 formed extensive enough, at a reasonable cost, to 

 increase materially the flow of a river at its low stage ; 

 but this has been accomplished by damming the out- 

 lets of some lakes near the sources of the Volga and 

 Msta in Russia, extending the navigable period of 

 those rivers by nearly three months; whilst a similar 

 improvement has been effected in the Upper Mississippi 

 by raising the water-level of several lakes near the 

 head-waters of the river, a system which might be 

 considerably extended in this case, owing to the 

 immense number of lakes existing near its sources. 

 The formation of reservoirs at intervals along a river 

 valley would greatly reduce the flood discharge by 

 impounding the flood-waters, but the conditions are 

 rarely favourable; and the cost of construction, and 

 the extent of land submerged, present insuperable 

 obstacles to the adoption of this system, merely for 

 the mitigation of floods, in the great majority of cases. 

 Several reservoirs, however, have been constructed in 

 Europe for storing up water for water-power for in- 

 dustrial purposes, as well as for the mitigation of 

 floods, with successful results, as, for instance, the 

 Furens and Ternay reservoirs in France, and the 

 Dahlhausen reservoir on the Wappen in Germany, the 

 provision for floods being effected by keeping the 

 reservoir drawn down to a definite extent below its 

 full water-level for iheir reception. 



The third part, relating to the canalisation of rivers, 

 occupies one hundred and forty-one pages, or rather 

 more than half the regular text of the book, and is 

 divided into ten chapters, the three first dealing with 

 locks and lock gates, the fourth with fixed dams on 

 rivers, and the remainder with the various types of 

 movable weirs, which constitute the more novel and 

 most interesting portion of the subject. Though the 

 first movable weir appears to have been the bear-trap 

 weir erected in 1818 across the Lehigh River in the 

 United States, consisting of two gates or shutters 

 turning on horizontal axes on the sill, and one resting 

 on the edge of the other, the principal types of movable 

 weirs were gradually introduced in France between 

 1834 and 1885; and most of these French forms have 

 been reproduced, on a larger scale, on some of the 

 rivers of the United States ; whilst the American bear- 

 trap weir was adopted, with improvements, at Laneu- 

 ville-au-Pont on the River Marne, in France, about the 

 middle of the nineteenth century. 



The object of these movable weirs is to leave the 

 channel of a river quite unimpeded in flood-time for 

 the passage of the flood discharge, and occasionally of 

 vessels when the lock is submerged, whilst retaining 

 the water-level of the river above it at a sufficient 

 height for navigation in dry weather; and the 

 three chief French types are the Needle Weir, the 



