198 GEOLOGY. 



If the main stream again swings over to the point where the tribu^ 

 tary issues from its valley, the tributary stream and all its affected 

 tributaries again become eroding streams. Thus scour-and-fill are 

 not confined to the valley of the main stream. 



River-lakes. — While rivers are in general hostile to lakes, they 

 sometimes give origin to them. Oxbow lakes (Fig. 182 and PL XVI), 

 due to the cut-offs of meandering streams, have already been referred 

 to. Lakes formed in the same way have other forms (PL XVI and Fig. 

 183). Rivers also give rise to lakes through the deposits they make. 

 If a main stream obstructs its tributaries by deposition at their debou- 

 chures, their lower courses are ponded and converted into lakes. The 

 lakes along the tributaries to the Red River of Louisiana have already 

 been cited as examples. If a tributary brings more load to its main 

 than the latter can carry away, the detritus constitutes a partial dam, 

 ponding the river and causing it to expand into a lake above. Such is 



Fig. 188. — Longitudinal section of an incipient delta made of coarse material. 



the origin of Lake Pepin already referred to. In mountain regions, 

 the alluvial cones of tributary valleys sometimes pond their mains. 



Rivers may be dammed in other ways, as by lava flows, by land- 

 slides, by glacial drift, etc. In all such cases, lakes may come into exist- 

 ence, but they are not due primarily to the activity of the river itself. 



Deltas.^ — Where a stream enters standing water, or a slower stream, 

 a special form of plain, the delta, is sometimes built up (Figs. 169, 187, 

 and 188) . Deltas and alluvial fans have much in common, and their only 

 notable differences are those imposed by the differences in the condi- 

 tions of deposition. The current of the stream is checked, but not 

 altogether stopped, at its immediate debouchure. If it carries abun- 

 dant sediment, much of it will be promptly dropped where the decrease 

 in velocity is first felt. Such flow as there is beyond the debouchure 



1 For an excellent discussion of deltas, see Gilbert, Fifth Ann. Rept. U. S. Geol. 

 Surv., pp. 104-8. Also Lake Bonneville, Monograph I, U. S. Geol. Surv. (same article). 



