Page:Popular Science Monthly Volume 70.djvu/105

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101
GLACIAL EROSION IN ALASKA

depth. For a stream valley to pass from youth to maturity, even under the most favorable conditions, requires a great lapse of time.

The form of river valley to be expected in such a mountainous country as the coast of British Columbia and Alaska, would therefore depend largely upon the length of time that the streams had been working to cut the valleys. Had the stream action been brief, we should expect to find profound gorges; had it been long, broader valleys and the more gentle slopes of maturity. If, as is the case in Alaska, the same valleys have some of the characteristics of youth and some of maturity, a special explanation must be sought.

A second characteristic which results from the normal development of stream valleys is the accordance in grade between main and tributary streams. No matter how fast the main stream may be lowering its valley, even though it be a Colorado River, the side streams, including even weak tributaries, lower their mouths at approximately the same rate that the main stream deepens its valley. This feature is so well established as a normal condition of valley development, that it may be stated as a law that, under normal conditions of stream development, tributary valleys enter main valleys approximately at grade. That this is not the case in many instances in Alaska will be shown below.

A third feature normally developed during the formation of stream valleys is that of a somewhat winding course with overlapping spurs, alternating first on one side then on the other. Because of this

 

PSM V70 D105 Grenville channel through a large tributary valley.jpg

Fig. 2. Large Tributary Valley entering Grenville Channel from the East, below the Sea Level. Note steepened lower slope on left side of tributary valley. Photograph by Lawrence Martin.