gilbert: transportation of debris 157 



is like that of eolian dunes, the current eroding their upstream 

 faces and depositing the eroded material on the downstream 

 faces. With any progressive change of conditions tending to 

 increase the load, the dunes eventually disappear and the debris 

 surface becomes smooth. The smooth phase is in turn succeeded 

 by a second rhythmic phase, in which a system of hills travel 

 upstream. These are called antidunes, and their movement is 

 accomplished by erosion on the downstream face and deposition 

 on the upstream face. Both rhythms of debris movement are 

 initiated by rhythms of water movement. 



Application of formulas. While the principles discovered in 

 the laboratory are necessarily involved in the work of rivers, 

 the laboratory formulas are not immediately available for the 

 discussion of river problems. Being both empiric and complex, 

 they will not bear extensive extrapolation. Under some circum- 

 stances they may be used to compare the work of one stream 

 with that of another stream of the same type, but they do not 

 permit an estimate of a river's capacity to be based on the deter- 

 mined capacities of laboratory streams. The investigation 

 made an advance in the direction of its primary goal, but the 

 goal was not reached. 



Load versus energy. The energy of a stream is measured by 

 the product of its discharge (mass per unit time), its slope, and 

 the acceleration of gravity. In a stream without load the 

 energy is expended in flow resistances, which are greater as 

 velocity and \dscosity are greater. Load, including that carried 

 in suspension and that dragged along the bed, affects the energy 

 in three ways. (1) It adds its mass to the mass of the water 

 and increases the stock of energy pro rata. (2) Its transporta- 

 tion involves mechanical work, and that work is at the expense 

 of the stream's energy. (3) Its presence restricts the mobihty 

 of the water, in effect increasing its viscosity, and thus consumes 

 energy. For the finest elements of load the third factor is more 

 important than the second; for coarser elements the second is the 

 more important. For each element the second and third together 

 exceed the first, so that the net result is a tax on the stream's 

 energy. Each element of load, by drawing on the supply of 



