NOTE ON CONDITIONS AT THE HEAD OF FLOOD 



PLAINS 



JOHN L. TILTON 



Flood plains are said to exist along the courses of rivers where 

 in time of flood deposits are laid down on the flooded areas. 

 Following such a flood plain up-stream the plain is said to become 

 narrower till reduced to zero. In working along tributary ravines 

 in Warren and Clarke* counties, Iowa, a different relation is noted 

 that it appears to the writer should be emphasized. Here in the 

 course of four to six miles gradations may be seen from the flat 

 surface of the upland to the alluvial flood plain by the rivers. Start- 

 ing with the upland, the rain water flowing along gentle depressions 

 previously made begins to erode in spots, first by undercutting the 

 sod here and there, the process of undercutting working headward 

 uniting several of the eroded patches till a small gully results. 

 Somewhat further downstream the volume of storm water is suf- 

 ficient to overflow the small trench and take short cuts from one 

 bend to the next, eroding a shallow channel by the side of and above 

 the small trench. A little mud is deposited, but the dominant effect 

 is here erosional. Following down this portion of a flood plain 

 where erosion is in excess of deposition the flood plain gradually 

 becomes wider, and gradually changes into a flood plain of the 

 recognized type, where deposition is in excess of erosion, and where 

 meanders begin to be evident. This gradual change is a common 

 feature along the chief ravines in southcentral Iowa. Degradation 

 is in progress near the heads of the ravines and aggradation is 

 marked along the lower courses of the same ravines. In texts 

 generally head water erosion and erosion by mountain and other 

 streams is described, and the building up of flood plains in the lower 

 courses of rivers, but I do not find the gradation from one extreme 

 to the other treated. Along large rivers this change may not be 

 noticeable for many miles ; but here in southcentral Iowa it may be 

 seen in the course of a few hours' walk. 



Department of Geology, 

 Simpson College. 



•See Geology of Clarke county. Iowa, chapter on Physiography. Iowa 

 Geological Survey, vol. XXVII. 



