No abstract

Persistent distribution patterns of woody vegetation within the bottomland forest of Passage Creek, Virginia, were related to fluvial landforms, channel geometry, streamflow characteristics, and sediment-size characteristics. Vegetation patterns were determined from species presence a~ oJ:>se~ed in transects and traverses on landforms developed along the stream. Distinct species distnbubonal patterns were found on four common fluvial geomorphic landforms: depositional bar, active-channel shelf, floodplain, and terrace. Independent hydrologic characteristics (flow duration a':ld floo? fr~q~:~ency) were _dete~i~ed for each of the landforms. Vegetation data were analyzed by bmary discnmmant analysis, pnnc1pal components analysis, and detrended correspondence analysis. Results and related field observations suggest that certain species are significantly associated with specific fluvial landforms. Vegetation patterns appear to develop more as a result of hydrologic processes associated with each fluvial landform rather than from sediment-size characteristics. Flood disturbance may be an important factor in maintaining the vegetation patterns, which may therefore be used as indicators for particular hydrogeomorphic site conditions. F!G. 2. Block diagram showing fluvial landforms. From the lowest, the features are: CB = channel bed, DB = depositional bar, AB =channel-shelf bank, AS= channel shelf, FB =floodplain bank, FP =floodplain, T1 =lower terrace, T" =upper terrace, and HL = hillslope. From Osterkamp and Hupp (1984).

Abstract:The response of rivers and riparian forests to upstream dams shows a regional pattern related to physiographic and climatic factors that influence channel geometry. We carried out a spatial analysis of the response of channel geometry to 35 dams in the Great Plains and Central Lowlands, USA. The principal response of a braided channel to an upstream dam is channel-narrowing, and the principal response of a meandering channel is a reduction in channel migration rate. Prior to water management, braided channels were most common in the southwestern Plains where sand is abundant, whereas meandering channels were most common in the northern and eastern Plains. The dominant response to upstream dams has been channelnarrowing in the southwestern Plains (e.g., six of nine cases in the High Plains) and reduction in migration rate in the north and east (e.g., all of twelve cases in the Missouri Plateau and Western Lake Regions). Channel-narrowing is associated with a burst of establishment of native and exotic woody riparian pioneer species on the former channel bed. In contrast, reduction in channel migration rate is associated with a decrease in reproduction of woody riparian pioneers. Thus, riparian pioneer forests along large rivers in the southwestern Plains have temporarily increased following dam construction while such forests in the north and east have decreased. These patterns explain apparent contradictions in conclusions of studies that ft-mused on single rivers or small regions and provide a framework for predicting effects of dams on large rivers in the Great Plains and elsewhere. These conclusions are valid only for large rivers. A spatial analysis of channel width along 286 streams ranging in mean annual discharge from 0.004 to 1370 cubic meters per second did not produce the same clear regional pattern, in part because the channel geometries of small and large streams are affected differently by a sandy watershed.

Geometry, channel-sediment, and discharge data were collected and compiled from 252 streamflow-gaging stations in the Missouri River basin. The stations, with several exceptions, have at least 20 years of streamflow records and represent the complete ranges of hydrologic and geologic conditions found in the basin. The data were analyzed by computer to yield simple and multiple power-function equations relating various discharge characteristics to variables of channel geometry and bed and bank material. The equations provide discharge as the dependent variable for the purpose of making estimates of discharge characteristics at ungaged sites. Results show that channel width is best related to variables of discharge, but that significant improvement, or reduction of the standard errors of estimate, can be achieved by considering channel-sediment properties, channel gradient, and discharge variability. The channel-material variables do not have uniform effects on width-discharge relations and, therefore, are considered as sediment-data groups, or stream types, rather than as terms in multiple power-function equations. Relative to streamflow, narrowest channels occur when streams of steady discharge transport sufficient silt and clay to form stable, cohesive banks but have a small bed-material load of sand and coarser sizes. Stable channels also are associated with relatively large channel gradients, relatively large channel roughness, and armoring of bed and bank by coarse particle sizes. The widest, most unstable channels are ones that apparently transport a large bedmaterial load of sand sizes. The downstream rates of change of width with discharge reflect these trends, indicating that a given bed-material load necessitates a minimum width for movement of tractive material. Comparisons of standard errors of estimate given here with similar results from regional studies are variable. It is assumed, however, that a benefit of this study is that the use of the equations is not limited to the Missouri River basin. Besides the principal utility of estimating discharge characteristics of ungaged streams, the equations given here can be used for the design of artificial channels and can be used as a basis of predicting channel changes resulting from upstream alterations of the basin or channel.

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.