644 DYNAMICAL GEOLOGY. 



such rivers as the Sacramento. In the case of these broad plains, the valley is seldom 

 one of erosion simply, but generally a geosynclinal trough, or an interval between sep- 

 arate mountain ranges. When a stream crosses a series of synclinal valleys, the flood- 

 plain generally expands as it enters each, and contracts at the passage from one to the 

 other. 



The surface of a flood-plain is only approximately flat. (1) The margin along a 

 stream is often higher than the part back of it; (2) some portions are frequently within 

 the reach of only the very highest freshets; (3) others are quite low, and are sometimes 

 occupied by ponds of water or lagoons, fed from the river by percolation through the 

 soil. The variation of height from these sources is often equal to two thirds of the 

 whole average height of the flood-plain above the river. The surface is sometimes 

 changed much in height during freshets, by the wearing away of one part and the in- 

 crease of others. 



The height and pitch of the flood-plain are essentially that of the stream at flood- 

 height, and will, therefore, be affected by the causes mentioned on page 637. It will 

 be comparatively low, toward the ocean. It will be diminished by any abrupt expan- 

 sion of the river-valley, by which the waters spread laterally to great distances, and 

 consequently have diminished vertical height. Conversely, the height will be increased 

 by a narrowing of the valley, and especially before the entrance of a contracted gorge. 



While, therefore, there is a general parallelism between a stream at low water and 

 its flood-plain, there are wide variations from this parallelism. 



The occurrence of waterfalls in the course of a stream causes the flood-plain above 

 to stand at a higher level than that below, equal at least to the height of the fall, and 

 somewhat above this height if the fall occurs in a gorge, which would set the waters 

 back during a flood. 



If the erosion of some thousands of years or less, deepen the bed of a stream fifty 

 feet, the flood-plain would sink correspondingly to a lower level; and thus, in the lapse 

 of time, without other geographical change than the one mentioned, a terrace would be 

 formed, some portion of the old plain being left, as would naturally happen, at its 

 former height. If a waterfall were gradually obliterated, the flood-plain would undergo 

 a corresponding change. If the barrier that caused the existence of a lake along a 

 river were removed, there would be a sinking of the river's channel, and a sinking by 

 erosion also of the flood-plain. If, from any cause — as a mountain-slide — a barrier 

 were thrown across a stream, and a lake made, the flood-waters -would stand at a cor- 

 responding!)- higher level than before, and would spread more widely, making new 

 flood-plains, above the former level. If the progressing erosion be very much less on 

 one part of a stream than on another (from the nature of the country, or that of the 

 rocks, etc.), the changes in the level of the later flood-plain would have the same 

 differences. Small streams, working the same length of time, would, of course, sink 

 their channels by erosion less than the large ones to which they are tributary, provided 

 the pitch be the same and the bed similar in material; and even a large pitch will not 

 often compensate for a very great difference in the amount of water. 



These are changes in the flood-plain which may take place from the ordinary inci- 

 dents to which rivers are exposed. 



Finally, if a continent undergo an elevation which is greatest about the headwaters 

 of the stream, or if an equable elevation and the pitch of the bottom off its mouth is 

 large, the pitch of the river is increased, and new erosive power is given it; and, with 

 the progress of the elevation, new flood-plains would form, at lower and lower levels. 

 This subject is already explained on page 558. The only case in which the river would 

 not have a greater pitch after such an elevation, is when the coast-region, added by the 

 elevation, slopes seaward at the same angle with that of the stream before the elevation, 

 or at a less angle than this. 



Topographical Effects of Erosion. — The topographical effects of 

 erosion depend on several conditions, — as (1) the durability of the 

 rocks, (2) their structure, and (3) their stratification. 



