Rot, Byron. 1995. The interaction of valley constraint, riparian landform, and riparian plant community size and age upon channel configurations of small streams of the western Cascade Mountains, Washington. M.S.
Biophysical factors influencing channel configuration were measured for 21 sites in mature to old forests of the western Cascades Mountains, Washington. The overall goal was to understand the patterns between channel configuration, valley constraint, riparian landform, channel type, and the riparian plant community size and age. Valley constraint was described in terms of the ratio of valley width to channel width. Riparian landform was delineated into four classes, three fluvially derived and organized by elevation above the channel, with the fourth class, slope, originating from non-fluvial processes. The three fluvial landforms were: floodplain (< 1 m), low terrace (1-3 m), high terrace (> 3 m); with the fourth class, slope (> 20% gradient and >15 m in width). The riparian plant community was characterized by the overstory species composition, density, and stand age, and understory species composition and percent cover. Stream channels were classified using a conceptual model based upon their expected response to changes in sediment supply and discharge.
Both successional processes and fluvial disturbances control riparian plant community composition based upon landform class. Floodplain landforms are dominated by deciduous species, especially red alder (62% of stems), while conifers dominated the other landforms. Conifers are found on low terrace landforms, specifically Pacific silver fir (Abies amabilis) and western hemlock (Tsuga heterophylla; 77% of stems). High terrace landforms support Douglas-fir (Pseudotsuga menziesii) and western hemlock (72% of stems), while slope landforms supported mostly western hemlock and some Douglas-fir (73% of stems). In addition, the relative importance of Douglas fir decreased with stand age for the three coniferous landforms indicating the effects of successional processes.
Valley constraint significantly influenced LWD volume within forced pool-riffle channels with the volume increasing as a power function of decreasing valley constraint (r2 =0.58). No relationship was found with the other channel types. The presence of off-channel habitat for aquatic organisms increased exponentially with decreasing valley constraint for all channel types (r2 =0.71).
Within the stream channel, the diameter of LWD was related to the age of the riparian forest. In old growth stands (>300 yrs), LWD diameter was greater than average riparian forest diameter for all sites. A mixed relationship between LWD and riparian forest diameter in younger stands reflected a mixture of LWD from previous stands, smaller suppressed stems from existing stands, and a wide range of diameters contributed by bank erosion. Finally, LWD diameter increased overall with stand age (r2=0.34).
This study was important for several reasons. First, it correlated fish habitat (channel configuration) to biophysical factors across multiple temporal and spatial scales. Second, the importance of successional processes within the riparian forest to channel configuration was also discussed. Finally, the composition of the riparian plant community was related to riparian landform. As a whole, the results suggest that the creation and maintenance of fish habitat results from the complex interaction of a variety of biophysical factors.