18 PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY. 
causes it to withdraw from former banks of erosion; the water there 
becomes shallow, and determines an area of deposition (away from 
the formerly eroded bank) building a sand bar which in time becomes 
an island. The deposition cannot be continuous with the deserted 
bank, as the thread of the current has been so suddenly withdrawn to 
the opposite side of the river. In consequence the island grows within 
the channel of the river near the bank from which the current has been 
withdrawn by its new deflection. Whenever out of adjustment, the 
river swings with unusual vigor, and yet is unable to re-establish itself 
until assisted at times of excessive flood, w r hen the river suddenly in¬ 
creases its erosion and so withdraws rapidly from the existing sand 
bar, depositing another sand bar (text-figure G). The partition type 
of plain would be deposited until the new meanders were well ad¬ 
justed to the new deflection. Considering the nature and the number 
of the causes which influence the course of streams, it is evident that 
a river will rarely have an opportunity to develop symmetrical, syste¬ 
matic meanders. But there will always be some portion of the river 
producing a new series of curves, and in consequence, within such por¬ 
tions of the river the flood plain will be forming by the partition process. 
Of these controls, change of volume is probably the most influential 
in bringing about deflection of the stream. Any change of volume 
alters the radius and arc of curvature of the meanders, thus disturbing 
the former adjustment and initiating the establishment of another 
series of curves. Furthermore, many chance obstructions such as 
ledges, fallen trees, ice blocks, etc., encountered by the sweeping flood, 
throw the current into widely divergent courses. New channels are 
carved, and the short-cut or cut-off may be the result. The floods 
which at least annually sweep through the river channel are forceful 
agents of erosion, tearing away the unconsolidated material until the 
passing of the highest water, when in consequence of the decreased 
velocity the deposition of the transported material instantly begins 
in the form of sand bars. After the attainment of symmetrical curves 
the deposited material is regularly added to the extending meander 
lobes. During the development of a new series of curves, the exces¬ 
sive floods cause an unusual amount of erosion and a correspondingly 
sudden withdrawal from the filling-in bank. Moreover, the current 
of every stream is at least yearly thrown more or less out of adjust¬ 
ment by change of volume. 
The short-cut and cut-off may be cited as causing the river suddenly 
