43f» J. T. Jutson — Sheet-flows or Sheet-floods. 



namely: the rill type, the smooth-bottomed valley type, 

 and the furrowed-floor type. They appear to be rather 

 agents of deposition than of erosion. The term "sheet- 

 flows" is nsed in preference to "sheet-floods," so as to 

 inclnde all kinds of wide, shallow, moving bodies of 

 water, no matter how gently they may flow. 



Description. 



Sheet-flows, as noted above, may be divided into three 

 classes, which vary according to the natnre of the gronnd 

 over which they flow. The divisions are perhaps some- 

 what arbitrary, bnt the types exist and the classification 

 is convenient for reference and description. 



1. Rill type. — This type consists of the broad sheets 

 of water which flow down the smooth surface of the long, 

 undissected gently-sloping sides of shallow, wide, open 

 slowly-corrading valleys. There is no limit as to the 

 width of water that may flow, except the length of a val- 

 ley side, and the area affected by the fall of rain, which 

 has caused the occurrence. The water is not actually 

 continuous across the line of flow, but the sheet is made 

 up of countless rills of varying width, and not separated 

 by more than a few feet. These rills may be merely films 

 of water not more than one inch in depth or they may 

 reach a depth of perhaps six inches, this variation being 

 due to the very slight inequalities of the ground. The 

 flow is so widespread and shallow that even with a mod- 

 erate slope, and on soil-covered areas, the water usually 

 has no power of cutting even the smallest trench or 

 furrow ; the surface of the ground remains quite smooth 

 and unbroken, although the spaces between the small 

 trees and shrubs are commonly bare of vegetation. 



The water will however, if possible, concentrate itself, 

 as is shown where ruts are worn in the surface by cart 

 wheels. It takes possession of these ruts and deepens 

 them, to form a narrow shallow channel, the width of 

 which seldom exceeds the width between the two wheel 

 ruts, and the depth of which is scarcely ever more than 

 2 feet, so little power of excavation does the water pos- 

 sess on these broad gentle slopes. In fact, in many 

 places the channel tends to become filled by drifting sand, 

 which the flow of water is unable to remove. The mile- 

 upon-mile of very gently-sloping soil-covered ground 



