Report of Sub‐Committee on Land Forms of the Committee on Dynamics of Streams, 1941–42

Abstract

The Sub‐Committee on Land Forms of the Committee on Dynamics of Streams of the American Geophysical Union has given special study to the causes and controlling factors of stream‐meanders. Members of the Committee and their colleagues have supervised studies on this and related problems. Model studies of the meanders were made at the United States Waterways Experiment Station at Vicksburg, Mississippi, and GERARD H. MATTHES, a member of the Committee, reports that:“Stream‐meanders were made the subject of extended experimental study at the United States Waterways Experiment Station, Vicksburg, Mississippi, during the past year. Several different materials were tried out, but the principal experiments were conducted in a bed of pulverized coal 125 feet long by 40 feet wide. The objective has been to ascertain the relative importance of the various basic factors involved. By maintaining the coal‐bed at a uniform gradient throughout and permitting the model‐stream to select its own hydraulic gradients two of the five basic variables were maintained practically constant, namely, the valley‐slope and the resistance of the materials composing bed and banks. [In this connection see GERARD H. MATTHES, Basic aspects of stream‐meanders, Trans. Amer. Geophys. Union, Pt. III, pp. 632–636, 1941.] Variations in water‐discharge and bed‐load discharge were the principal variables experimented with. Toward the close of the season a new factor was introduced which hitherto has not received the recognition that appears due it, namely, vegetation. After unsuccessful attempts to simulate vegetation through artificial expedients, it was found possible to cause actual plants (Panicum clandestinum) to grow in the coal‐bed, and provide a fair simulation of willows in the prototype. The results obtained indicate that dense vegetation, such as willows and cottonwoods, along the banks of a stream and particularly on convex bars, exerts a marked influence on bed‐load movement during high stages by promoting the upward as well as riverward growth of convex bars, thereby increasing the caving of the concave banks opposite. At the time the tests were discontinued for the duration of the cold season, it had become apparent that in the case of alluvial streams carrying appreciable sediment‐loads the vegetation as a factor affecting stream‐bed morphology, is of much greater importance than is commonly attributed to it.