Grant, G. E.; Swanson, F. J. 1995. Morphology and processes of valley floors in mountain streams, western Cascades, Oregon. In: Costa, John E.; Miller, Andrew J.; Potter, Kenneth W.; Wilcock, Peter, eds. Natural and anthropogenic influences in fluvial geomorphology: the Wolman volume. Geophysical Monograph 89. Washington, DC: American Geophysical Union: 83-101.
Development of valley floor landforms and channel morphology in headwater streams is strongly influenced by processes external to the channel. The distribution of landforms on valley floors of two fifth-order mountain streams in western Oregon exhibit distinctive reach-to-reach variations, as defined by a valley floor width index: the ratio of approximate width of Holocene valley floor (surfaces less than or equal to 3m in height) to active channel width. This variation corresponds to the distribution of bedrock outcrops and hillslope landforms, including large landslides and alluvial fans, which constrain the channel. Lowest valley floor width index values (less than or equal to 1.3) occur where the channel is incised in bedrock or where landslides or alluvial fans encroach upon the valley floor, commonly forcing the channel against bedrock in the opposite valley wall. Highest valley floor width index values (greater than or equal to 4.0) occur upstream of these constrained reaches where valley floors aggrade and little bedrock is exposed. Widths of valley floor landforms of different origins do not increase uniformly with increasing valley floor width; active channel widths remain relatively constant while reaches with greater valley floor width generally exhibit greater widths of floodplains created by fluvial processes and/or debris flows. Hillslope and tributary processes also influence the channel gradient and determine the size and density of boulders in the channel. Highest boulder densities and steepest channels occur where channels are bordered by alluvial fans, terraces containing debris-flow deposits, and active or inactive large landslides. Reaches differ in their susceptibilities to reworking by fluvial and non-fluvial processes. The distribution of landforms of fluvial origin varies in relation to reach-averaged shear stresses generated during large floods. The pattern of landforms of debris-flow origin is determined by the valley floor geometry, location of debris-flow producing tributaries, and the longitudinal sequence of reaches. changes in valley floor morphology in mountain streams occur rapidly and episodically during infrequent, intense floods, in contrast to the more gradual, fluvial reworking of valley floor sediment in low-gradient alluvial streams.