56 IOWA ACADEMY OF SCIENCES. 



paratively quiet water on the down stream side. Around 

 and into this from each side come eddies, bearing sediment 

 in suspension which lose their swiftness, drop their load 

 and thereby start a sand bar. This bar develops by addi- 

 tions mainly on the down stream end, but also on both 

 sides. 



The deflection of the current by the obstruction turns 

 the stronger current slightly against shore and tends to 

 start a meander, leaving the island on inner side of curve 

 and increasing deposit. 



The swifter eddy swinging from the side of the stronger 

 current, deposits in a curve from the lower end and shore- 

 ward on the opposite side. A tendency to stagnation, 

 therefore, results on that side and causes a deposit in a 

 curve from the upper end along the edge of the current. 

 The sand bar thus becomes crescent shaped and the first 

 stage is ended. 



In the second stage the silting goes on rapidly until the 

 horns of the crescent reach the shore and enclose a lagoon 

 of stagnant water. 



During the third and last stage the lagoon is filled up 

 as are other flood plain lakes, by direct deposit of sediment 

 from winds and high water, and by plant encroachment, 

 until all trace of the lagoon is lost and the island has been 

 added to the flood plain. 



The study of the single island, together with the obser- 

 vations on others in all stages of the process described 

 leads us to the conclusion that it is a type common to 

 valley and plain tracts of rivers, and that flood plains 

 receive definite accretions by means of sand bars. 



