Channel Narrowing and Vegetation Development Following a Great Plains Flood

Abstract

Streams in the plains of eastern Colorado are prone to intense floods following summer thunderstorms. Here, and in other semiarid and arid regions, channel recovery after a flood may take several decades. As a result, flood history strongly influences spatial and temporal variability in bottomland vegetation. Interpretation of these patterns must be based on understanding the long—term response of bottomland morphology and vegetation to specific floods. A major flood in 1965 on Plum Creek, a perennial sandbed stream, removed most of the bottomland vegetation and transformed the single—thread stream into a wider, braided channel. Channel narrowing began in 1973 and continues today. In 1991, we determined occurrences of 150 vascular plant species in 341 plots (0.5 m2) along a 7—km reach of Plum Creek near Louviers, Colorado. We related patterns of vegetation to elevation, litter cover, vegetative cover, sediment particle size, shade, and year of formation of the underlying surface (based on age of the excavated root flare of the oldest woody plants). Geomorphic investigation determined that Plum Creek fluvial surfaces sort into five groups by year of formation: terraces of fine sand formed before 1965; terraces of coarse sand deposited by the 1965 flood; stable bars formed by channel narrowing during periods of relatively high bed level (1973—1986); stable bars similarly formed during a recent period of low bed level (1987—1990); and the present channel bed (1991). Canonical correspondence analysis indicates a strong influence of elevation and litter cover, and lesser effects of vegetative cover, shade, and sediment particle size. However, the sum of all canonical eigenvalues explained by these factors is less than that explained by an analysis including only the dummy variables that define the five geomorphically determined age groups. The effect of age group is significant even when all five other environmental variables are specified as covariables. Therefore, the process of postflood channel narrowing has a dominant influence on vegetation pattern. Channel narrowing at Plum Creek includes a successional process: annual and perennial plants become established on the channel bed, sediment accretes around the vegetation, and increasing litter cover, shade, and scarcity of water eliminate species that are not rhizomatous perennials. However, successional trajectories of individual surfaces are modified by flow—related fluctuations of the bed level; surfaces deposited by the 1965 flood have had distinct sediment and vegetation since their formation. Species richness is highest on surfaces dating to 1987—1990; the many species restricted to this transitory assemblage are perpetuated by flood—related fluctuations in channel width. Since the 1965 flood, seedling establishment of the dominant trees (genus Populus) has occurred only on low surfaces formed during channel narrowing. Thus, the flood has indirectly promoted Populus establishment over a 26—yr period.

Intercontinental Similarities in Riparian-Plant Diversity and Sensitivity to River Regulation

“…The reason is probably that coarse-grained soils are indicative of areas that are more disturbed by wave and flow action, opening sites for colonization and allowing competitively inferior species to survive. In contrast, Friedman et al (1996) found that in a semiarid stream, exotics dominated on fine-textured, moist and human-disturbed sediments, whereas native plants dominated on coarse-grained, xeric sediments deposited by a large flood.…”

Cottonwood understory zonation and its relation to floodplain stratigraphy

“…Riparian systems in the semiarid western United States show high spatial and temporal variation in soil nutrient dynamics (Boggs and Weaver 1994;Vought et al 1994), soil texture (Lyon and Gross 2005), hydrology (Toner and Keddy 1997), and vegetation patterns (Boggs and Weaver 1994;Lyon and Gross 2005;Wintle and Kirkpatrick 2007) driven by fluvial geomorphic processes (Friedman et al 1996;Patten 1998). Vertical and horizontal distance from the river channel (thalweg) is responsible for much of the variation in local abiotic and biotic conditions (Merigliano 2005).…”

Spatio-temporal patterns along a primary succession on alluvial sediments

“…This indicates a high standing plant biomass, although this was not measured. However, where stable conditions do persist for more extended periods of time, pioneer softwood communities (as white willow stands) may be replaced by hardwood forests, often including species of Acer, Ulmus and Fraxinus [26][27][28].…”