LEVEES. 99 



located wider apart in the neighborhood of tributaries of great flow in order to provide 

 a greater area for the increased discharge, thus forming reservoirs in which is gathered 

 not only the surplus water, but tremendous quantities of sediment as well. In regard 

 to the result of this method De Mas says:* "The controlling levees have been spaced 

 so that in the localities where affluents abound the major bed forms a sort of reservoir, 

 in which are stored not only the floods, but also the deposits carried by turbid waters. 

 It follows from this arrangement that the major beds of the Po and of its affluents in 

 their lower parts serve as regulators to each flood descending from the mountains; 

 and after having distributed it they lead it to the sea through a contracted outlet which 

 modifies the velocity and the disastrous effects caused by the raising of the flood-level 

 above Panaro. This distribution was first brought to attention when Lombardini 

 stated two remarkable facts which are shown also in the report of the engineer Baum- 

 garten. The first is that the discharge of a flood is very nearly the same at Tessin, 

 at Cremona, and in the vicinity of Ferrara. The second is that while all its affluents 

 together discharge 528,000 cubic feet per second, the discharge of the Po is about 

 176,000 feet for the same unit of time. The greatest part of this remarkable result 

 should be attributed to natural circumstances. It is certain that the affluents from 

 the Apennines discharge before those from the Alps; and there is no doubt that the 

 lakes Maggiore, Como, Garda, and others retard the most powerful torrents. Thus 

 it was found that at Lake Como the maximum discharge in the flood of September, 

 1829, was at the entrance 68,501 cubic feet per second, and only 28,389 cubic feet at 

 the exit. It will be seen, therefore, that the flood was diminished in passing through 

 the lake in the ratio of 2.4 to i. But it is equally certain that in the establishment 

 of levees whose effects are to produce certain contrary results natural laws must not 

 be neglected, and a knowledge of these must govern us accordingly. Thus, according 

 to the same experienced engineer, the volume stored between the levees of the Po and 

 its affluents, from Casale to the sea, is 66,739,000,000 cubic feet, which corresponds to 

 more than four days' discharge of the river at the rate of 181,280 cubic feet per second. 

 In reality the hand of man has created a vast regulating reservoir which affects the 

 regime of the river in the same way that the lakes cited above affect its affluents." 



The Theiss.t The Theiss flows through the plain of Hungary, which in general 

 consists of a layer of vegetable earth overlying a thick stratum of compact black clay. 

 The former constitutes the banks and the latter the bed of the stream. Like most 

 rivers the slope is great in the mountainous portion, diminishing continuously in the 

 plain. The heights of the floods increase as the slope decreases. They come in the 

 spring, and are caused by the melting of snow in the Carpathian Mountains. They 

 are slow, often taking several weeks in which to reach their full height, and their dura- 

 tion at the highest level is generally several days. The maximum discharge at Szegedin 

 is given as 123,200 cubic feet per second. The river always maintains a splendid 

 navigable depth. 



* Rivifcres 4 Courant Libre. t Annales des Fonts et Chauss^es, 1890. 



