November i6, 1888.] 



SCIENCE. 



233 



into play. Using still the flood-terrain as above described, let it be 

 supposed that the river is turned against it in such a manner that 

 the whole mechanical energy of the stream is directed against it, 

 and suppose further that as fast as the banks are torn down by the 

 impact of the water and the sapping of the banks the material is 

 promptly carried away through the agency of great declivity, then 

 the whole terrain would be carried away in less than ten years. 



Next, let the rate of transportation under maximum conditions 

 be illustrated, and still let the described flood-terrain be used for 

 this purpose. Suppose that the terrain could be loaded upon the 

 Mississippi in such a manner that the waters are constantly sup- 

 plied to their utmost capacity. Now it has been observed in Utah, 

 and again in Colorado in the case of certain bad-land streams, 

 that under most favorable conditions water is capable of transport- 

 ing its own volume of load. The Mississippi River annually dis- 

 charges into the Gulf an average of one million cubic feet per sec- 

 ond. If this volume of water were loaded to its utmost capacity, 

 as described above, the flood-terrain of the Mississippi would be 

 ■discharged into the Gulf in one year. 



The rate of corrasion is subject to many interdependent variable 

 conditions, Only the laws of the first order have been presented. 

 There is still a great number of conditions of a second order to be 

 •considered ; but they do not in any material way vitiate the laws 

 already stated. The facts and principles that have been presented 

 are those which the engineer must use in planning and construct- 

 ing irrigation works. They are also of importance in dealing with 

 the regimen of rivers for the purpose of improving navigation, and 

 ■for the still more important purpose of protecting flood-plains from 

 overflow. It is proposed here to call attention to some of the en- 

 gineering methods which have been used to control rivers for .the 

 protection of flood-plains. Those selected for mention in this 

 manner are as follows. 



The banks of the stream may be protected from lateral corrasion 

 by revetment, but such protection will be sufiicient only to the ex- 

 tent to which it is applied ; for thorough protection both banks 

 must be revetted throughout the whole length of the fiood-plain 

 reach. And further, the revetment must be carried below the level 

 of possible vertical corrasion or the revetment will be undermined. 

 By this method the channel is protected from the choking which 

 arises from the deposition of materials brought in from upper 

 reaches, lateral tributaries, and local erosion. In a bank-protected 

 channel along a flood-plain reach there is a constant tendency to 

 distribute the obstructing deposits evenly along the bottom, as the 

 lower declivities are sites of deposits and the higher declivities 

 present conditions of increased vertical corrasion. In a river with 

 uniform channel and uniform volume of water the deposition is 

 uniformly diminished from head to foot, and such a stream builds 

 up its channel until a degree of declivity is reached sufficient to 

 •carry away all the supply of load. If the declivity is more than 

 sufficient to carry away the load supply, vertical corrasion is inau- 

 gurated and the channel is deepened. If the declivity is insuffi- 

 cient to carry away the supply of load, the deposit of sediment will 

 build up the channel, and destructive floods will be increased 

 thereby. Revetment, therefore, is efficient only on the condition 

 that the declivity is exactly sufficient to carry away the load and to 

 produce no further corrasion ; for if vertical corrasion be in- 

 creased, the revetment will be undermined and destroyed, and if 

 vertical corrasion is insufficient, deposits will be made and floods 

 will result. The practical problem, therefore, is to decide whether 

 the declivity is or is not sufficient to preserve the channel. This 

 problem is always solved by nature, and its solution is made per- 

 fectly plain. If the declivity of the flood-plain reach is sufiicient to 

 preserve the channel, the channel will be preserved, and there will 

 be no lateral corrasion. Every flood-terrain is such because the 

 ■channel of the stream has an insufficient declivity for its own pro- 

 tection. The very fact that corrasion is wholly lateral is in itself 

 an absolute demonstration that the declivity of the stream is insuf- 

 ficient for the protection of the channel. This arises from the fact 

 that the load once deposited remains, as the channel does not pre- 

 sent conditions for its reloading : revetment, therefore, is necessa- 

 rily futile, except for local and temporary purposes. 



If portions of the banks of a channel are revetted, the only result 

 •arising therefrom is to change the locus of lateral corrasion ; for, 



the total deposits remaining the same, the total lateral corrasion will 

 remain the same. If the whole channel is revetted, the whole chan- 

 nel will be built up thereby, and ever a greater volume of water 

 will be distributed over the flood-plain, until the channel is entirely 

 filled at its head, or built up to such a declivity that vertical corra- 

 sion will be sufficient to preserve the channel. 



There are four other methods that have been presented by engi- 

 neers and geologists still worthy of consideration, as they are more 

 or less efficient, either separately or conjointly. These are as fol- 

 lows. 



1. The channel of the stream may have its banks and bars re- 

 moved, and it may be deepened by river ploughs. To be efficient, 

 the clearing of the channel of its deposited obstructions must be 

 complete. The effect of clearing a lower reach is not extended to 

 an upper reach, but the effect of clearing an upper reach is to in- 

 crease the obstructions of the lower. For this reason the channel 

 must be cleared its entire length throughout the region to be pro- 

 tected from floods at one effort. 



2. The channel of the river may be shortened. By this method 

 the declivity of the stream will be increased, the velocity of the cur- 

 rent increased, and the waters more rapidly discharged. At the 

 same time the channel of the stream will be deepened progres- 

 sively from the foot to the head of the reach, where the stream 

 runs through alluvial formations ; but wherever the stream has its 

 bed in indurated rocks the progress of stream deepening will be 

 retarded. 



The shortening of the channel may be accomplished by two 

 methods. 



a. By establishing a nearer outlet. 



6. By utilizing and promoting cut-off reaches. 



3. The headwaters and tributaries may be impounded in reser- 

 voirs at flood time and held until low water, and the volume 

 through the year may thus be more or less equalized. 



4. The headwaters and tributaries of a river may have their 

 waters drawn off into settling basins, and thus they may be caused 

 to discharge the sediment they carry, which is the material which 

 forms the deposits and chokes the channel, and also the instrument 

 of lateral corrasion. 



It is manifest that, the storage of water and the discharge of 

 sediment may be accomplished by the same agency. 



It is the purpose here merely to mention the principal efficient 

 methods of controlling rivers in their flood-plain reaches. Every 

 river presents problems more or less peculiar to itself, and the ap- 

 plication to special cases of the laws which have been set forth is 

 one of great interest and of profound importance. 



J. W. Powell. 



COMMERCIAL GEOGRAPHY. 

 The Care of our Forests. 



IN the annual report of the Department of Agriculture, B. E. 

 Fernow, chief of the forestry division, dwells most emphatically upon 

 the necessity of adopting a sound policy regarding our forests. 

 His interesting report is accompanied by a map showing the distri- 

 bution of forests in the Rocky Mountains, where they sen-e the im- 

 portant purpose of regulating the flow of springs and streams. Mr. 

 Fernow's weighty arguments and urgent demands for better care of 

 our forests ought to attract the most speedy attention of our legis- 

 lators. He says, — 



" It has become evident, in spite of the enormous supplies which 

 seemed to be available, that our natural forests are being rapidly 

 reduced, both by an increased demand and by wasteful practices ; 

 and it is now safe to say that the annual consumption of wood and 

 wood-products is at least double the amount reproduced on our 

 present forest area. The forest, under proper management, is ca- 

 pable of furnishing continuous crops, and therefore, as a source of 

 constant supply, demands national legislation. 



" It has become evident, that with the unrestrained scourge of fire 

 and the destruction by herding, and other malpractices now prev- 

 alent, and in the absence of all rational forest management, not 

 only is the remaining forest deteriorated in material value, but large 

 tracts of land are converted into absolute deserts or useless bar- 



