56 Kansas Academy of Science. 



this way a stream from a high mountainous area crossing a wide 

 plain may have migrated over hundreds of miles laterally. Excel- 

 lent examples of this may be observed at the present time in 

 streams reaching the Pacific coast from the Coast Range mountains. 

 It is universally true that the streams in the steeper parts are still 

 cutting their channels deeper, while on the plains between the 

 mountains and the ocean they are building up their channels with 

 debris brought from above, and are shifting from place to place as 

 above mentioned. The question, therefore, of whether or not a 

 stream will cut its channel deeper or will fill up the channel and 

 elevate itself to positions actually farther from the center of the 

 earth than those previously occupied is dependent entirely upon 

 the somewhat intricate and delicate balancing of velocities and 

 loads. 



FLOOD-PLAINS. 



The building-up process at the bottom of the river channel and 

 the widening of the bluff lines soon produce an almost level valley 

 area known as the flood-plain. In times of high water a river covers 

 the entire flood-plain and a variable amount of sand and silt and 

 mud are deposited, thereby building up the general level of the 

 valley. Some streams have built up their flood-plains for more 

 than 100 feet, as shown by the depth" of the flood-plain material. 

 In general, the depth is greater near the mouth of a stream, but 

 irregularities of surface produce great irregularities in the thick- 

 ness of the flood-plain material throughout a flood-plain area. The 

 materials usually are laid down iii broad, horizontal layers well 

 stratified, but sometimes local irregularities occur where washouts, 

 sink-holes, etc., are made in the flood-plain material during times 

 of high water. 



It should be noted that the very presence of a river flood-plain 

 is evidence that the river has covered the entire flood-plain area. 

 Earth movements often change such conditions, allowing a stream 

 to begin cutting its channel anew after it has built up a former 

 flood-plain. Such cases are rare in the Mississippi valley, however, 

 and in eastern Kansas none have been found. The highest ground 

 existing in any of the eastern Kansas river valley flood-plains was 

 created by flood waters from the stream and remain to-day a silent 

 witness to the fact of great floods having overflowed the entire flood- 

 plain area, no matter what the stream may be. 



We may say, therefore, in discussing any particular part of a 

 stream, that in its early stages, in its youthfulness, it is deepening 

 its channel, but when the period of maturity has been reached it 



