An Empirical Classification of Flood-Plain Streams

Abstract

F LOOD-PLAIN streams may be classified according to the geological work they have accomplished on the flood plain. The relationship between a stream's work and the observable floodplain features is, in a general way, quite clear as long as one remains in the realm of geomorphology and avoids hydrodynamics. Few surficial features are more complex than these elongate masses of alluvium, yet there are few whose immediate origin is more apparent. Sufficiently detailed information being available, a classification of flood-plain structure is thus a system for classifying stream activity. Such a system is, in effect, a grouping of flood-plain streams according to the work accomplished in times of high water. It is then that streams possess the most energy; it is then that the geological environment undergoes its greatest changes. During low water most streams do little more than modify in a trivial way the features formed during the last flood. To classify a stream according to its activity during times of slight volume is, therefore, to emphasize that which is relatively unimportant. A useful classification should facilitate the ready description of both the common and the unusual flood-plain conditions and, thus, of common and unusual stream work. An orderly, recurrent event, such as channel deepening, and an accidental one, such as a change of climate, have their effect, not merely in the channel, but also in the formation of the surrounding plain. In presenting a system based on attributes of the flood plain the writer does not deny the need for a study of flood-plain history in general. Although such studies underlie the commonly used system, its principal concepts-youth, maturity, and age-are perhaps, in their connotations, unduly remote from the facts of observation. For example, those sections of a stream that are old have not necessarily been mature, and mature sections are by no means always found down-valley from those in youth. Accordingly, in this paper streams are classified locally, on the basis of their flood-plain structure; and, except for a few minor instances, no attempt is made to show the sequence of form that a given flood plain will follow in the course of its history. The system is, in short, an empirical classification of flood-plain streams in general, though it points out clearly that, at a given place, the genetic connection between a stream and its flood plain is very close. Whereas the use of a genetic classification would involve the assumption that the object of study reached its present 593