The Torrential Invertebrate Fauna

Abstract

be seen that at a velocity of 50 cm/sec. the current will remove all particles less than 5 mm in diameter. At this and higher velolocities there will therefore be a stony bottom, and hence I shall designate 50 cm/sec. as their lower limit of swift-flowing water. It might perhaps seem to be rather a low limit. At a rough judgement, however, most people are inclined to considerably overestimate stream velocities. In a waterfall water moves at about 6 m/sec.; at this velocity, air-resistance and acceleration due to gravity are in equilibrium. 6 m/sec. is thus the- theoretical upper limit for flowing water, but in proper water-courses and even in rushing mountain streams velocities over 3 m/sec. rarely occur. By comparison it should be borne in mind that the mechanical effect of the current is not proportional to the velocity itself, but to the square of the velocity. Thus a. current of 3 mi/sec.