642 DYNAMICAL GEOLOGY. 



633. It will be comparatively low towards the ocean. It will be diminished 

 by any abrupt expansion of the river-valley, by which the waters spread late- 

 rally 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 correspondingly higher level than before, and would spread more widely, 

 making new flood-plains above the former level. If the progressing erosion 

 be very much lees 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 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 ordi- 

 nary incidents to which rivers are exposed. 



Finally, if a continent undergo an elevation, the pitch of the river is in- 

 creased and new erosive power is given it ; and with the progress of the eleva- 

 tion new flood-plains would form at lower and lower levels. This subject is 

 already explained at length on page 555. 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. 



2. Transportation by rivers. 



The transporting power of running water is mentioned on page 635. 



The materials transported are (1) stones, pebbles, sand, and clay; 

 (2) logs and leaves from the forests, and sometimes trees that have 

 been torn up or dislodged by the current; (3) mollusks, worms, 

 insects, attached to the logs or leaves ; (4) occasionally larger ani- 



